Nitrogen-fixing (N2) cyanobacteria provide bioavailable nitrogen to vast ocean regions but are in turn limited by iron (Fe) and/or phosphorus (P), which may force them to employ alternative nitrogen acquisition strategies. The adaptive responses of nitrogen-fixers to global-change drivers under nutrient-limited conditions could profoundly alter the current ocean nitrogen and carbon cycles. Here, we show that the globally-important N2-fixer Trichodesmium fundamentally shifts nitrogen metabolism towards organic-nitrogen scavenging following long-term high-CO2 adaptation under iron and/or phosphorus (co)-limitation. Global shifts in transcripts and proteins under high CO2/Fe-limited and/or P-limited conditions include decreases in the N2-fixing nitrogenase enzyme, coupled with major increases in enzymes that oxidize trimethylamine (TMA). TMA is an abundant, biogeochemically-important organic nitrogen compound that supports rapid Trichodesmium growth while inhibiting N2 fixation. In a future high-CO2 ocean, this whole-cell energetic reallocation towards organic nitrogen scavenging and away from N2-fixation may reduce new-nitrogen inputs by Trichodesmium, while simultaneously depleting the scarce fixed-nitrogen supplies of nitrogen-limited open ocean ecosystems.
Walworth N. G., Fu F.-X., Lee M. D., Cai X., Saito M. A., Webb E. A. & Hutchins D. A., in press. Nutrient co-limited Trichodesmium as nitrogen source or sink in a future ocean. Applied and Environmental Microbiology. Article (subscription required).