Mutual interference of phosphorus starvation and diazotrophy in the cyanobacteria Nostoc sp. PCC 7120

The generalized response of the cyanobacterium Nostoc sp. PCC 7120 to and its recovery from phosphorus (P) starvation stress differ drastically under diazotrophic and non-diazotrophic growth modes. In nitrogen (N) -replete medium, Nostoc sp. PCC 7120 cells were resilient even to prolonged P starvation when its growth was supported by mobilization of cellular reserves of P (polyphosphate) and glycogen on the background of accumulation of nitrogen and carbon reserves (mainly cyanophycin). The P-starving cells quickly recovered upon re-feeding with inorganic phosphate (P_i). Under diazotrophic conditions, P starvation essentially diminished the fixation of dinitrogen. As a result, most of the vegetative cells comprising the trichomes of the cyanobacterium died and decomposed while other cells retained their structural integrity but did not divide. In turn, the latter fell into two categories: some of them showed signs of nutrient starvation; while the other became dormant but did not display the signs of starvation. They resembled neither akinete nor chlorotic cells but were similar to arthrospores lacking a thickened sheath. Re-feeding with P_i triggered a quick resuscitation of the dormant vegetative cells manifested by mobilization of their internal reserves, resumption of the cell growth and division. These processes took place faster than the formation of heterocytes with well-developed envelope (thus, nitrogenase activity recovered by the 7th day after re-feeding of the cells with P_i). The results provide a deeper insight into the mechanisms of stress tolerance in Nostoc sp. PCC 7120 and modulation of the cyanobacterial productivity in natural ecosystems and artificial cultivation facilities by nitrogen and P availability.