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The writer is grateful to acknowledge the source of this information as based upon the work of Dr. Anthony E. Walsby, as attributed to him in various scientific references. Dr Walsby has been engaged upon this field of research for more than fourty years.

Gas Vacuoles and Gas Vesicles

Although many types of cells grow in direct contact with the atmosphere and may depend on the air for their supply of carbon, oxygen, and nitrogen, gas-filled spaces are rarely found inside the protoplasmSubstances making up a cell including its exterior membrane. of living cells. There are, however, certain prokaryotic organisms which possess gas-filled structures of a more or less permanent nature. These structures are known as gas vacuoles. They were first reported towards the end of the nineteenth century as occurring in a few speciesA taxonomic category subordinate to a genus (or subgenus) and superior to a subspecies or variety, composed of individuals possessing common characters distinguishing them from other categories of individuals of the same taxonomic level. In taxonomic nomenclature, species are designated by the genus name followed by a Latin or Latinized adjective or noun. of bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen. and blue-green algae.

History

In 1895, Ahlborn, Klebahn and Strodtmann showed in the “Hammer, Cork and Bottle” experiment that gas vacuoles could be made to disappear under moderate pressure.

The "Hammer, Cork and Bottle" experiment, showing the results of collapsing the gas vacuoles in the blue-green aalga Microcystis aeruginosa. (a) The decrease in turbidity after striking the cork in the left hand bottle (the bottle on the right is untreated). (b) The loss of buyancy apparent in the treated sample after allowing it to stand for a couple of hours.

Advance in Microscope Power

Klebahn’s suspicions on the existence of a membrane were confirmed in a rather unexpected way by electron microscope studies that were carried out many years later. In 1965 Bowen and Jensen, using thin-sectioning techniques, found that, in five species of blue-green algae they investigated, the gas vacuoles were made up of stacks of cylindrical, electron-transparent structures which they termed "gas vesicles.".

It was evident that the membrane offered far less resistance to gaseous diffusion than even a thin film of surrounding water, and from the results of manometric and pressure-rise experiments that have been carried out with N2, O2, CO2, CO, H2, Ar, and CH4 with gas vesicles of blue-green algae and bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen., it seems likely that the membranes are freely permeable to all types of gases. Two important consequences of the permeabilitya measure of a porous material's ability to allow fluids or gases to flow through its pores. An important property of rocks that determines how much and how rapidly fluids or gases can move through them; for example, how much water can be pumped from an aquifer (see: porosity). are firstly that the vacuoles cannot be used to store gas, and secondly that they cannot be inflated by gas. However, the external membrane of the vesicle is not permeable to fluid as they are inherently repellent to water, and the surface tension of fluids prevents egress into the vesicles’ rigid structure.

Gas vesicles in a freeze-fractured cell of Prosthecomicrobium pneumaticum showing the straight-sided cone, the ribs in the cone and cylinder, and the prominent central rib where the two identical halves of the structure may be joined back-to-back. Reproduced from reference Walsby, 1978, with permission from the publisher. (Micrograph by D. Branton and A. E. Walsby.) Magnification, x210,000. (image 469x450)

Physical Features of the Gas Vesicle

The unique feature of the gas vesicle membrane is that it encloses a hollow, gas-filled space, and this poses three questions: how does the gas get inside the membrane; what prevents the membrane from collapsing into the hollow space; and what prevents water from accumulating inside it?

First Evidence of Ultrasound Effect

In 1969, using a modified Warburg apparatus Walsby substantiated that the gas vesicle membrane is highly permeable to gases. He also found that after equilibration of the gas-vacuolate suspension under atmospheric pressure, no pressure change was recorded in the closed system on collapsing the gas vacuoles with an ultrasonic pulse from a transducer located underneath the Warburg flask and then re-equilibrating. Since the gas released was being transferred to an overlying gas phase whose volume was increased by a volume equal to that of the gas vacuoles, the lack of pressure change indicated that the gas had been present in the vacuoles at atmospheric pressure.

