Planktonic nitrogen fixation in lakes is strongly related to lake trophic status, with moderate and high rates usually occurring only in eutrophic lakes. Among eutrophic lakes, nitrogen fixation is related to the ratio of nitrogen loading to phosphorus loading to the lake; significant nitrogen fixation by planktonic organisms generally occurs only when the N: P ratio of the nutrient loading is near or below the Redfield ratio of 16: 1. In contrast, nitrogen fixation by planktonic organisms is generally low in estuaries even when the N: P ratio of nutrients inputs is low.

The tendency toward less nitrogen fixation by plankton in estuaries and coastal marine ecosystems than in lakes subject to similar loadings of nitrogen and phosphorus may be due to a lower availability in oxic seawater of one or more trace elements required for nitrogen fixation, such as molybdenum and iron. Iron concentrations are generally lower in estuarine waters and seawater than in most lakes. And although molybdenum concentrations in seawater are actually higher than in lakes, molybdenum availability is probably lower, since sulfate inhibits molybdate assimilation by microbes. Molybdate is the primary form of molybdenum in oxic seawater, and the ratio of sulfate: molybdate is greater than in lakes. However, even in lakes sulfate is several orders of magnitude more abundant than molybdenum, and the ratio of dissolved sulfate to dissolved molybdenum typically is much greater than the ratio of sulfur to molybdenum apparently required by nitrogen-fixing cyanobacteria. Consequently, assimilation of molybdate by cyanobacteria is probably an energetically expensive process in all natural waters, but more so in seawater than in freshwaters. High concentrations of dissolved organic matter are known to favor blooms of cyanobacteria, perhaps by increasing iron and/or molybdenum availability through chelation.

The primary controls on nitrogen fixation in sediments, wetlands, macrophyte beds, and cyanobacterial mats may be different from those for fixation by planktonic organisms. Both molybdenum and iron are probably more available in these systems than in oxic waters, since reducing conditions and high DOC concentrations will increase iron solubility and favor the stability of reduced forms of molybdenum; sulfate should not inhibit the assimilation of these reduced molybdenum compounds. Consequently, nitrogenase synthesis (and, therefore, nitrogen fixation) in wetlands and in sediments may be less energetically expensive than in oxic water columns. A major control on nitrogen fixation in sediments may be repression of nitrogenase synthesis by high concentrations of ammonium, a factor less important to planktonic fixation because of the much lower concentrations of ammonium generally found in water columns than in sediments.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1988.33.4part2.0688

Nitrogen fixation in freshwater, estuarine, and marine ecosystems. 2. Biogeochemical controls1

Cell quotas of microcystin (QMCYST; femtomoles of MCYST per cell), protein, and chlorophyll a (Chl a), cell dry weight, and cell volume were measured over a range of growth rates in N-limited chemostat cultures of the toxic cyanobacterium Microcystis aeruginosa MASH 01-A19. There was a positive linear relationship between QMCYST and specific growth rate (m), from which we propose a generalized model that enables QMCYST at any nutrient-limited growth rate to be predicted based on a single batch culture experiment. The model predicts QMCYST from m, mmax (maximum specific growth rate), QMCYSTmax (maximum cell quota), and QMCYSTmin (minimum cell quota). Under the conditions examined in this study, we predict a QMCYSTmax of 0.129 fmol cell 21 at mmax and a QMCYSTmin of 0.050 fmol cell 21 at m5 0. Net MCYST production rate (RMCYST) asymptotes to zero at m5 0 and reaches a maximum of 0.155 fmol cell 21 day 21 at mmax. MCYST/dry weight ratio (milligrams per gram [dry weight]) increased linearly with m, whereas the MCYST/protein ratio reached a maximum at intermediate m. In contrast, the MCYST/Chl a ratio remained constant. Cell volume correlated negatively with m, leading to an increase in intracellular MCYST concentration at high m. Taken together, our results show that fast-growing cells of N-limited M. aeruginosa are smaller, are of lower mass, and have a higher intracellular MCYST quota and concentration than slow-growing cells. The data also highlight the importance of determining cell MCYST quotas, as potentially confusing interpretations can arise from determining MCYST content as a ratio to other cell components.

https://aem.asm.org/content/aem/67/1/278.full.pdf

Cellular microcystin content in N-limited Microcystis aeruginosa can be predicted from growth rate.

Publisher Summary Besides the many species representative of the major eukaryotic algal phyla, the freshwater phytoplankton comprises a number of cyanobacteria (or blue-green algae) These aggregations have earned recognition in their own collective term, “water blooms” This chapter reviews the aspects of ecology of planktonic cyanobacreria in relation to the factors governing their buoyancy, population dynamics, and perennation in natural limnetic environments The buoyant properties of cyanobacteria are assessed in relation to the Stokes equation, drawing distinctions between the differential contributions of size, density, and form resistance among individual genera Density is altered through the erection or collapse of gas vesicles, their dilution by growth and through the maintenance of glycogen ballast The outcome of buoyancy-controlled movements in natural fields of turbulent mixing is described in empirical terms The chapter also reviews the growth requirements of cyanobacteria with respect to temperature, light, and nutrients together with their responses to deficiencies of supply The outcome of interactions between buoyancy regulation and limiting resources are proposed in a series of hypothetical model environments The circumstances under which excess buoyancy is acquired and expressed in surface bloom formation are re-examined Adaptations for perennation and re-establishment of populations after bloom formation or other adverse conditions are reviewed The evolutionary strategies of planktonic cyanobacteria are deduced

