The Interrelations of Cell Turgor Pressure, Gas-vacuolation, and Buoyancy in a Blue-green Alga

The increase in pressure required to collapse gas vacuoles onsuspending the cells of the blue-green alga Anabaena flos-aquaein hypertonic sucrose solutions shows the turgor pressure tovary over the range of 265 to 459 KN m^–2 under differentculture conditions. The cell turgor increased at a rate of upto KN m^–2 h^–1 on transferring the alga from lowto high light intensity. This rise appears to be a result ofthe accumulation of photosynthate, as it is dependent on thepresence of carbon dioxide in the gas phase and is inhibitedby DCMU. Experiments using ¹⁴CO² indicate that the increasedrate of photosynthesis during the high light exposure is easilysufficient to account for the observed turgor rise. The rise in turgor can bring about collapse of sufficient ofthe alga's gas vacuoles to destroy its buoyancy. Higher turgorpressures, and consequently a lower degree of gas vacuolationand buoyancy, were maintained when the alga was kept at highlight intensitives for a week and more. The significance ofthis behaviour is discussed in relation to stratification ofplanktonic blue-green algae in natural habitats.     

The Role of Potassium in the Control of Turgor Pressure in a Gas-Vacuolate Blue-Green Alga

The contribution of K⁺ accumulation to cell turgor pressurewas investigated in the gas-vacuolate blue-green alga Anabaenaflos-aquae. The cell turgor pressure, measured by the gas vesiclemethod, drops in cells suspended in culture medium depletedof K⁺ but rapidly rises again, by 100 kPa or more, when K⁺ isresupplied. A similar though rather slower rise in turgor pressureis supported by an equivalent concentration of Rb⁺. The internalK⁺ concentration rose from 66 to 91 mM when K⁺ was suppliedat an external concentration of 0.4 mM. This rise was light-dependent.Greater increases in internal K⁺ concentration and turgor pressureoccurred when K⁺ was supplied at a higher concentration, 3.6mM. In both cases over 60% of the observed turgor pressure risecould be accounted for by accumulation of K⁺. The turgor pressurerise supported by light-stimulated K⁺ uptake can cause collapseof enough of the alga's gas vesicles to destroy its buoyancy.The effect of K⁺ availability on buoyancy regulation by planktonicblue-green algae is discussed.     

Buoyancy regulation and vertical migration by Oscillatoria rubescens in Crooked Lake,Indiana

Filaments of Oscillatoria rubescens stratified in the metalimnion of Crooked Lake, Indiana at depths of 6–9 m, where the incident light intensity averaged 2% of the surface intensity. Buoyancy (due to gas vesicles) was regulated in response to light intensity, and increased turgor pressure generated at high light intensity could contribute to the collapse of gas vesicles. Filaments exposed to irradiances of 20–50 µE m^-2 s^-1 had neutral buoyancy. As nutrient availability was increased (by resuspending filaments in nutrient-rich water from the hypolimnion or by preventing CaCO₃ precipitation with a calcium chelator), higher light intensities were necessary for buoyancy loss and increased turgor.

A series of traps were placed in the lake to intercept floating and sinking filaments. Migration activity (both floating and sinking) was greatest 1 m above the most dense concentration of O. rubescens. These results, together with vertical profiles of primary production, suggest that maximum production by O. rubescens occurred above the population maximum in the water column.     

REGULATION OF GAS VESICLE CONTENT AND BUOYANCY IN LIGHT- OR PHOSPHATE-LIMITED CULTURES OF APHANIZOMENON FLOS-AQUAE (CYANOPHYTA)1

Measurements of the gas vesicle space in steady-state light or phosphate-limited cultures of Aphanizomenon flos-aquae Ralfs, strain 7905 showed that gas vesicle content decreased as energy-limited growth rate increased hut was the same at several phosphate-limited growth rates. Upon a decrease in growth irradiance, gas vesicle content did increase in phosphate-limited cultures, hut the cultures remained nonbuoyant as long as P was limiting. Buoyant, energy-limited cultures lost their buoyancy in less than 2 h when exposed to higher irradiances. The primary mechanism for buoyancy loss was the accumulation of polysaccharide as ballast. Collapse of gas vesicles by turgor pressure played a minor role in the loss of buoyancy. When cultures were exposed to higher irradiances, cells continued to synthesize gas vesicles at the same rate as before the shift for at least 1 generation time. The amount of ballast required to make individual filaments in the population sink varied 4-fold. This variation appears to be due to differences in gas vesicle content among individual filaments.     