Other Factors Causing Vesicle Collapse

As is to be expected for a structure whose only component is a proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds., the strength of the vesicle is affected by a range of chemical and physical factors which affect the conformations and interactions of proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds. molecules. The vesicles become progressively weaker in temperatures in excess of 40 C, in the presence of agents which compete for hydrogen bonds, and others (such as chloroform) which destroy hydrophobic bonding. Electron paramagnetic resonance studies indicate that irreversible conformational changes take place under similar sets of conditions. The gas vesicles of both blue-green algae and halobacteria show optimalThe most favorable condition in regard to an environmental factor. stability at around pHp(otential of) H(ydrogen); Scale used to measure the alkalinity or acidity of a substance through the determination of the concentration of hydrogen ions in solution. A pH of 7.0 is neutral. Values below 7.0, to a minimum of 0.0, indicate increasing acidity. Values above 7.0, to a maximum of 14.0, indicate increasing alkalinity. 7.5 and demonstrate marked weakening and even spontaneousHappening or arising without apparent external cause; self-generated. Arising from a natural inclination or impulse and not from external incitement or constraint. Unconstrained and unstudied in manner or behavior. Growing without cultivation or human labor; indigenous. collapse at pHp(otential of) H(ydrogen); Scale used to measure the alkalinity or acidity of a substance through the determination of the concentration of hydrogen ions in solution. A pH of 7.0 is neutral. Values below 7.0, to a minimum of 0.0, indicate increasing acidity. Values above 7.0, to a maximum of 14.0, indicate increasing alkalinity. values above 10 and below 4. They show different behaviors in solutions of varying salt concentrationThe amount of a component in a given area or volume., however, for while those of the alga are unaffected by concentrated solutions of monovalent cations, those of the halobacteria become considerably stronger. Which obviously represents an adaptation to the salinityConcentration of dissolved salts found in a sample of water. Measured as the total amount of dissolved salts in parts per thousand. Seawater has an average salinity of about 34 parts per thousand (ppt), alternatively, measured as Specific Conductance or Specific Conductivity expressed in microSiemens per centimeter (µS/cm) normalized to a temperature of 25 degrees Celsius. Pure water is reckoned to be 0 µS/cm, and ocean seawater at 50,000 µS/cm. of the cellA cell is the smallest self-functioning unit found in living organisms. Each cell is enclosed by an outer membrane or wall and contains genetic material (DNA) and other parts to carry out its life functions. Some organisms such as bacteria consist of only one cell, but most of the organisms found on the Earth are made up of many cells. environment.

Collapse at Defined Range of Pressure

In 1971, Walsby also reported that gas vesicles collapse instantaneously at a well defined range of pressure, when this is applied to them in aqueous suspension as a hydrostatic pressurePressure exerted by the weight of water bearing down.

Rigid Protein Structure

Hollow, and bounded by single membranes only 2 nm thick, the vesicles were reported to have the form of cylinders, of constant diameter (75 nm) but variable length (200 to 1,000 nm), with conical ends. Bowen and Jensen could not detect these cylindrical structures in algae which had been subjected to a pressure sufficient to destroy their gas vacuoles, (by centrifuge) and they correctly concluded that the vesicles had collapsed flat to the double, membranous elementsA molecule composed of one type of atom. Chemists have recognized or created 112 different types of elements. Two or more different elements form a compound. which they found in their place. There are at least two types of proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds. present in the cyanobacterial gas vesicle; a small hydrophobic proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds. (GvpA) forms the ribs of the main structure, and a larger, more hydrophilic proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds. (GvpC) stabilizes the structure.

Walsby posits that the vesicle must, be a self-erecting structure, and suggests that it starts life as a cluster of particles, which are orientated in such a way that as more are added, a hollow structure is formed. Juvenile forms possess only the conical end pieces. The energy required to increase the size of the structure under the pressures impinging on it would have to be derived from the assembly process, but the gas would enter the enclosed space by passive diffusion. Changes to external environmentall of the external factors that may act on an organism, either plant or animal, or on a natural community. For example: gravity, air, wind, sunlight, moisture, temperature, soil, and other organisms are some of the environmental factors that may affect the life processes of an organism. may also affect the process of formation. As Waaland and Branton reported, in 1969, in blue-green alga Nostoc muscorum, gas vesicle production could be induced on dilution of its culture medium.

There seems to be no significant (inverse) correlation between the length and strength of the blue-green algal vesicles (which do not vary in diameter), and this suggests that, unlike cylinders of homogeneous construction, little support is derived from the ends along their length, i.e., the main strength of the structure resides in the individual ribs and there is little lateral support from one rib to the next. The variation in critical pressure of the algal vesicles probably reflects, therefore, variations in the strength of the wall material. Irregularities in assembly of the structure may be one cause of this; another is that the proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds. is weakened by attack from proteolytic enzymes, as has been demonstrated by experiments with isolated vesicles.

First Indication of Ultrasound as an Algae Control

The conclusion to Walsby’s 1972 paper is interesting as indicating a possible role for ultrasound as an algae control technology.

“Finally, it is suggested that the information we have gathered on the stability of gas vesicles under various conditions might also be employed in their destruction. If gas vacuoles are so important to the success of planktonic blue-green algae which form waterblooms, we might be able to control these nuisance organisms by collapsing their vacuoles. Pressures generated by explosions have been found effective in this respect, and field trials are in progress; but it is hoped that fundamental studies on these curious structures might lead to less catastrophic solutions.”

A comprehensive list in the literature named: Lyngbya sp., Anabaena flos aqua, and Microcystis aeruginosa, among others, to have gas vesicles, which are examples of speciesA taxonomic category subordinate to a genus (or subgenus) and superior to a subspecies or variety, composed of individuals possessing common characters distinguishing them from other categories of individuals of the same taxonomic level. In taxonomic nomenclature, species are designated by the genus name followed by a Latin or Latinized adjective or noun. present in the waters of Kings Bay/ Crystal River today.

Professor Walsby's foresight is affirmed today by the treatment of cyanobacteriaBacteria that have the ability to photosynthesize. with ultrasound emissions.

