Anthony E. Walsby

Cyanobacterial dominance: The role of buoyancy regulation in dynamic lake environments

Abstract The interactions of size, shape, and density of cyanobacteria result in a 5‐order of magnitude difference in flotation or sinking rates which, in turn, influence the extent of their disper...

The Permeability of Heterocysts to the Gases Nitrogen and Oxygen

Heterocysts of the cyanobacterium Anabaena flos-aquae retina gas vacuoles for several days after differentiation. It is demonstrated that the rate of gas diffusion into a heterocyst that is near an overlying gas phase can be determined approximately from observations on the rate of gas pressure rise required to collapse 50% of its gas vacuoles. The mean permeability coefficient (α) of heterocysts O2 and N2 was found to be 0.3s-1. From this it was calculated that the average permeability (k) of the heterocyst surface layer is about 0.4 μm s-1 (within a factor of 2). This is probably within the range that could be provided by a few layers of the 26-C glycolipids in the heterocyst envelope. It is likely, but not proven, that the main route for gas diffusion is through the envelope rather than through the terminal pores of the heterocyst. From measurements of cell nitrogen content (2.7 pg). doubling time (3 days) and heterocyst: vegetative cell ratio (1:24) it was calculated that the average heterocyst fixed 5.9 x 10-18 mol N2 s-1; this must equal the diffusion rate of N2 inside the average heterocyst that was 22% below the outside air-saturated concentration. the maximum N2 fixation rate allowed by the estimated permeability coefficeint would be 2.7 x 10-17 mol s-1 per heterocyst, slightly greater than the maximum calcualted N2 fixation rate. The observed permeability coefficient is low enough for the oxygen concentration in the heterocyst to be maintained close to zero by the probable rate of respiration, providing an anaerobic environment for nitrogenase. The rate of O2 diffusion will limit the N2-fixation rate in the dark by limiting the rate at which ATP is supplied by oxidative phosphorylation.

The gas vesicles, buoyancy and vertical distribution of cyanobacteria in the Baltic Sea

The mean pressures required to collapse gas vesicles in turgid cells of cyanobacteria from the Baltic Sea were 0·91 MPa (9·1 bar) in Aphanizomenon flos-aquae, 0·83 MPa in Nodularia sp. collected from the main deep basins and 0·34 MPa in Nodularia from shallower coastal regions. The gas vesicles were strong enough to withstand the depth of winter mixing, down to the permanent halocline (60 m in the Bornholm Sea, 90 m in the Eastern Gotland Sea) or to the sea bottom (30 m or less in the shallow Arkona Sea and Mecklenburg Bight). The cyanobacteria had low cell turgor pressures, within the range 0·08–0·18 MPa. The colonies were highly buoyant: the Aphanizomenon colonies floated up at a mean velocity of 22 m per day and the Nodularia colonies at 36 m per day. The colonies remained floating when up to half of the gas vesicles had been collapsed. In summer the cyanobacteria were mostly restricted to the water above the thermocline and in calm conditions their concentration increased towards the top of the water ...

A molecular and phenotypic analysis of Nodularia (Cyanobacteria) from the Baltic Sea

The filamentous diazotrophic cyanobacterium Nodularia forms water blooms each year in the Baltic Sea. Filaments isolated from such water blooms vary in their trichome width, degree of coiling, and properties of their gas vesicles; previously, these characters have been used to classify individuals to species level. To test the validity of such a phenotypic classification, we determined the nucleotide sequences for a region of the phycocyanin locus that includes a noncoding intergenic spacer (PC‐IGS), the IGS between two adjacent copies of the gvpA gene (which encodes the main structural gas vesicle protein) and the rDNA internal transcribed spacer (rDNA‐ITS), for 13 clonal Nodularia isolates from the Baltic Sea during August 1994. The complete 16S‐rDNA sequence was determined for three isolates and was found to be identical in each of them. Molecular sequences for noncoding regions of the genome were used to assign isolates to three groups on the basis of PC‐IGS, two groups on the basis of gvpA‐IGS, and three groups on the basis of rDNA‐ITS. No consistent correlation was found between genotype and any of the phenotypic features examined, and no link was found between any of these features themselves, indicating that these characters are not useful for placing Nodularia isolates into meaningful taxonomic groups. The PC‐IGS, gvpA‐IGS, and rDNA‐ITS genotypic groupings were not congruent. This might indicate that gene flow occurs between individuals in Nodularia populations.

