Akinete development in the planktonic blue-green algaAnabaena circinalis

Akinete development in Anabaena circinalis, which characteristically begins in vegetative cells two cells away from a heterocyst during the late exponential phase of growth in batch culture, coincides with the decline of nitrogenase activity and of heterocyst frequency. The trend of akinete production is inversely related to that of heterocyst differentiation. Akinete formation is suppressed by agitation and enhanced under undisturbed culture conditions. The latter permit surface accumulation of the buoyant filament material which leads to increased culture density, decreased light penetration into culture medium and hence to limitation of light-dependent reactions. The conditions resemble those present in surface blooms of buoyant planktonic populations in the natural habitat and in temperate eutrophic lakes in general. No conclusive evidence was obtained that the depletion of mineral nutrients is responsible for the initiation of akinete differentiation. Treatments like orthophosphate supplementation, the supply of an alternative carbon source (organic substrate) or the addition of culture filtrate (promoting the utilization of trace elements), which prolong the post-exponential phase of growth in culture, result in increased akinete production. Light limitation in a dense filament suspension, which primarily affects the nitrogen-fixing activities of heterocysts, is probably the most important external factor triggering the process of akinete differentiation. The possible role of heterocysts in the onset of sporulation is discussed in the light of these findings.