WATER-BLOOMS

1. Peculiarities in the ecology of planktonic blue-green algae are reviewed in relation to recent advances in understanding their physiological characteristics. 2. Dense water-blooms are always the result of buoyant migration of existing populations to the lake surface under calm weather conditions. The size of the population is the direct result of photoautotrophic growth, and is dependent upon light and the availability of inorganic nutrients; it is apparently enhanced by moderately high water temperatures, high pH, low oxygen tensions and possibly, the presence of organic solutes. The relative effectiveness of these factors is untested. 3. Buoyancy is imparted by gas vacuoles whose principal function is to regulate the position of the alga in the water column. Control is effected by two mechanisms: (i) ‘dilution’ of newly produced vacuoles during active cell division; (ii) changes in cell turgor-pressure acting on the gas-vacuole structure. Gas-vacuole production is greatest at low light intensities and the alga becomes more buoyant; at higher light intensities, increased turgor-pressure collapses the weaker vacuoles causing the alga to lose buoyancy. 4. Potentially, algae are able to poise themselves at an optimum point in the light gradient, usually towards the bottom of the euphotic zone, where the algae are likely to encounter the conditions most favouring their growth. 5. Different species of blue-green algae differ in the typical sizes of their colonies and, hence, in their rates of controlled movement. These differences are interpreted as hydrodynamic adaptations to the variations in turbulent water movements to which the algae are subject. 6. Populations of single-filamentous Oscillatoria agardhii and O. rubescens come to occupy the stable metalimnia of stratified lakes, provided that they are located within the euphotic zone. 7. The large stream-lined colonial forms occur mainly in polymictic lakes and in the unstable epilimnia of stratified lakes where light penetration is restricted to the superficial layers. These algae are adapted to sink or float rapidly to the optimum depth when turbulence subsides. Because of their potentially high rates of movement, it is the large colonial forms that commonly form blooms. 8. Bloom formation can occur when most of the algae possess excess buoyancy. Excess buoyancy is acquired when the photosynthetic rate is insufficient to develop the necessary turgor-pressure to cause collapse of the vacuoles. Photosynthesis may be sufficiently impaired under four circumstances: (i) during turbulent circulation of the population over a depth that significantly exceeds the euphotic depth; (ii) in the absence of light (e.g. at night): (iii) at limiting concentrations of carbon dioxide: and (iv) when the algal population is senescent. 9. Because bloom-formation depends upon the coincidence of persistent algal overbuoyancy with calm weather, its occurrence is incidental, and serves no vital function in the biology of blue-green algae. 10. Some possible causes for the occurrence of blue-green algal blooms in a relatively restricted range of water bodies are discussed. Large bloom-forming populations are probably restricted to moderately rich, mildly alkaline, thermally unstable lakes in all regions, except those which are permanently cold. Extremes of poverty or richness of nutrients, short water-retention times and low pH seem to be factors which select against planktonic blue-green algae.     

The impact of year-to-year changes in the weather on the dynamics of Daphnia in a thermally stratified lake

The factors influencing the seasonal dynamics of Daphnia in a thermally stratified lake (Esthwaite Water) are described and related to long-term changes in the weather. The Daphnia produced three cohorts in the year and the strength of the cohorts was determined by year-to-year variations in the physical characteristics of the lake and the abundance of edible algae. Food was most abundant in early summer when small, fast-growing flagellates were particularly common. In late summer, the phytoplankton community was dominated by large, inedible species but edible forms re-appeared when nutrients were entrained by wind mixing. Examples are presented to demonstrate the effect that year-to-year variations in the weather have on the growth of the phytoplankton and the dynamics of the Daphnia. In ‘good’ years, when the lake stratifies early and there are periods of episodic mixing in summer, there are two ‘pulses’ of edible algae and two strong cohorts of Daphnia. In ‘bad’ years when stratification is delayed and there is little episodic mixing, the growth of the edible algae is suppressed and the Daphnia produce two weak cohorts. The results are discussed in relation to the impact of intermediate disturbances on growth of phytoplankton and current theories of population regulation in Daphnia. The evidence suggests that the dynamics of the Daphnia in the lake are strongly influenced by seasonal variations in the mixing regime, the recycling of nutrients and the episodic growth of edible algae.     

