Blue-green algae have a completely different mechanism for moving up and down in the water column, involving the formation and collapse of gas-filled vacuoles within the cells. A comprehensive account of these
structures and their likely ecological significance may be found in the monograph on the blue-green algae by Fogg et al. (1973) and the reviews by Walsby (1975), Reynolds and Walsby (1975) and Oliver (1994).
The gas vacuoles (Fig. 12.18) are cylindrical structures about 0.07 mm in diameter and of variable length, 0.3 to 0.4 mm being common but up to
2 mm being recorded. There are many in each cell. The membrane by which they are bounded is composed of protein, just one protein species, molecular weight 20.6 kDa, being present.1427 The gas mixture within the vacuole is the same as that in the surrounding solution and at the same partial pressures. When the turgor pressure within the surrounding cytoplasm rises above a certain critical value, the gas vacuoles collapse. New vacuoles are formed by de novo synthesis rather than by re-inflation of collapsed vacuoles: small vesicles are formed first which then increase in length. Formation of gas vacuoles increases the buoyancy of the blue-green algal cells, making them float towards the surface: collapse of the vacuoles makes the cells sink. It is the gas-vacuole volume/cell volume ratio that determines whether, and how fast, the cells rise or sink, or remain at the same depth.
Vacuole formation is favoured by dim light, by a lack of inorganic carbon (CO2, HCO3) for photosynthesis, and by a sufficiency of inor-
ganic nitrogen. , , , Under these circumstances photosynthesis proceeds rather slowly and the availability of nitrogen both facilitates the synthesis of vacuole membrane and diverts the products of photosynthesis towards the manufacture of cell materials rather than sugar accumulation. Having formed vacuoles, the cells float up towards the surface. With increased light intensity and CO2 availability (resulting from diffusion of atmospheric CO2), the rate of photosynthesis increases. If, as will commonly be the case, mineral nitrogen concentration decreases towards the surface, photosynthesis leads increasingly to sugar accumulation rather than cell growth, turgor pressure within the cell increases and, eventually, the vacuoles begin to collapse. Depending on how far this process goes the cells will either simply slow down in their upward motion and eventually come to a halt at a particular depth, or will begin to actually sink down again towards the deeper layers where nitrogen concentrations are higher. Thus gas-vacuole buoyancy control can be used to enable the blue-green algal population to find a particular depth that suits it and this is what seems to happen to forms such as Oscillatoria: these occur as small-diameter single filaments with high resistance to movement through the water, respond slowly to buoyancy changes, and so normally remain in a layer at a particular depth, usually several metres. Species with large colonies, on the other hand, can sink or rise rapidly through the water and since marked buoyancy changes can take place within time periods as short as an hour or so, can respond rapidly to changing conditions during the day. It is in the forms such as Microcystis and Anabaena, with large colonies, that the algae will frequently rise all the way up to the surface under still conditions, forming the surface scums so commonly seen on eutrophic lakes in the summer. They are redistributed again from the surface either by collapse of the gas vacuoles or by wind action.
While it appears likely that in most cases the advantage conferred on blue-green algae by buoyancy control resides in the ability of the cells to locate themselves at a depth where light intensity is optimal, there is some evidence that the more rapidly moving forms, such as Microcystis and Anabaena, can indeed periodically sink down through the thermocline where, like the flagellates referred to earlier, they can take up phosphate before rising up to photosynthesize again in the illuminated, but nutrient-depleted, surface layer.432 In hypertrophic Hartbeespoort Dam (South Africa), nutrients are always present in excess, but wind speeds and water turbulence are low, and enormous populations of M. aeruginosa develop. Zohary and Robarts (1989) attribute the success of this alga to its ability, by means of its strong buoyancy, to maintain itself in the shallow diurnal mixed layer that forms each day under the intense sunlight incident on this subtropical impoundment. Other, non-buoyant, species sediment through the density gradient and are lost. Modelling studies also indicate that the formation of diurnal mixed layers greatly favours positively buoyant algae such as Microcystis.601 Being trapped near the surface in intense sunlight is, of course, a considerable hazard to phytoplankton. The Hartbeespoort Microcystis, however, appeared to be well adapted to intense light: it had a low cellular chlorophyll content and a high photo-synthetic saturation irradiance (Ek up to 1230 mmol photons m~2s_1).1505
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