Showing the effects of exposure to ultrasound after 21 days on Microcystis ichthyoblabe, stock culture treated (Left) untreated (Right).
© Kyoto University, Yoshihisa Shimizu 15 August 2003. (image 800x467)


Forces Acting upon the Gas Vesicle

Diagram summarizing the source of pressure which may act on a gass vesicle inside a protasyotic cell: (a) atmospheric pressure; (h) hydrostatic pressurePressure exerted by the weight of water bearing down due to depth; (t) cell turgor pressure; (c) pressure from surface tension at the cell wall; (s) pressure from surface tension at the gas vesicle wall; (g) pressure of the vacuole gas. (reproduced from reference 108 with permission from The Royal Society, London)

A Model for the Gas Vesicle: a Pair of Flowerpots

Walsby suggests:When we consider structures in bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen.,invisible to the naked eye, we draw analogies with everyday objects that we can see and handle. Take a pair of those old-fashioned earthenware flowerpots (but without drainageholes in the bottoms), coat the inner surfaces with a film of oil, and glue them together rim to rim.

This pot is permeable to gas and therefore contains air at atmospheric pressure, although it does not need the air inside to maintain the hollow space. Now submerge the pots in water. Air continues to diffuse through the pores in the wall and can equilibrate to and fro between the enclosed gas space and the air dissolved in the surrounding medium. Liquid water will seep into the pores in the outer surface but will be held back from entering the oil-coated pores of the inner surface by surface tension. If the oil-coated pores are small enough, surface tension will even prevent water from being forced inside the chamber when a hydrostatic pressurePressure exerted by the weight of water bearing down is applied to the water. This pressure will be borne by the rigid wall with little decrease in volume. Of course, if sufficient pressure is applied, the wall will break and the gas-filled space will be lost.

The flowerpot analogy is especially useful as an aid to understanding the mechanical properties of the gas vesicle, but with some of the other properties there are problems of scale. According to the laws of physics, surface tension will be much more effective in excluding liquid water from the minuscule gas vesicle than from pores of the flowerpot, and gas will fill the gas vesicle very much faster

Subsequent Notes By Walsby

Mechanical properties at the molecular level. In making the flowerpot analogy, I commented that some of the physical properties encounter problems when modeled at different sizes. The mechanical properties do not suffer in this way as long as the model is "constructed" to scale throughout. Nevertheless, it is rather fascinating to see the theory of mechanics apply to a structure that has a wall that is only one moleculeMinute particle that consists of connected atoms of one or many elements thick (or two where GvpC crosses).

A check on the validity of these analyses has been provided by comparing two properties of the proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds. that makes the gas vesicle wall with those of nylon. This is a polymer of -[CO-NH-(CH2)n]- forming p-chains which are hydrogen bonded in the same way as polypeptide - sheet. Young’s modulus of the densest nylon is 4 GPa, and the yield stress of this material is 83 MPa ; these values are close to those of the gas vesicle proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds., 3.8 GPa for Young’s modulus and 78 MPa for the yield stress. In mechanical terms, a coil pot made of nylon ribs, suitably stiffened with fibers, would refine our model of the gas vesicle.

Using titanium, a hollow cylinder could be constructed that was stiffer, stronger, and lighter than the gas vesicle: forging structures from metal, however, is not within the repertoire of organic

  1. Relating to an organism.
  2. Derived from an organism.
evolution.

Arrangement of Gas vesicles in a cyanobacteriaBacteria that have the ability to photosynthesize.

Transverse section of a dividing cell of the cyanobacterium Microcystis sp. showing hexagonal stacking of the cylindrical gas vesicles. (Micrograph by H. S. Pankratz.) Magnification, x31,500. (image 665x700)

Functions of the Gas Vesicle and Influencing Factors

Buoyancy.

Twenty two years later Walsby summarizes in a 1994 paper, that gas vesicles occur almost exclusively in procaryotes from aquatic habitats. Their function is to provide buoyancy, which allows aerophilic bacteria to float into oxygenated surface waters and enables cyanobacteriaBacteria that have the ability to photosynthesize. to float up toward the light; some cyanobacteria and photosynthetic bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen. regulate buoyancy provided by their gas vacuoles and they stratify in layers below the water surface. The mechanisms of buoyancy regulation in cyanobacteria may involve modulation of gas vesicle gene expression and the destruction of gas vesicles by turgor pressure (which can be measured by using gas vesicles as pressure probes); they also involve counteracting effects of changes in carbohydratesIs an organic compound composed of carbon, oxygen, and hydrogen atoms. Some examples are sugars, starch, and cellulose. and other dense substances. These mechanisms are affected by light, and this explains how these organisms regulate their vertical distribution in natural waters.

Intact gas vacuoles can provide buoyancy because they have an overall density much less than that of the aqueous medium in which the various organisms grow. The probable density of the constituent gas vesicles can be calculated from estimates of the density of the membrane, its thickness, and the overall shape of the structure. Assuming the gas vesicles of blue-green algae to be cylinders of mean length 370 nm and width 71 nm capped by right cones with a solid angle of 70°, and the mean thickness of the membrane to be 1.62 nm, then the ratio of membrane volume to the total volume of the structure is calculated to be 1.0:10.6.