The properties and buoyancy-providing role of gas vacuoles in Trichodesmium Ehrenberg

All three species of the marine blue-green alga Trichodesmium collected in the Sargasso and Caribbean seas were found to possess gas vacuoles. The constituent gas vesicles were much stronger than those found in any freshwater blue-green alga, the mean critical collapse pressures being 12 bars in T. erythraeum, 34 bars in T. contortum and 37 bars in T. thiebautii. This great strength is obviously an adaptation to the hydrostatic pressures at the depths to which these organisms occur in the ocean. In each case the gas vesicles are far too strong to be collapsed by rising cell turgor pressure, though gas-vacuolation could be slowly regulated by the differential growth of gas vesicles and cells. Since the vesicles are of a similar shape and size to those in other species, the vesicle wall material must be stronger. The majority of Trichodesmium colonies collected were positively buoyant, and in all cases tested the buoyancy was dependent on the presence of gas vacuoles. The buoyancy is important in increasing the residence time of these slowly growing algae in the euphotic zone and it is responsible for the surface water-blooms which they form.     

Interrelations of photosynthetic behaviour and buoyancy regulation in a natural population of a blue-green alga

The photosynthetic activity of Anabaena cirdnalis and associated changes in buoyancy were determined from prepared suspensions exposed in the natural light field of Crose Mere. The observations are related to variations in subsurface irradiance and temperature. Parallel experiments, aimed at trapping algal colonies undertaking controlled vertical movements within the lake system, are also described. Buoyancy loss and downward migration are clearly associated with specific photosynthetic rates: rates as low as 1.8 mg O₂ (mg chlorophyll a) h⁻¹ are shown to be sufficient to effect buoyancy loss, while movements in the lake tend towards a depth where rates of 5–7 mg O₂ (mg chlorophyll a)⁻¹ h⁻¹ are possible. These rates are significantly less than those possible at light saturation. The effect of increasing temperature is to depress the population in the light-gradient. The significance of this response is discussed in relation to the growth of natural populations of blue-green algae.     

Gas vesicle formation and buoyancy regulation in Pelodictyon phaeoclathratiforme (Green sulfur bacteria)

Gas vesicle formation and buoyancy regulation in Pelodictyon phaeoclathratiforme strain BU1 (Green sulfur bacteria) was investigated under various laboratory conditions. Cells formed gas vesicles exclusively at light intensities below 5 mol · m^-2 · s^-1 in the stationary phase. No effect of incubation temperature or nutrient limitation was observed. Gas space of gas vesicles occupied always less than 1.2% of the total cell volume. A maximum cell turgor pressure of 330 kPa was determined which is comparable to values determined for cyanobacterial species. Since a pressure of at least 485 kPa was required to collapse the weakest gas vesicles in Pelodictyon phaeoclathratiforme, short-term regulation of cell density by the turgor pressure mechanism can be excluded.Instead, regulation of the cell density is accomplished by the cease of gas vacuole production and accumulation of carbohydrate at high light intensity. The carbohydrate content of exponentially growing cells increased with light intensity, reaching a maximum of 35% of dry cell mass above 10 mol · m^-2 · s^-1. Density of the cells increased concomitantly. At maximum density, protein and carbohydrate together accounted for 62% of the total cell ballast. Cells harvested in the stationary phase had a significantly lower carbohydrate content (8–12% of the dry cell mass) and cell density (1010–1014 kg · m^-3 with gas vesicles collapsed) which in this case was independent of light intensity. Due to the presence of gas vesicles in these cultures, the density of cells reached a minimum value of 998.5 kg · m^-3 at 0.5 mol · m^-2 · s^-1.The cell volume during the stationary phase was three times higher than during exponential growth, leading to considerable changes in the buoyancy of Pelodictyon phaeoclathratiforme. Microscopic observations indicate that extracellular slime layers may contribute to these variations of cell volume.     

The buoyancy-providing role of gas vacuoles in an aerobic bacterium

The heterotrophic, freshwater bacterium Prosthecomicrobium pneumaticum Staley possesses sufficient gas vacuoles to render it buoyant at all stages of growth. Although the cells have a turgor pressure of about 300 kPa, there is no evidence that this pressure is important in causing collapse of the constituent gas vesicles. A mutant of the bacterium, which produced only 0.2% of the amount of gas vacuoles produced by the wild type, was isolated. It always sank in liquid culture. Wild type and mutant bacteria grew at the same rate in shaken culture, but in static culture the wild type, which floated to the liquid surface grew more quickly than the mutant, which sank. Other competition experiments suggested that the advantage gained in floating at the surface was simply that oxygen was more readily available there to this obligate aerobe. Similar advantages may benefit gas vacuolate forms in natural habitats.A second mutant was isolated which produced about 40% fewer gas vacuoles than the wild type in corresponding stages of growth, insufficient to provide buoyancy, and unlikely to be of selective value. The occurrence of this mutant suggests there may be duplication of the gas vacuole gene.Abbreviations T turbidity - PST pressure sensitive turbidity - kPa kilo-Pascals (100 kPa=1 bar)     

