Phosphate modulates elemental and macromolecular composition of unicellular nitrogen‐fixing cyanobacteria

Abstract

The C : N : P of nitrogen‐fixing cyanobacteria regulates phosphate‐limited nitrogen fixation in the ocean yet the causes and range in variation C : N : P is not well constrained. Here we quantify the C : N : P and underlying macromolecular composition of Crocosphaera . Crocosphaera is a nitrogen‐fixing cyanobacterium that substantively contributes to elemental biogeochemistry in the (sub)tropical oceans. We observed variation in C : N : P and macromolecular composition over the photoperiod, across two different sized phenotypes and with phosphate concentration. In both phenotypes, C : N is ~ 1.3‐fold higher in the day than the night due to increasing carbohydrate in the smaller phenotype and increasing lipid in the larger phenotype during the day and increasing protein in both phenotypes at night. Detectable levels of surface adsorbed phosphorus were only observed at night. The two sized phenotypes differed in macromolecular allocation: the smaller phenotype had a higher proportion of carbon to DNA, RNA, and chlorophyll a while the larger phenotype was higher in lipids relative to carbohydrates. The largest variation in C : N : P arose with changes in phosphate concentration. Decreasing phosphate in the media from 50 to 0.5  μ M induced a 27‐fold decrease in cellular P, increasing molar N : P from ~ 2 to between 41 and 64 and C : P from ~ 15 to between 335 and 484 across the phenotypes. Much of these changes can be attributed to changes in polyphosphate content and secondarily DNA content. The accumulation and storage of polyphosphate under higher phosphate concentrations could temporarily increase Crocosphaera 's capacity to fix nitrogen when phosphate is scarce.