Effect of Colonial Forms

Walsby establishes that the function of the vesicles is to afford buoyancy to the blue-green alga. it is interesting to note that, while most other planktonic algae tend towards the unicellular habit or small, divided forms giving slow sinking rates, many gas vacuolate blue-green algae have adopted streamlined colonial forms which give high rates of flotation; viz., the flakes of Aphanizomenon, balls of Microcystis and Coelosphaerium, and rafts of Anabaena (L.). Reynolds has shown that the observed flotation potential of blue-green algae gathered from waterblooms is correlated above all else with colony size.

Levels of Nutrient Concentration

It is likely that the buoyancy provided by gas vacuoles has some more subtle significance to many planktonic blue-green algae than merely that of carrying them to the water surface. The same must be true of gas vacuoles in the photosynthetic bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen., since these organisms are obligate anaerobes which would not survive exposure to light in the aerated medium of water surfaces. Lund, 1969, has pointed out that the buoyancy provided by gas vacuoles will produce "forced convection" as the alga (or bacterium) moves with respect to the water mass, and that as with sedimenting algae this may be of importance in maintaining around the cells steep diffusion gradients of nutrientsAny food, chemical element or compound an organism requires to live, grow, or reproduce.. Lund also remarked on the paradox of algae with gas vacuoles maintaining a position someway below the lake surface. That this even occurred in the waters under ice, in which turbulenceAn eddying motion of the atmosphere that interrupts the flow of wind. is completely absent, demonstrated that the alga could not be lighter than water and provided the first suggestion that gas vacuoles might be important in regulating buoyancy.

Variation in the ratio of cell volume to gas vacuole volume may be brought about by changes in the respective rates at which cell materials and intact gas-vacuoles (i) are produced, and (ii) disappear. Such changes will, in theory, result in variation of the cell density and will determine whether the cell sinks or floats, and how rapidly it does so. If the changes are correlated with physical or chemical conditions which form vertical gradients in natural waters, they may provide the cells with a means of regulating their position in the vertical water column. There are a number of conditions which may vary with depth, and, in particular, light intensity, temperature, salinityConcentration of dissolved salts found in a sample of water. Measured as the total amount of dissolved salts in parts per thousand. Seawater has an average salinity of about 34 parts per thousand (ppt), alternatively, measured as Specific Conductance or Specific Conductivity expressed in microSiemens per centimeter (µS/cm) normalized to a temperature of 25 degrees Celsius. Pure water is reckoned to be 0 µS/cm, and ocean seawater at 50,000 µS/cm., oxygen tension and pressure may form stable gradients.

Light Intensity

The degree of gas vacuolation in various blue-green algae has been reported to respond in different ways to light intensity, but in three planktonic forms which have been investigated, by Walsby,Anabaena flos-aquae ( L.), Oscillatoria redekei, and Microcystis aeruginosa, gas vacuoles are recorded as being more abundant in alga grown under low light. Two mechanisms have been proposed to explain this.

The first is that the formation of gas vacuoles may proceed independently of light intensity, with the result that as this increases they become diluted out by the greater vegetative growth that this sustains. Smith and Peat (1967) found that gas-vacuolation of Anabaena flos-aquae was least in the early exponential phase when growth of the alga was most rapid (on the bottom); and Lehmann and Jost (1971) demonstrated a similar response in Microcystis aeruginosa by the gas vesicle-counting method. They concluded that the decrease in gas vesicle numbers must have resulted from dilution by cell growth, regarding the disappearance of these structures as being unlikely because of their "high stability." It has been pointed out, on the other hand ( Walsby, 1969), that the fall in gas-vacuolation may be a direct response (perhaps by the second mechanism) of the cells to the higher light intensities which exist in young cultures.

The second mechanism is that the alga disposes of intact gas vesicles in high light intensity by collapsing them. It has been found that when Anabaena flos-aquae is grown under low light intensity its cells have a lower turgor pressure and a proportion of the gas vesicles with a lower critical pressure than when the alga is grown under high light. When the alga was transferred from low to high light intensity, the cell turgor pressure was found to increase in instances from less than 3 to more than 4.5 atm (290 to 460 kN m-2) and at a rate of nearly 1 atm (94 kN m- 2) per hr. The rise in turgor pressure, which results from the increased accumulation of photosynthate, was found to be sufficient to collapse enough of the alga’s gas vesicles to destroy its buoyancy. The loss of buoyancy was often quite rapid, being detectable within half an hour, and nearly complete at two hours. Sometimes rather more gas vesicles appeared to be lost than could be accounted for by the rise in turgor alone and while this may have been an artifact of measurement it is also possible that some vesicles collapsed as a result of their becoming weakened in a changing cellular environmentall of the external factors that may act on an organism, either plant or animal, or on a natural community. For example: gravity, air, wind, sunlight, moisture, temperature, soil, and other organisms are some of the environmental factors that may affect the life processes of an organism.. Walsby found that several factors affect the strength and stability of isolated gas vesicles, and two of these, decrease in pHp(otential of) H(ydrogen); Scale used to measure the alkalinity or acidity of a substance through the determination of the concentration of hydrogen ions in solution. A pH of 7.0 is neutral. Values below 7.0, to a minimum of 0.0, indicate increasing acidity. Values above 7.0, to a maximum of 14.0, indicate increasing alkalinity. (as might occur with the production of organic

  1. Relating to an organism.
  2. Derived from an organism.
acids) and exposure to proteolytic enzymes, might be important in weakening them inside cells.

