Nitrogen fixation by cyanobacteria supplies critical bioavailable nitrogen to marine ecosystems worldwide; however, field and lab data have demonstrated it to be limited by iron, phosphorus and/or CO 2. To address unknown future interactions among these factors, we grew the nitrogen-fixing cyanobacterium Trichodesmium for 1 year under Fe/P co-limitation following 7 years of both low and high CO 2 selection. Fe/P co-limited cell lines demonstrated a complex cellular response including increased growth rates, broad proteome restructuring and cell size reductions relative to steady-state growth limited by either Fe or P alone. Fe/P co-limitation increased abundance of a protein containing a conserved domain previously implicated in cell size regulation, suggesting a similar role in Trichodesmium. Increased CO 2 further induced nutrient-limited proteome shifts in widespread core metabolisms. Our results thus suggest that N 2-fixing microbes may be significantly impacted by interactions between elevated CO 2 and nutrient limitation, with broad implications for global biogeochemical cycles in the future ocean.
Cyanobacterial nitrogen fixation supplies bioavailable nitrogen to marine ecosystems, but the mechanisms governing iron and phosphorus co-limitation in elevated CO 2 remain unknown. Here, the authors show a complex cellular response to co-limitation characterized by changes in growth, cell size, and the proteome.