Cyanobacterial Water-Blooms

Pseudanabaena 7409 is a chromatically adapting cyanobacterium which photocontrols the synthesis of both phycoerythrin and phycocyanin [Tandeau de Marsac (1977) J. Bacteriol. 130, 82-91]. Phycobilisomes, isolated from cells grown in either green or red light, have been dissociated and the component biliproteins purified and characterized.



Phycobilisomes isolated from cells grown in green light were composed of allophycocyanin B, allophycocyanin, two phycocyanin subunits (one α-type and one β-type subunit), phycoerythrin and eight uncolored polypeptides. When dissociated phycobilisomes were chromatographed on DEAE-cellulose at pH 5.5, most of the phycocyanin was recovered as part of a large (17.3 S) multiprotein complex with phycoerythrin (molar ratio 1:1). This complex also contained five of the uncolorerd polypeptides found in innntact phycobilisomes isolated from cells grown in green light.



Phycobilisomes isolated from cells grown in red light were composed of allophycocyanin B, allophycocyanin, four phycocyanin subunits (two α-type and two β-type subunits), and six uncolored polypeptides. When these phycobilisomes were dissociated, the phycocyanin was recovered as a large (21.0 S) multiprotein comples which was composed of the four phycocyanin subunits types and four uncolored polypeptides. Thia complex was morphologically identical to the rod-like stacks of discs about 6x12nm which form the peripheral rods of intact phycobilisomes.



Each of the four phycocyanin subunits found in the complex isolated from the phycobilisomes of cells grown in red light was purified to homogeneity and characterized. Amino acid compositions of the four subunits indicated that each subunit was a unique gene product. Two of the subunits of the complex were apparently identical to those of the phycocyanin purified from phycobilisomes isolated from cells grown in green light. These studies suggest that one pair of phycocyanin subunits was synthesized constitutively (i.e. irrespective of the light wavelength to which the cells were exposed during growth) while the synthesis of the second pair of phycocyanin subunits was specifically induced during growth in red light.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1981.tb05624.x

Effects of chromatic illumination on cyanobacterial phycobilisomes. Evidence for the specific induction of a second pair of phycocyanin subunits in Pseudanabaena 7409 grown in red light.

Toxic blooms of Microcystis aeruginosa are a regular feature of numerous impoundments throughout southern Africa. In the more eutrophic impoundments, bloom formation can reach serious proportions, often resulting in cattle kills. The extent of the problem in South Africa is taken into perspective. As a result of the variability in appearance of the natural populations it is not possible to identify the toxic strains from microscopical identification alone, but samples collected from various impoundments over a period of four years indicated that toxicity, when present, was always associated with a form described by Kom£rek (1958) as Microcystis aeruginosa forma aeruginosa and never with the form described as M. aeruginosa forma flos-aquae. Laboratory experiments supplemented with field measurements indicated that M. aeruginosa can grow well over a range of light intensities, giving it a competitive advantage over other algae. The alga adapts physiologically to different light intensities by changing its gas vacuole and pigment composition. At high light intensities there is an increase in gas vacuole content, while the reverse is true at low light intensities. The pigment and gas vacuole changes enable M. aeruginosa to grow rapidly under a variety of conditions and partly explains the success of this alga in many turbid South African impoundments.

Studies on the Ecology, Growth and Physiology of Toxic Microcystis Aeruginosa in South Africa

Physiological evidence was obtained for a light shielding role for gas vacuoles inMicrocystis aeruginosa Kuetz. emend. Elenkin, by comparing photosynthetic oxygen evolution, growth behaviour and pigment composition of cells with intact or collapsed gas vacuoles. The oxygen evolution rates were strongly dependent on cell concentration, a maximum rate for cells with intact gas vacuoles occurring at about 1.4×109 cells/ml and for cells with collapsed gas vacuoles at about 2.5×109 cells/ml. By using light saturation curves for oxygen evolution, it was estimated that at low light intensities up to 30% of the photosynthetically useable light was shielded at a cell concentration of 6×108 cells/ml. Collapsing the gas vacuoles twice daily did not alter the initial growth rate of the cultures, but enabled them to reach a higher final cell density. Collapsing of gas vacuoles during growth for about four generations resulted in a lower level of all acetone soluble pigments with a greater relative reduction in carotenoids than in chlorophyll a. Collapse of the gas vacuoles does not alter the cell volume. Various optical interactions which could account for light shielding are discussed.

Physiological effects of the presence and absence of gas vacuoles in the blue-green alga, Microcystis aeruginosa Kuetz. emend. Elenkin.

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Physiological effects of the presence and absence of gas vacuoles in the blue-green alga, Microcystis aeruginosa Kuetz. emend. Elenkin.