It is plain that any response which results in decreased gas-vacuolation with increasing light intensity could enable the alga to select a position on a vertical light gradient, and there are several accounts of blue-green algae doing this in freshwater lakes ( J. W. G. Lund). Fogg has argued that the conditions which exist in the discrete layers occupied by these algae may be those most suitable for their growth. In fact it has been observed that as inorganic nutrientsAny food, chemical element or compound an organism requires to live, grow, or reproduce. become depleted in certain freshwater lakes, the population maxima of gas-vacuolate blue-green algae (but not other forms) move down. This suggests that the algae station themselves at a depth where the mean light intensity does not exceed that required to maintain growth at a level which is limited by other factors. It has been proposed that just such a response would be provided by the mechanism described above in which the rise in turgor pressure results from the amount of carbon fixed in photosynthesisIs the chemical process where plants and some bacteria can capture and organically fix the energy of the sun. This chemical reaction can be described by the following simple equation:
6CO2 + 6H2O + light energy >>> C6H12O6 + 6O2
The main product of photosynthesis is a carbohydrate, such as the sugar glucose, and oxygen which is released to the atmosphere. All of the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical combining of carbon dioxide and water with sunlight. This chemical reaction is catalyzed by chlorophyll acting in concert with other pigment, lipid, sugars, protein, and nucleic acid molecules. Sugars created in photosynthesis can be later converted by the plant to starch for storage, or it can be combined with other sugar molecules to form specialized carbohydrates such as cellulose, or it can be combined with other nutrients such as nitrogen, phosphorus, and sulfur, to build complex molecules such as proteins and nucleic acids. Also see chemosynthesis. It is said that photosynthesis gives rise to three quarters of the world supply of oxygen that we breathe.
exceeding that which can be assimilated by the cells (Dinsdale and Walsby, 1972). The collapse of gas vacuoles under rising turgor pressure is sufficiently rapid to explain another curious behavior pattern of blue-green algae, that of diurnal migrationThe moving of one species or a group of species from one area to another. to and from the water surface during night and day, respectively (Ganf, 1939 and Sirenko et al ,1968). This behavior would also seem to be connected with avoiding high incident light intensities, particularly in the case of the tropical lake described by Ganf.

Gas Vesicle Width

It is in the natural environmentall of the external factors that may act on an organism, either plant or animal, or on a natural community. For example: gravity, air, wind, sunlight, moisture, temperature, soil, and other organisms are some of the environmental factors that may affect the life processes of an organism. of ponds, lakes, and seas that gas vesicles have evolved; it is proposed that two counteracting factors have been involved in the selection of their width. The amount of gas space enclosed by gas vesicle proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds. increases with the width of a gas vesicle. There should therefore have been selection for wide gas vesicles, which provide buoyancy with greater economy than narrow ones. For mechanical reasons, however, the critical collapse pressure of a gas vesicle varies inversely with its width. The highest pressure to which organisms will be exposed in their natural habitatThe place or set of environmental conditions in which a particular organism lives. must therefore have set an upper limit on the width of their gas vesicles. Put simply, the greater the depth of the water column in a lake or sea (and the higher the cell turgor pressure), the narrower the gas vesicle must be to withstand the pressure.

Mistaken Postulate

The writer has postulated elsewhere that floating mats of algae sink following heavy rain because the rain pounding on the mats destroys buoyancy as gas trapped in the mat folds are released into the atmosphere by the pounding of the raindrops. In light of Walsby, I am now of the opinion that the heavy rain produces some other effect to weaken at least some of the vesicles so that they collapse. This reduces the overall buoyancy of the mat to the degree that it is unable to continue to float at the surface of the water. The effects could involve any of those mentioned by Walsby, vis., pressure upon the vesicles by the rain pounding, increases in dissolved oxygenMeasures the amount of gaseous oxygen dissolved in an aqueous solution. Oxygen gets into water by diffusion from the surrounding air, by aeration (rapid movement), and as a waste product of photosynthesis. at the surface due to the rain drop aeration, possibly causing chemical changes and action to weaken the proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds. structures. The cyanobacteriaBacteria that have the ability to photosynthesize. of the mat would, grow new vesicles so that floating of the mat could recur.

Some interesting micrographs are shown from a paper of Anthony E. Walsby of 1994.

Phase-contrast light micrographs of filaments of the cyanobacterium Anabaena flos-aquae before (a) and after (b) collapse of their gas vesicles by application of a pressure of 1.0 MPa (10 bar). The cells are approximately 5,um wide.

Whatever the causes of variation in critical pressure are, it is clear that the blue-green algae require gas vesicles which are stronger than those of the halobacteria. Firstly, they inhabit deeper waters than the shallow brine pools occupied by the salt bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen.. But, more important, they generate cell turgor pressure of several atmospheres. This has been demonstrated,in the absence of any other suitable method for prokaryotic organisms, by using the gas vesicles themselves as devices for measuring hydrostatic pressurePressure exerted by the weight of water bearing down. Thus it appears that the gas vesicles in these different groups of organisms are adaptedTo be accustomed to the natural factors that are in a given area and to be able to survive these factors, being either positive or negative. to withstand the pressures to which they are likely to be subjected.

