Physiological, biochemical and molecular responses of the halophilic cyanobacterium Aphanothece halophytica to Pi-deficiency

We studied the responses of a halophilic cyanobacterium Aphanothece halophytica at surplus (normal composition of growth medium containing 125 mu M PO43-), sufficient (the minimum concentration supporting optimal growth, 22 mu M PO43-) and deficient (no external supply of Pi) concentrations of inorganic phosphate (Pi). The cyanobacterium was able to grow well in Pi-deficient conditions until the end of incubation (14 days), though at a marginally reduced rate. The cellular P-quota in Pi-surplus cells at the end of incubation was 2.7 times that of their initial P-quota (0.75 mu mol mg protein(-1)), and remained fairly high (0.442 mu mol mg protein(-1)) even in Pi-deficient medium. However, cultures growing in Pi-sufficient medium (22 mu M PO43-), upon transfer to Pi-deficient medium, exhibited a rapid decline in cellular P level. Furthermore, cells growing in Pi-surplus medium showed a rapid efflux of P into the external medium. Aphanothece halophytica exhibited a biphasic phosphate transport system involving both high- (K-s 2.06 mu M) and low-affinity (K-s 17.85 mu M) transporters. Cyanobacterial cells maintained a basal level (constitutively expressed and not affected by Pi availability) of alkaline phosphatase (APase) activity, which increased 5-7-fold under Pi-deficiency. Supplementation of phosphate to the medium caused gradual decline in the enzyme activity to the basal level. Pi-deficient cells showed an enhanced level of transcripts for PPi-dependent glycolytic enzymes. Though moderate, Pi-deficiency affected the respiration, photosynthetic rate and electron transport chain activity negatively. PS II activity was most sensitive to Pi-deficiency, followed by PSI and whole chain. Pi-replete A. halophytica cells showed a single high-affinity nitrate transport system. However, deficiency of Pi reduced the nitrate and nitrite reductase activities.