REGULATION OF GAS VESICLE CONTENT AND BUOYANCY IN LIGHT-OR PHOSPHATE-LIMITED CULTURES OF APHANIZOMENON FLOS-AQUAE (CYANOPHYTA)1

Measurements of the gas vesicle space in steady-state light or phosphate-limited cultures of Aphanizomenon flos-aquae Ralfs, strain 7905 showed that gas vesicle content decreased as energy-limited growth rate increased but was the same at several phosphate-limited growth rates. Upon a decrease in growth irradiance, gas vesicle content did increase in phosphate-limited cultures, but the cultures remained nonbuoyant as long as P was limiting. Buoyant, energy-limited cultures lost their buoyancy in less than 2 h when exposed to higher irradiances. The primary mechanism for buoyancy loss was the accumulation of polysaccharide as ballast. Collapse of gas vesicles by turgor pressure played a minor role in the loss of buoyancy. When cultures were exposed to higher irradiances, cells continued to synthesize gas vesicles at the same rate as before the shift for at least 1 generation time. The amount of ballast required to make individual filaments in the population sink varied 4-fold. This variation appears to be due to differences in gas vesicle content among individual filaments.     

The development and vertical distribution of populations of gas-vacuolate bacteria in a eutrophic,monomictic lake

The role of gas vacuoles in the vertical stratification of planktonic bacteria is analysed. Measurements made with certain gas-vacuolate bacteria in laboratory culture suggest that only colonial forms could sink or float fast enough to form population maxima in lakes by vertical migration from other depths. It is suggested that in the case of individual cells the importance of the buoyancy provided by gas vacuoles is to minimise sinking rates and thereby to increase residence times of the organisms at depths where conditions support their growth.Changes in the vertical distribution of a number of gas-vacuolate bacteria were followed throughout the year in a monomictic, eutrophic lake (Crose Mere, Shropshire). All were restricted to the anaerobic hypolimnion which developed in summer. The various species formed maxima at different depths and times. With some of them (e.g. species of Thiopedia, Pelonema and Brachyarcus) growth was necessary to explain their development. In others (e. g. species of Pelodictyon and two colourless bacteria) vertical migrations might also have contributed to their development.     

Regulation of blue-green algal buoyancy and bloom formation by light,inorganic nitrogen,C02, and trophic level interactions

In highly eutrophic ponds, buoyancy of the gas-vacuolate blue-green alga Anabaenopsis Elenkinii (Miller) was regulated by complex interactions between chemical and physical parameters, as well as by biological interactions between various trophic levels. Algal buoyancy and surface bloom formation were enhanced markedly by decreased light intensity, and to a lesser extent by decreased CO₂ availability and increased availability of inorganic nitrogen. In the absence of dense populations of large-bodied Cladocera, early season blooms of diatoms and green algae reduced light availability in the ponds thus creating conditions favorable for increased buoyancy and bloom formation by A. Elenkinii. The appearance of blue-green algal blooms could be prevented by a reduced density of planktivorous fish, which allowed development of dense cladoceran populations. The cladocerans limited the growth of precursory blooms of diatoms and green algae, and given the resulting clear-water conditions, buoyancy of A. Elenkinii was reduced, and blue-green algal blooms never appeared.     

Analysis of growth, composition and thickness of the cell walls of transgenic tobacco plants expressing a yeast-derived invertase

Summary Transgenic tobacco (Nicotiana tabaccum L. cv. Samsun NN) expressing a yeast invertase in the vacuole provides a novel tool for studying the role of turgor, osmotic pressure, and cell wall properties during cell expansion. The plants used showed increased osmolarity and an increased cell size in young leaves. Their advantage is that they allow long-term analysis and undisturbed conditions. Cell expansion rate was maximal in leaf six of the transgenic plants and in leaf eleven of wild-type plants. Turgor rose to 0.52 ± 0.04 MPa (n=45) and 0.35 ± 0.03 MPa (n=45) in transgenic and wild-type plants, respectively. It was maximal where elongation rates were highest. Thus, elevated cell expansion rate was, at least in part, related to an enhancement in turgor. However, comparison between turgor and relative expansion rates showed that higher turgor pressures were required to achieve similar cell expansion rates in transformed plants as in the wild-type. This finding underlines the importance of the yield threshold and, thus, of the cell wall in growth regulation. This conclusion is further supported by the observation that the cell walls of transgenic plants were up to 77% thicker than the wild-type, but not qualitatively modified.     