Mode of gas vesicle collapse.

Gas vesicles collapse when exposed to a sufficient hydrostatic pressurePressure exerted by the weight of water bearing down. The central cylinder collapses to a flat rectangular envelope; the conical end caps collapse to sectors of circles that, because of their curved edges, form a tangential contact with the ends of rectangle. As the cylinder collapses, its cross-section must deform throughout its length and splits must propagate parallel to one another on diametrically opposite sides along the entire length, to enable the collapsed structure to lie flat. Evidence that the structure does lie flat is provided by its tendency to form scrolls or to fold. The regularity of the collapsed structures must stem from the crystalline perfection of the wall. Thin-walled cylinders (gas vesicle walls are one or two molecules thick) constructed of high-strength materials collapse by instability failure. At a given pressure the cylinder goes out of round and buckles, so that the forces become concentrated in the more highly curved regions. A "runaway" situation develops, and the structure collapses. Such instability does not occur when the excess pressure is on the inside, because the wall is uniformly under tension and no buckling results. This may, perhaps, be more easily understood by returning to the analogy of our rod of gas vesicle proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds.. Under tension the rod keeps straight and remains intact until the extensional yield stress is reached. Under compression the rod bends and then snaps at a much lower load. The flattened form of the collapsed gas vesicle clearly indicates instability failure, and this is confirmed by the finding that the critical collapse pressure is much lower than the pressure at which gas vesicles explode.

Critical Pressure

The critical pressure (or critical collapse pressure) is the minimum difference in pressure between the outside and the inside of the gas vesicle that causes the structure to collapse . The only source of pressure on the inside is the gas, which is always in equilibrium with the gas dissolved in the suspending medium. The gas vesicle gas is therefore usually air at the ambient pressure and balances the overlying air pressure. Consequently, the critical pressure (pa) can be functionally defined as the gauge pressure at which a gas vesicle collapses in a suspension (previously) equilibrated with air.

Variation in Shape

Part of the variation in critical pressure in different speciesA taxonomic category subordinate to a genus (or subgenus) and superior to a subspecies or variety, composed of individuals possessing common characters distinguishing them from other categories of individuals of the same taxonomic level. In taxonomic nomenclature, species are designated by the genus name followed by a Latin or Latinized adjective or noun. can be explained by the variation in cylinder radius of the gas vesicles. From the theory of mechanics (see Buckling Pressure, below), it is expected that the collapse pressure will vary inversely as some function of radius. Surveys of gas vesicles in eight genera of cyanobacteriaBacteria that have the ability to photosynthesize. in which the mean cylinder radius varied from 31 nm in an Oscillatoria strain to 55 nm in Dactylococcopsis salina showed (empirically) that the mean critical pressure was inversely correlated with radius.

The same general trend holds when extrapolated in either direction: the narrowest gas vesicles recorded (r = 23 nm) occur in Trichodesmium thiebautii, in which the highest critical pressures have been measured (pc = 3.7 MPa ); the widest occur in halobacteria, in which r can exceed 100 nm and pc = 0.09 MPa. Part of the variation in critical pressure within each species can also be attributed to variation in cylinder width. In Anabaena flos-aquae the variation in cylinder radius accounts for about half of the critical pressure variation. No similar correlation has been found between the critical pressure and the length of the gas vesicles. It has been concluded that the individual ribs derive little support from the ends of the structure via their neighbors.

Buckling pressure

For an unstiffened cylinder, there is a theory that relates the buckling pressure to Young’s modulus of the wall material. Accordingly, the Microcystis gas vesicle would have a predicted buckling pressure of 0.18 MPa. This value is considerably lower than the mean critical pressure of the intact gas vesicle (about 0.8 MPa) but quite close to the mean critical pressure (0.23 MPa) of gas vesicles that have had the outer proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds., GvpC, removed. It can be envisaged that each rib has a weak spot in a different position and therefore, when under pressure, tends to go out of round at a different position. Such distortion is resisted by the straps of GvpC molecules, which bind the ribs together and thereby provide a sort of corset that prevents the gas vesicle from bulging out. The advantages of providing stiffening to postpone buckling should increase with cylinder width.

Yield stress

It has been explained that the gas pressure inside a gas vesicle can be raised by infiltration with gas from an overlying gas phase and that gas vesicles can be equilibrated under high gas pressures. If the overlying gas pressure is then lowered, the gas pressure inside the gas vesicle will exceed the pressure on the outside. If the overlying gas pressure is lowered rapidly enough, the pressure difference between the inside and outside may be sufficient to cause the gas vesicle to explode. No buckling occurs with excess pressure on the inside (just as no buckling occurs with a rod under tension); consequently, failure of the gas vesicle will not occur until the yield pressure is reached.

The yield pressure has been determined so far only with the gas vesicles of Microcystis sp., which have a meanpy of 4.3 MPa (43 bars). This is five times greater than the mean critical pressure of these gas vesicles. It confirms that collapse under external pressure occurs through instability failure due to buckling, even though this is postponed (also approximately fivefold) through the stiffening provided by GvpC. A quantitativeRelating to or expressed as a specified or indefinite number or amount. analysis of the amount of gas present supports the conclusion that the gas vesicles would have exploded and formed bubbles within the cells (251).

