Gas-vacuole content /%

3 The relationship between sinking velocity (ws) or floating velocity (—ws) of cells of Anabaena circinalis and their gas-vacuole content as a proportion of cell volume. After Reynolds (1972) and redrawn from Reynolds (1984a).

tially linear relationship between density (and buoyant velocity) and the gas-vacuole content. It is 'potential' in so far as other items in the complement of cell materials affect the density, and the velocity is sensitive to size and form resistance. Measurements of the gas-vesicle content required to gain neutral buoyancy vary between 0.7% and 2.3% of the cell volume (Reynolds and Walsby, 1975).

To conclude this very brief overview of the buoyancy provision that gas vacuoles impart, it is relevant to the ecology of these organisms to refer to the mechanisms of buoyancy regulation. To be continuously buoyant is arguably advantageous in deep, continuously mixed water layers. In small, possibly sheltered lake basins and in larger ones at low latitudes, where the variability in convective mixing is highly responsive to diurnal heat income and net nocturnal heat loss, it is biologically useful to be able to alter or even reverse buoyancy. There are at least three ways in which the planktic Cyanobacteria do this. The relative content of gas vesicles is, in the first instance, the outcome of the balance between their assembly and their collapse. As cells simultaneously grow and divide, gas vesicles will be 'diluted out by growth' unless the cellular resource allocation to their assembly keeps pace. There is plenty of evidence (reviewed in Reynolds, 1987a) that the processes are not closely coupled and that the relative content of vesicles increases during slow (especially light-limited) growth and decreases during rapid growth. It is also apparent that for the species with the strongest vesicles, this is the main mechanism of control and, of course, it operates at the scale of generation times. For the species with weaker vesicles that are vulnerable to collapse in the face of rising turgor generated by low-molecular-weight carbohydrates, there is a rather more responsive mechanism of reversing buoyancy. Cells floating into higher light intensities photosynthesise more rapidly, raise cell turgor, collapse vesicles, lose buoyancy, sink back where the cycle can start again. The cycle of buoyant adjustments can operate on a diel basis and bring about daily migratory cycles over depth ranges of 2-4 m, cells accumulating near the surface by night and at greater depths by the end of the daylight period (Reynolds, 1975; Konopka et al., 1978). Such behaviour may be invoked to explain the diel migratory cycles of Anabaena spp. (reported by Talling, 1957a; Pushkar', 1975; Ganf and Oliver, 1982) and Aphanizomenon (Sirenko et al., 1968; Horne, 1979), or imitated in laboratory mesocosm (Booker et al, 1976; Booker and Walsby, 1981).

The third mechanism can also result in fairly fine control of buoyancy trimming in Cyanobac-teria with gas vesicles of intermediate strength, such as those of Microcystis, beyond the scope of the turgor-collapse mechanism. Here, the buoyancy provided by a coarsely variable complement of robust gas vesicles is countered by a finely variable accumulation of photosynthetic polymers (chiefly glycogen) of high molecular weight (Kromkamp and Mur, 1984; Thomas and Walsby, 1985). So long as approximate neutral buoyancy is maintained, relatively small differences in the glycogen content take the average density of the colonies either side of neutral buoyancy, in response to insolation and photosyntetic rate. The large size attained by colonies magnifies the small differences in density (as predicted in the modified Stokes equation, 2.16), allowing Micro-cystis colonies to migrate on a diel basis in stable water columns and to recover vertical position very rapidly after disruptive storms. Apart from the first detailed descriptions of this behaviour in shallow, tropical Lake George (Ganf, 1974a), similar adjustments, the ability of Microcystis to attain this control on its buoyancy, is apparent from the studies of Okino (1973), Reynolds (1973b, 1989a), Reynolds et al. (1981) and Okada and Aiba (1983).

No less striking is the formation of persistent plate-like layers in the stable metalimnia of certain relatively deep lakes: the plankters may be almost lacking from the water column but for a band of 1 m or rather less, where they remain poised, often at quite low light intensities. The behaviour has been known for many years from alpine lakes in central Europe (Findenegg, 1947; Thomas, 1949, 1950; Ravera and Vollenweider, 1968; Zimmermann, 1969; Utkilen et al, 1985a) and generally involves the solitary filaments of Planktothrix of the rubescens-prolifica-mougeotii group but it is also known from small, stratifying continental lakes elsewhere (Juday, 1934; Atkin, 1949; Eberley, 1959, 1964, Lund, 1959; Brook et al., 1971, Gorlenko and Kuznetsov, 1972; Walsby and Klemer, 1974) and to involve other genera (Lyngbya, or Planktolyngbya, Spirulina: Reynolds et al., 1983a; Hino et al., 1986). The ability to maintain station is attributable to close regulation of gas-vesicle content but the very low light intensities suggest that this is regulated by allocation. Zimmermann's (1969) study showed that, through the season, P. rubescens moves up and down in the water column of Vierwaldstattersee, mainly in response to changes in the down-welling irradiance. The cells are able to maintain biomass or even to grow slowly in situ and the behaviour has been interpreted as a sort of 'aestivation', to escape the period of minimal resource supply. However, the recent season-long investigation by Bright and Walsby (2000) of the P. rubescens stratified in the Zurichsee, points to a sophisticated set of adaptations to gain positive growth in the only region of the lake where a small nutrient base and a low light income are simultaneously available. The ability to control organismic density is crucial to the exploitation of the opportunity.

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