Inorganic carbon and pH dependency of Trichodesmium’s photosynthetic rates

We established the relationship between photosynthetic carbon fixation rates and pH, CO2 and HCO3 concentrations in the diazotroph Trichodesmium erythraeum IMS101. Inorganic 14C-assimilation was measured in TRIS-buffered ASW medium where the absolute and relative concentrations of CO2, pH and HCO3 were manipulated. First, we varied the total dissolved inorganic carbon concentration (TIC) (< 0 to ~ 5 mM) at constant pH, so ratios of CO2 and HCO3 remained relatively constant. Second, we varied pH (~ 8.54 to 7.52) at constant TIC, so CO2 increased whilst HCO3 declined. We found that 14C-assimilation could be described by the same function of CO2 for both approaches but showed different dependencies on HCO3 when pH was varied at constant TIC than when TIC was varied at constant pH. A numerical model of Trichodesmium’s CCM showed carboxylation rates are modulated by HCO3 and pH. The decrease in Ci assimilation at low CO2, when TIC was varied, is due to HCO3 uptake limitation of the carboxylation rate. Conversely, when pH was varied, Ci assimilation declined due to a high-pH mediated increase in HCO3 and CO2 leakage rates, potentially coupled to other processes (uncharacterised within the CCM model) that restrict Ci assimilation rates under high-pH conditions.

Boatman T. G., Mangan N. M., Lawson T. & Geider J. R., 2018. Inorganic carbon and pH dependency of Trichodesmium’s photosynthetic rates. Journal of Experimental Botany ery141. doi:10.1093/jxb/ery141. Article.

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