Inorganic carbon and pH dependency of photosynthetic rates in Trichodesmium

Increasing atmospheric CO₂concentrations are leading to increases in dissolved CO₂and HCO 3 - concentrations and decreases in pH and CO 3 2- in the world's oceans. There remain many uncertainties as to the magnitude of biological responses of key organisms to these chemical changes. In this study, we established the relationship between photosynthetic carbon fixation rates and pH, CO 2, and HCO 3 - concentrations in the diazotroph, Trichodesmium erythraeum IMS101. Inorganic 14 C-assimilation was measured in TRIS-buffered artificial seawater medium where the absolute and relative concentrations of CO 2, pH, and HCO 3 - were manipulated. First, we varied the total dissolved inorganic carbon concentration (TIC) (<0 to ∼5 mM) at constant pH, so that ratios of CO₂and HCO 3 - remained relatively constant. Second, we varied pH (∼8.54 to 7.52) at constant TIC, so that CO₂increased whilst HCO 3 - declined. We found that 14 C-assimilation could be described by the same function of CO₂for both approaches, but it showed different dependencies on HCO 3 - when pH was varied at constant TIC than when TIC was varied at constant pH. A numerical model of the carbon-concentrating mechanism (CCM) of Trichodesmium showed that carboxylation rates are modulated by HCO 3 - and pH. The decrease in assimilation of inorganic carbon (Ci) at low CO 2, when TIC was varied, was due to HCO 3 - uptake limitation of the carboxylation rate. Conversely, when pH was varied, Ci assimilation declined due to a high-pH mediated increase in HCO 3 - and CO₂leakage rates, potentially coupled to other processes (uncharacterised within the CCM model) that restrict Ci assimilation rates under high-pH conditions.