Gas vesicle collapse in cells: turgor pressure.

The collapse of gas vesicles inside cells is governed by the same principles that apply to isolated gas vesicles, but there are several factors in cells (such as temperature, pHp(otential of) H(ydrogen); Scale used to measure the alkalinity or acidity of a substance through the determination of the concentration of hydrogen ions in solution. A pH of 7.0 is neutral. Values below 7.0, to a minimum of 0.0, indicate increasing acidity. Values above 7.0, to a maximum of 14.0, indicate increasing alkalinity., salinityConcentration of dissolved salts found in a sample of water. Measured as the total amount of dissolved salts in parts per thousand. Seawater has an average salinity of about 34 parts per thousand (ppt), alternatively, measured as Specific Conductance or Specific Conductivity expressed in microSiemens per centimeter (µS/cm) normalized to a temperature of 25 degrees Celsius. Pure water is reckoned to be 0 µS/cm, and ocean seawater at 50,000 µS/cm., surfactants, and enzymes) that may affect the stability of the gas vesicle proteinOrganic substances primarily composed of carbon, hydrogen, nitrogen, and some other minor elements which are arranged in about 20 different compounds known as amino acids. The various amino acids found in a protein are linked together by peptide bonds. and hence the critical pressure of the structures. The most important factor, however, is cell turgor pressure.

The cells of bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen. are usually distended by a turgor pressure that equalizes the difference in water potential between the outside and inside of the cell. This turgor presses out on the cell wall and in on the gas vesicle and contributes to pressures causing collapse of the structures. The first measurement of turgor pressure per se (i.e., rather than solute potential) in bacteria was made by using gas vesicles as pressure sensors. The mean critical pressureof gas vesicles was determined in cells suspended in hypertonic sucrose solution, which removed the turgor pressure; the mean apparent critical pressure of the gas vesicles was measured in another sample of the turgid cells, suspended in the dilute culture medium; the turgor pressure could then be calculated from the difference in these two measurements.

The critical collapse pressure of gass vesicles isolated from Anabaena flos-aquae varies from about 0.45 to 0.85 MPa (solid symbols). Gas vesicles inside the turgid cells are collapsed by application of 0.05 to 0.45 MPa (open symbols) because they are already subjected to a cell turgor pressure of about 0.4 MPa. The difference between the two curves gives the turgor pressure. The turgor pressure falls slightly as the gas vesicles are collapsed (see below for an explanation). Redrawn from Walsby (239) with permission from the publisher.

Writer's Note

Although Professor Walsby used this arrangement to measure turgor pressure, its importance today is that it reveals the hydrostatic pressurePressure exerted by the weight of water bearing down required to collapse a gas vesicle. And that it is within the capability of Ultrasound emissions in use to control algae today.

This simple measurement gives a result that, for cyanobacteriaBacteria that have the ability to photosynthesize., is usually within a few percent of the original turgor pressure. There are a number of refinements that may be needed to give more precise values in these and other organisms, the most important of which concerns the loss of turgor pressure as gas vesicles are collapsed.

For this review the principal interest of turgor pressure is its role in the regulation of gas vesicles themselves; in some organisms, gas vesicles can be collapsed when the turgor pressure rises. Cell turgor pressure is not static but increases with the concentrationThe amount of a component in a given area or volume. of ions or other small molecules in the cytoplasmAll of the protoplasm in a cell except for what is contained in the nucleus. Measurements by the gas vesicle method have shown that turgor pressure rises in cyanobacteriaBacteria that have the ability to photosynthesize. exposed to high photon irradiances; about half of the rise in turgor can be attributed to an increase in the concentration of low-molecular-weight photosynthetic products, and about half can be attributed to the light-stimulated uptake of potassium salts. Such light-dependent rises in turgor may be an inevitable consequence of increased photosynthetic activity and perhaps occur in all cyanobacteriaBacteria that have the ability to photosynthesize.. They have now been demonstrated in a number of gas-vacuolate species of Anabaena ,Aphanizomenon, Gloeotrichia, Nostoc , Oscillatoria , and Microcystis . Gas vesicle regulation by cell turgor is discussed below (see Gas Vesicle Collapse by Turgor Pressure).

Gas vesicle versus flagellum. The buoyancy provided by gas vesicles enables bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen. to rise up toward the water surface, to remain in suspension, or to perform vertical migrations in water. In theory, swimming by means of flagella might provide an alternative means of doing the same. Flagellar movement is more versatile, of course, because it also permits movement in the horizontal plane and tactic or phobic responses to directional stimuli. Why, then, do some microorganismsExtremely small organism that can only be seen using a microscope. have gas vesicles rather than flagella? Walsby suggests three possible reasons: (i) that the genetic information for production of flagella does not occur in some groups of procaryotes; (ii) that there are difficulties in sustaining upward migration by flagellar swimming; and (iii) that for some procaryotes, gas vesicles provide a cheaper or faster means of transport.

