AKINETE DIFFERENTIATION IN ANABAENA CIRCINALIS (CYANOPHYTA) 1

Three lines of evidence established conclusively that phosphorus limitation triggered akinetes to differentiate in Anabaena circinalis Rabenhorst. First, akinetes differentiated when phosphorus was limited, but not when nitrogen, inorganic carbon, iron, trace elements, or light were limited, or when dissolved oxygen concentration was increased. In the phosphorus limitation experiment, akinetes appeared first in the 0 mg P‐L −1 cultures, and the higher the initial concentration of phosphorus was, the longer it took for akinetes to differentiate. Second, akinete differentiation commenced when Q p fell to the same critical concentration in all cultures. The critical Q p for akinete differentiation in A. circinalis was 0.3‐0.45 pg P·cell −1 , and there was no significant difference between cultures grown with 0.6, 0.2, 0.06, or 0 mg P · L −1 ( F = 5.48, of = 3, P > 0.05). Similarly, there were no significant differences between P cultures in internal cellular soluble reactive phosphorus (SRP) concentration ( F = 0.63, df = 3, P > 0.05) or external SRP per cell in the medium ( F = 5.16, df= 3, P > 0.05) when akinete differentiation commenced. Both were between 0.01 and 0.07 pg SRP‐cell −1 . A thorough literature search indicates that this information has not been reported previously. The third line of evidence came from electron micrographs, which illustrated that polyphosphate was present in trichomes prior to akinete differentiation but was absent in trichomes with akinetes indicating that phosphorus reserves were depleted when akinetes differentiated. Lipid globules (carbon reserve) and cyanophycin granules (nitrogen reserve) increased in number in trichomes with akinetes, compared to trichomes without akinetes. Thus, the ratio of internal P:C:N was different in trichomes with akinetes compared to trichomes without akinetes and may be important in activating akinete‐differentiating genes.