A new CBIOMES study shows that phosphorus availability can dramatically reshape the chemistry of Crocosphaera, a key nitrogen-fixing cyanobacterium, with implications for nutrient cycling across the global ocean.
Reporting by Helen Hill for CBIOMES News
Nitrogen-fixing cyanobacteria help sustain productivity across vast regions of the ocean by converting atmospheric nitrogen into forms that other organisms can use. But this process often depends on the availability of another nutrient: phosphorus. In a recent Limnology and Oceanography study, CBIOMES researchers Sing-how Tuo, Ying-Yu (Ruby) Hu, Andrew Irwin, Michael Follows, and Zoe Finkel show that phosphorus availability profoundly influences the elemental and biochemical composition of Crocosphaera, an important marine nitrogen-fixing cyanobacterium.
Tuo et al examined two Crocosphaera phenotypes under different phosphate concentrations and across day-night cycles. They found that cellular chemistry is far from fixed. Carbon-to-nitrogen ratios increased during the day and declined at night, reflecting shifts in how the cells allocate resources. The smaller phenotype accumulated carbohydrates during daylight hours, while the larger phenotype accumulated lipids. Both increased protein content at night, when nitrogen fixation is most active.
The two phenotypes also displayed distinct strategies for storing and investing resources. Smaller cells devoted a greater share of carbon to DNA, RNA, and chlorophyll, whereas larger cells invested relatively more in lipids. These differences suggest that even closely related organisms can use different biochemical approaches to thrive in the same environment. The biggest changes, however, occurred when phosphate availability was reduced. Lowering phosphate concentrations from 50 μM to 0.5 μM produced a 27-fold decline in cellular phosphorus content. As a result, N:P and C:P ratios rose dramatically, demonstrating how strongly nutrient availability can reshape cellular stoichiometry.
Much of this shift was driven by changes in polyphosphate, a phosphorus storage compound that acts as a reserve for future use. When phosphorus is abundant, Crocosphaera can accumulate polyphosphate and later draw upon those stores when conditions become nutrient-poor. The authors found that variation in polyphosphate content was the primary driver of changes in elemental composition.
These findings have important implications for marine biogeochemistry. Many ocean models rely on relatively fixed elemental ratios, yet this study shows that the chemistry of key nitrogen-fixing microbes can vary substantially depending on nutrient conditions. By altering how carbon, nitrogen, and phosphorus are stored within cells, phosphate availability influences not only microbial physiology but also broader nutrient cycling in the ocean.
Ultimately, the study reveals that phosphorus is more than a limiting nutrient. It regulates the internal organization of one of the ocean’s most important nitrogen-fixing microbes, linking environmental nutrient supply to the elemental composition of marine ecosystems. By highlighting the central role of polyphosphate storage, the work provides new insight into how microscopic cellular decisions can shape global biogeochemical cycles.
Image credit: Wikipedia
Publication:
Sing-how Tuo, Ying-Yu Hu, Andrew J. Irwin, Michael J. Follows, Zoe V. Finkel (2026), Phosphate modulates elemental and macromolecular composition of unicellular nitrogen-fixing cyanobacteria, Limnology and Oceanography, doi: 10.1002/lno.70475