Bloom formation of Gloeotrichia echinulata and Aphanizomenon flos-aquae in a shallow,eutrophic, Danish lake

Over a period of four years, the seasonal periodicity of dominant phytoplankton species in a shallow, eutrophic Danish lake changed markedly. Cyanophytes prevailed during the summer period of all four years. In the first three years, species of Microcystis, Anabaena and Aphanothece dominated, whereas in the fourth year of investigation, these algae were replaced by Gloeotrichia echinulata (J. E. Smith) Richter and Aphanizomenon flos-aquae (L.) Ralfs. The most striking environmental differences in the fourth year as compared with the previous three years, were an increase in tranparency, from about 0.5 meter in 1989–1991 to more than 2 metres preceding the summer maximum in 1992, and a simultaneous occurrence of low oxygen concentrations. A collapse of the fish population was followed by an increased proportion of large Cladocerans in the zooplankton. Improved light conditions at the bottom and grazing pressure from large Cladocerans favoured growth of the large colony forming blue-green algae, Gloeotrichia echinulata and Aphanizomenon flos-aquae. These species germinate from resting spores in the sediment and are able to sustain some growth there before migration to the lake water. The transfer of algal biomass from the bottom sediment to the water phase was accompanied by a marked increase in concentrations of particulate phosphorus and nitrogen in the entire lake.     

Controlling the Growth of Microcystis Using Surged Artificial Aeration

This paper investigates the effect of lake mixing, due to artificial hypolimnetic aeration, on the succession and ecological composition of selected phytoplankton species in Tegeler See, Berlin. Special attention is given to Microcystis, a nuisance bloom-forming blue-green algae and its competitor (in this lake) Ceratium, a dinoflagellate. Due to the installation configuration, the aerators frequently destratified the lake, especially when the stratification was weak. Five years representing three different operating schemes of the 15 aerators (continuous aeration, non-aeration and surge aeration) in this lake were used to determine possible aeration strategies in controlling Microsystis. In order to ease correlation of the mixing status in the lake with its yearly successions of phytoplankton growth a non-dimensional parameter called the Lake number was utilised. This parameter represents a balance between: i) forcing due to wind and aerator discharge and ii) potential energy acquired from the stratification of the lake. The results indicate that surged operation of the aerators is effective in suppressing massive growth of Microcystis. Due to their ability to adapt to the mixing/stability state of the water column, prolonged aeration or prolonged stable stratified conditions have little effect in controlling this algae.     

A MODEL APPROACH FOR SIZE-SELECTIVE COMPETITION OF MARINE PHYTOPLANKTON FOR FLUCTUATING NITRATE AND AMMONIUM1

Phytoplankton size-selective competition for fluctuating nutrients was studied with the use of a numerical model, which describes nitrate and ammonium uptake, nitrate reduction to ammonium, and growth as a function of cell she under fluctuating nitrogen limitation. The only size-dependent parameter in the model was the cell nutrient quota. Related to this, the cell surface area per biomass was negatively correlated to cell volume, and the vacuole volume per biomass ratio was positively correlated to cell volume. Simulations showed an inverse correlation between the maximum specific growth rate and cell size under steady-state conditions. With nitrate as the limiting nitrogen source, nitrogen quotas were always higher than with ammonium at the same specific growth rate. Net passive transport of ammonium due to unspecific diffusion of ammonia across the plasma membrane decreased the affinity for ammonium, whereas the affinity for nitrate was not influenced. Transient state-specific ammonium uptake was not dependent on cell size. However, small algae always have the highest specific growth rate in ammonium-controlled systems according to our model. Transient state nitrate uptake rate was positively correlated to cell size because larger algae have a higher vacuole volume per biomass, in which nitrate can be stored. Despite their lower maximum growth rate, larger algae became dominant during simulations under fluctuating nitrate supply when amplitude of and the period between nitrate pulses were high enough. Results from model simulations were qualitatively validated by earlier observations that large diatoms become dominant under fluctuating conditions when nitrate is the main nitrogen source.     

