Calcium and phosphate regulation of nitrogen metabolism in the cyanobacterium Spirulina platensis under the high light stress

High light stress (40 W/m(2))-induced alterations in the nitrogen assimilat ory enzymes in Spirulina platensis were studied under the Ca2+ and phosphat e (Pi)-supplemented as well as starved conditions. Results revealed that ac tivities of nitrate reductase (NR), amino acid transferases (AST/GOT and AL T/GPT), and protease enzymes in the high-light-incubated cells were relativ ely higher under the Ca2+- and Pi-starved conditions, On the contrary, rela tive rates of glutamine synthetase (GS) and ATPase activities were lower in the Ca2+- and Pi-starved cells. But the Spirulina cells under the Ca2+- an d Pi-added conditions showed enhanced activity of both GS and ATPase enzyme s. During the high-light stress, a decline in the GS activity, particularly under the Ca2+- and Pi-starved conditions, was indicative of a nitrogen st arvation-like condition. This could be one of the reasons for induction of the NR and protease enzymes. A higher rate of GS activity was recorded unde r both the Ca2+- and Pi-supplemented conditions, perhaps owing to the enhan ced rate of ATPase activity in such conditions. But a declining pattern of both NR and protease activities in the presence of Ca2+ and Pi, despite the higher rate of ATPase activity, might involve some other mechanism like th e protein-kinase system.