  1. Occurrence. In some groups of bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen. flagella are unknown, and the gas vesicle may provide the only available means of vertical movement. Flagella have not been found in cyanobacteriaBacteria that have the ability to photosynthesize., for example, although Waterbury et al. have discovered a few marine speciesA taxonomic category subordinate to a genus (or subgenus) and superior to a subspecies or variety, composed of individuals possessing common characters distinguishing them from other categories of individuals of the same taxonomic level. In taxonomic nomenclature, species are designated by the genus name followed by a Latin or Latinized adjective or noun. that are capable of swimming. In other groups, such as the purple and green sulfur bacteria, both gas vesicles and flagella are present, although particular species usually have exclusively either one or the other of these structures). Chemotactic swimming while floating would usually be self-defeating. Halobacteria possess both gas vesicles and flagella contemporaneously, although they seem to use them for different purposes. As discussed above, the gas vesicles may function in buoying cells to the oxygenated layers of the brine surface; the flagella are used in phototaxis. Walsby observed that the buoyant cells of gas-vacuolate halobacteria accumulate at the surfaces of liquid cultures by floating up, but gas-vacuoleless mutants do not accumulate at the surface by swimming.

  2. Sustained upward movement. The gas vesicle can provide sustained upward transport for days on end and move cells over considerable depths. The prime function of the flagellum in bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen. is executing chemotaxis. Most flagellate bacteria have no means of sustaining a vertical swimming direction over long periods. The only exceptions appear to be phototrophs that swim phototactically up a vertical light gradient and magnetotactic bacteria that usually swim downward in seeking the Earth’s magnetic pole. No magnetotactic forms have been reported from the planktonMinute plant (phytoplankton) and animal organisms (zooplankton) that are found in aquatic ecosystems.. Some purple sulfur bacteria that lack gas vesicles use flagellar swimming for vertical migration in lakes where other gas-vacuolate purple sulfur bacteria are simultaneously stratifying.

  3. Cost. Perhaps one of the reasons why the flagellum has not replaced the gas vesicle in groups where both may occur is the burden of operating each of the respective structures. Following an ingenious cost-benefit analysis developed by Raven for assessing the performance of various organellesIs a specialized structure found in cells that carry out distinct cellular functions., Walsby concluded that there would be benefit for gas vesicles in slowly growing cyanobacteriaBacteria that have the ability to photosynthesize. and flagella in rapidly dividing heterotrophic bacteria.

  4. Velocity of movement. A final factor that should be mentioned in comparing the flagellum and gas vesicle is the velocity of movement. High flotation velocities, which are purely a consequence of size (Stokes’s law), are necessary for the daily migration over depths of many meters in stratified lakes. Colonies are formed not only by planktonic cyanobacteriaBacteria that have the ability to photosynthesize. but also by some of the purple bacteriaSimple single celled prokaryotic organisms. Many different species of bacteria exist. Some species of bacteria can be pathogenic causing disease in larger more complex organisms. Many species of bacteria play a major role in the cycling of nutrients in ecosystems through aerobic and anaerobic decomposition. Finally, some species form symbiotic relationships with more complex organisms and help these life forms survive in the environment by fixing atmospheric nitrogen. (Thiopedia spp.) and methanogenic archaeaIs a group of recently discovered organisms that resemble bacteria. However, these organisms are biochemically and genetically very different from bacteria. Some species of the domain Archaea live in the most extreme environments found on the Earth. (Methanosarcina spp.).

The above diagram by Walsby is a summary of the factors involved in the regulation of gas vesicle content and buoyancy in planktonic cyanobacteriaBacteria that have the ability to photosynthesize.. The signs indicate whether the factor increases (+) or decreases (-) as a result of the change (+ or -) in the preceding factor. (image 800x600)

Writer’s Note:

It seems clear that ultrasound will collapse gas vesicles and this will cause the cyanobacteriaBacteria that have the ability to photosynthesize. to sink, but not necessarily kill them. Also, pressures from ultrasound emissions will likely, if strong enough, disrupt their structure sufficiently for them to die. Although “grounded” the algae will try to grow fresh vesicles so that they may take in more nutrientsAny food, chemical element or compound an organism requires to live, grow, or reproduce. and rise in order to engage in photosynthesisIs the chemical process where plants and some bacteria can capture and organically fix the energy of the sun. This chemical reaction can be described by the following simple equation:
6CO2 + 6H2O + light energy >>> C6H12O6 + 6O2
The main product of photosynthesis is a carbohydrate, such as the sugar glucose, and oxygen which is released to the atmosphere. All of the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical combining of carbon dioxide and water with sunlight. This chemical reaction is catalyzed by chlorophyll acting in concert with other pigment, lipid, sugars, protein, and nucleic acid molecules. Sugars created in photosynthesis can be later converted by the plant to starch for storage, or it can be combined with other sugar molecules to form specialized carbohydrates such as cellulose, or it can be combined with other nutrients such as nitrogen, phosphorus, and sulfur, to build complex molecules such as proteins and nucleic acids. Also see chemosynthesis. It is said that photosynthesis gives rise to three quarters of the world supply of oxygen that we breathe.
. With a continual exposure to ultrasound emissions, sufficient only to sink them, the effort expended of the algae to grow new vesicles will weaken them over time, since the emissions collapse the vesicles even as they grow anew. Thus cyanobacteriaBacteria that have the ability to photosynthesize. growth will likely be kept in check until their demise through lack of food as an effective algae control technology.

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