RECRUITMENT OF BENTHIC MICROCYSTIS (CYANOPHYCEAE) TO THE WATER COLUMN: INTERNAL BUOYANCY CHANGES OR RESUSPENSION?1

In some lakes, large amounts of the potentially toxic cyanobacterium Microcystis overwinter in the sediment. This overwintering population might inoculate the water column in spring and promote the development of dense surface blooms of Microcystis during summer. In the Dutch Lake Volkerak, we found photochemically active Microcystis colonies in the sediment throughout the year. The most vital colonies originated from shallow sediments within the euphotic zone. We investigated whether recruitment of Microcystis colonies from the sediment to the water column was an active process, through production of gas vesicles or respiration of carbohydrate ballast. We calculated net buoyancy, as an indication of relative density, using the amounts and densities of the major cell constituents (carbohydrates, proteins, and gas vesicles). Carbohydrate content of benthic Microcystis cells was very low throughout the year. Buoyancy changes of benthic Microcystis were mostly a result of changes in gas vesicle volume. Before the summer bloom, net buoyancy and the amount of buoyant colonies in the sediment did not change. Therefore, recruitment of Microcystis from the sediment does not seem to be an active process regulated by internal buoyancy changes. Instead, our observations indicate that attachment of sediment particles to colonies plays an important part in the buoyancy state of benthic colonies. Therefore, we suggest that recruitment of Microcystis is more likely a passive process resulting from resuspension by wind‐induced mixing or bioturbation. Consequently, shallow areas of the lake probably play a more important role in recruitment of benthic Microcystis than deep areas.     

Role of light, carbon dioxide and nitrogen in regulation of buoyancy, growth and bloom formation of Anabaena flos-aquae

The availability of light, CO₂ and NH₄-N interacted to controlbuoyancy and growth of the gas vacuolate blue-green alga, Anabaenaflos-aquae. At high light intensities algal growth rates werehigh; however, the alga was non-buoyant regardless of the availabilityof CO₂ or NH₄-N. The mechanism for buoyancy loss involved increasedcell turgor pressures at higher light intensities which resultedin collapse of gas vacuoles. At lower light intensities algalgrowth rates and cell turgor pressures were reduced and buoyancywas controlled by the availability of CO₂ and inorganic nitrogen.Carbon dioxide limitation increased buoyancy, while reducedinorganic nitrogen availability reduced buoyancy. Mechanismsfor buoyancy regulation at low light intensities involved changesin cellular C/N ratios which appeared to affect the rate ofsynthesis and accumulation of protein-rich gas vacuoles. Algalspecific growth rates were combined with buoyancy data to forma single index (µ_bloom) to the rate of surface bloom formationof A.flos-aquae as a function of the availability of light,CO₂ and NH₄-N. The bloom formation index was enhanced with decreasedavailability of light and CO₂, and increased availability ofNH₄-N.     

LIGHT AND ELECTRON MICROSCOPY OF GRAZING BY POTERIOOCHROMONAS MALHAMENSIS (CHRYSOPHYCEAE) ON A RANGE OF PHYTOPLANKTON TAXA1

Grazing of fluorescent latex beads, bacteria, and various species of phytoplankton by Poterioochromonas malhamensis (Pringsheim) Peterfi (about 8.0 μm in diameter) was surveyed. The alga ingested fluorescent beads and various live or killed and nomnotile or motile organisms including bacteria, blue-green algae, green algae, diatoms, and chrysomonads. The size range of grazed prey was from 0.1 to 6.0 μm for latex beads and from 1.0 μm (bacteria) to about 21 μm (Carteria inverse) for organisms. As many as 17 latex beads (2.0 μm) or more than 10 Microcystis cells (5–6 μm) were ingested by a single P. malhamensis cell. Following such grazing, the cell increased in volume by up to about 30-fold. The range of cell volume of ingested prey was from 0.52 μm³ (bacteria) to about 3178 μm³(Carteria inversa). This study demonstrates for the first time that P. malhamensis is capable of grazing algae 2–3 times larger in diameter than its own cell and of grazing intact motile algae. Poterioochromonas malhamensis is an omnivorous grazer. Food vacuole formation and digestion processes were examined. The membrane that was derived from the plasma membrane and surrounded the prey disappeared sometime after ingestion. The food vacuole was then formed by successive fusion of numerous homogeneous vesicles accumulated around the prey. The prey was enclosed in a single membrane-bound food vacuole and then digested.