Calcium and Phosphate Regulation of Nitrogen Metabolism in the Cyanobacterium Spirulina platensis Under the High Light Stress

Abstract

High light stress (40 W/m²)-induced alterations in the nitrogen assimilatory enzymes in Spirulina platensis were studied under the Ca²⁺ and phosphate (Pi)-supplemented as well as starved conditions. Results revealed that activities of nitrate reductase (NR), amino acid transferases (AST/GOT and ALT/GPT), and protease enzymes in the high-light-incubated cells were relatively higher under the Ca²⁺- and Pi-starved conditions. On the contrary, relative rates of glutamine synthetase (GS) and ATPase activities were lower in the Ca²⁺- and Pi-starved cells. But the Spirulina cells under the Ca²⁺- and Pi-added conditions showed enhanced activity of both GS and ATPase enzymes. During the high-light stress, a decline in the GS activity, particularly under the Ca²⁺- and Pi-starved conditions, was indicative of a nitrogen starvation-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 under both the Ca²⁺- and Pi-supplemented conditions, perhaps owing to the enhanced 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 the protein-kinase system.