Physiological and Ultrastructural Responses of Nostoc sp. (Cyanobacteriota) Strains to Phosphorus Starvation under Non-Diazotrophic Conditions

Phosphorus (P) is an essential macronutrient central to the exchange and storage of energy and information in the cell. Due to its limited bioavailability, P often becomes a limiting nutrient in aquatic and terrestrial ecosystems hence the studies of responses to stress caused by P starvation cyanobacteria, the primary producers, are of considerable interest. Indeed, the availability of P is among main factors limiting diazotrophy in cyanobacteria. To gain a deeper understanding of the effect of P starvation on cyanobacteria in non-diazotrophic conditions, we studied a model system of two near—isogenic strains, Nostoc sp. PCC 7120 and Nostoc sp. PCC 7118 differ in their ability to form heterocysts. Specifically, we investigated the differences in the responses of these strains to P starvation by comparing their growth kinetics, photosynthetic pigment content, ultrastructural rearrangements of vegetative cells, and the expression profile of key genes of phosphorus metabolism. The tolerance of PCC 7120 to P starvation was higher than that of PCC 7118, which manifested itself in a higher growth rate, less profound ultrastructural changes (in particular, phycobilisomes as well as polyphosphate reserves were retained in the cells of PCC 7120). At the same time, the accumulation of cyanophycin, a depot of nitrogen and energy, increased several-fold in the cells of both strains during P starvation, but this increase was larger in PCC 7118 cells. Assumably, the increased resilience of the PCC 7120 to P starvation stems from its higher ability to accumulate intracellular reserves of P in the form of polyphosphates. Our findings suggest that the phenotypic differences between the strains Nostoc sp. PCC 7118 and Nostoc sp. PCC 7120 are not limited to the different ability to form heterocysts. A deeper understanding of the drivers of stress response phenotypic diversity in near-isogenic strains would require a comparative analysis of their whole-genome sequences.