Measurement of filamentous cyanobacteria by image analysis

A method of image analysis is described for measuring total filament lengths of cyanobacteria collected on membrane filters from samples of natural populations or cultures. Images of autofluorescent filaments viewed by epifluorescence microscopy were transferred to a computer screen by using a high resolution television camera and an image integrator to enhance sensitivity. The digitised images, saved as 0.4 Mbyte bitmap computer files, were analysed with an image analysis program. An evaluation is made of procedures for determining the total length of filaments from the detected area or perimeter of their skeletonised images. Methods are described for correcting errors arising from the orientation, crossing and overlapping of filaments.

The effect of temperature on recovery of buoyancy by Microcystis

Summary: Colonies of Microcystis in Abbots Pool, Avon, UK, were found to regulate their buoyancy according to light (photon flux density). The autumnal decline of the population was associated with an increase in the proportion of colonies that were non-buoyant, and with declining temperatures in the pond. Non-buoyant colonies taken from the pond regained buoyancy in the dark rapidly at 20°C but only slowly at 12°C and below. A laboratory strain of Microcystis behaved in a similar manner. Comparisons of the behaviour of this organism placed at 8°C and 20°C were made; in high photon flux density buoyancy was lost at both temperatures due to accumulation of dense carbohydrate. When transferred to the dark cells at 20°C became buoyant again as carbohydrate was utilized and more gas vesicles were made; at 8°C much less carbohydrate was used and no increase in gas-vacuolation occurred. The failure to regain buoyancy in the dark at low temperatures accounts for the loss of buoyancy and sedimentation of the Microcystis in Abbots Pool.

Photosynthesis and nitrogen fixation in a cyanobacterial bloom in the Baltic Sea

Daily integrals of photosynthesis by a cyanobacterial bloom in the Baltic Sea, during the summer of 1993, were calculated from the vertical distributions of light, temperature and the organisms in the water column and from photosynthesis/irradiance curves of picoplanktonic and diazotrophic cyanobacteria isolated from the community. The distribution of chlorophyll a in size-classes <20 µm and >20 µm was monitored over 9 days that included a deep mixing event followed by calm. Picocyanobacteria formed 70% of the cyanobacterial biomass and contributed 56% of the total primary production. Of the filamentous diazotrophs that formed the other 30%, Aphanizomenon contributed 28% and a Nodularia-containing fraction 16% of the primary production. For the whole population there was little change in standardized photosynthetic O2 production, which remained at about 31 mmol m−2 before and after the mixing event. There were differences, however, between the classes of cyanobacteria: in picocyanobacteria primary production hardly changed, while in Aphanizomenon it increased by 2.6 and in Nodularia it fell below zero. Total phytoplankton photosynthesis was strongly dependent on total daily insolation with the compensation point at a photon insolation of 22.7 mol m−2 d−1. Similar analyses of N2 fixation showed much less dependence on depth distribution of light and biomass: Aphanizomenon fixed about twice as much N2 as Nodularia their; their fixation exceeded their own N demand by about 12%. Together, these species contributed 49% of the total N demand of the phytoplankton population. Computer models based on the measured light attenuation and photosynthetic coefficients indicate that growth of the cyanobacterial population could occur only in the summer months when the critical depth of the cyanobacteria exceeds the depth of mixing.

CYTOMORPHOLOGICAL CHARACTERIZATION OF THE PLANKTONIC DIAZOTROPHIC CYANOBACTERIA TRICHODESMIUM SPP. FROM THE INDIAN OCEAN AND CARIBBEAN AND SARGASSO SEAS1

Trichodesmium Ehrenberg species were collected in the Caribbean Sea, Sargasso Sea, and coastal areas of Tanzania (Indian Ocean). The specimens were divided into five species on the basis of morphometric characters such as cell dimensions and colony formation: T. tenue Wille, T. erythraeum Ehrenberg, T. thiebautii Gomont, T. hildebrandtii Gomont, and T. contortum Wille. In addition, Trichodesmium sp., a spherical colony of uncertain taxonomic position was examined. The cell structure of each species was investigated by means of light, scanning electron, and transvnission electron microscopy. Particular attention was paid to the presence and ultrastructural arrangement of gas vacuoles and glycogen fiber clusters (GFCs). This resulted in identification of two major groups of species: 1) T. tenue, Trichodesmium sp. with spherical‐shaped colonies, and T. erythraeum with GFCs and more or less localized gas vacuoles; and 2) T. thiebautii, T. hildebrandtii, and T. contortum lacking GFCs and with gas vacuoles spread at random. The species within each group were further characterized with respect to the dimension of the gas vesicles, cylindrical bodies, scroll bodies, and a new cellular inclusion body, Differences in colony formation and cell dimensions correlated with specific ultrastructural characters in five of the six forms. This is the first ultrastructural study comparing different forms of Trichodesmium sampled at geographically remote areas and shows that one species appears identical regardless of the sampling site. Some of the species had not been investigated earlier, and probably more species are to be identified and analyzed.

