Interactions Between Changing pCO2, N2 Fixation, and Fe Limitation in the Marine Unicellular Cyanobacterium Crocosphaera

We examined the physiological responses of steady-state iron (Fe)-replete and Fe-limited cultures of the biogeochemically critical marine unicellular diazotrophic cyanobacterium Crocosphaera at glacial (19 Pa; 190 ppm), current (39 Pa; 380 ppm), and projected year 2100 (76 Pa; 750 ppm) CO2 levels. Rates of N2 and CO2 fixation and growth increased in step with increasing partial pressure of CO2 (pCO2), but only under Fe-replete conditions. N2 and carbon fixation rates at 75 Pa CO2 were 1.4–1.8-fold and 1.2–2.0-fold higher, respectively, relative to those at present day and glacial pCO2 levels. In Fe-replete cultures, cellular Fe and molybdenum quotas varied threefold and were linearly related to N2 fixation rates and to external pCO2. However, N2 fixation and trace metal quotas were decoupled from pCO2 in Fe-limited Crocosphaera. Higher CO2 and Fe concentrations both resulted in increased cellular pigment contents and affected photosynthesis vs. irradiance parameters. If these results also apply to natural Crocosphaera populations, anthropogenic CO2 enrichment could substantially increase global oceanic N2 and CO2 fixation, but this effect may be tempered by Fe availability. Possible biogeochemical consequences may include elevated inputs of new nitrogen to the ocean and increased potential for Fe and/or phosphorus limitation in the future high-CO2 ocean, and feedbacks to atmospheric pCO2 in both the near future and over glacial to interglacial timescales. By the end of this century, CO2 levels in the atmosphere and in surface seawater will roughly double, to 70–75 Pa (IPCC 2007). In the context of this anticipated doubling of the partial pressure of CO2 (pCO2), some culture and field studies have indicated that growth and carbon fixation by marine phytoplankton may increase (Kim et al. 2006; Fu et al. 2007; Hutchins et al. 2007). Anthropogenically driven changes in CO2 availability may thus exert a strong control on algal physiology, nutrient cycling, and ecological interactions. 1 Corresponding author ([email protected]).