Gas vesicle genes in Planktothrix spp. from Nordic lakes: strains with weak gas vesicles possess a longer variant of gvpC

In cyanobacteria of the genus Planktothrix:, there are three length variants of gvpC, the gene that encodes the outer protein of the gas vesicle. Sequence analyses indicated that the three allelic variants of gvpC differ principally in the presence or absence of a 99 nt and a 213 nt section. Strains with the new variant, gvpC(28), which encodes a 28 kDa form of GvpC, produce gas vesicles that collapse at the relatively low critical pressure (p(c)) of 0.61-0.75 MPa. The authors have identified 12 classes of gvp genotypes that differ in the number and arrangement of alternating gvpA-gvpC genes and in the presence of OmegaC, a fragment of gvpC. The gvpC(28) gene was found to be the most common variant of gvpC amongst 71 strains of Planktothrix: isolated from Nordic lakes: 34 strains contained only gvpC(28); 22 strains, which possessed only the shorter gvpC(20) gene, produced gas vesicles with a higher p(c) of 0.76-0.91 MPa; and 15 strains, which possessed both gvpC(20) and gvpC(28), also produced the stronger gas vesicles. Genotypes with only the gvpC(28) genes were more common amongst green Planktothrix: strains (33 out of 38) than red strains (one out of 33). It is suggested that there is competition between the strains producing the two types of gas vesicles, with the stronger forms favoured in lakes deeper than 60 m, in which the combination of cell turgor pressure and hydrostatic pressure can collapse the weaker gas vesicles. The fact that none of the Nordic lakes are deeper than 67 m would explain the absence of the gvpC(16)-containing strains that produce even narrower gas vesicles of p(c) 1.0-1.2 MPa, which are common in the much deeper Lake Zürich.

The effects of diel changes in photosynthetic coefficients and depth of Planktothrix rubescens on the daily integral of photosynthesis in Lake Zürich

Abstract: In late summer and autumn, before the vertical circulation reaches the thermocline, the phytoplankton population of Lake Zürich is dominated by the red-coloured filamentous cyanobacterium Planktothrix rubescens, which stratifies in the metalimnion at depths close to the photosynthetic compensation point. The filament volume concentration reached a maximum of 12 cm3 m-3; the depth of the maximum varied from 10.5 to 12.5 m. Changes in the depth distribution were attributed to a combination of (1) seiche movements, which raised or lowered the thermocline by up to 2 m over 36 h, and (2) flotation by the buoyant filaments relative to the isotherms, by up 0.4 m d-1. These changes caused a 2-fold change in insolation at the Planktothrix peak. Estimates were made of the daily integral of photosynthetic O2-production, ΣΣ(NP), by the population of P. rubescens over a period of four cloudless days. The estimates were calculated from measurements of surface irradiance (at 5-min intervals), vertical light attenuation, temperature, filament volume concentration and the photosynthesis/irradiance (P/I) curves of filaments concentrated from the metalimnion. Despite the similar, high insolation on each of the four days, the calculated values of ΣΣ(NP) varied from 9 to 53 mmol m-2 d-1, owing to the changing depth distribution of the filaments. Measurements of P/I curves of lakewater samples incubated at a depth of 11 m showed changes in the photosynthetic coefficients during the day. These also generated large changes in calculated values of ΣΣ(NP). The computer spreadsheet used to calculate ΣΣ(NP) was modified to incorporate time-based changes in the photosynthetic coefficients and vertical distribution of the organism. These refinements provide a more accurate description of photosynthesis by the deep-living P. rubescens, which adjusts its position by buoyancy regulation to exploit the light field in the metalimnion, where it outcompetes other phytoplankton.