CO₂ modulation of the rates of photosynthesis and light-dependent O₂ consumption in Trichodesmium

As atmospheric CO₂ concentrations increase, so too does the dissolved CO₂ and HCO₃⁻ concentrations in the world’s oceans. There are still many uncertainties regarding the biological response of key groups of organisms to these changing conditions, which is crucial for predicting future species distributions, primary productivity rates, and biogeochemical cycling. In this study, we established the relationship between gross photosynthetic O₂ evolution and light-dependent O₂ consumption in Trichodesmium erythraeum IMS101 acclimated to three targeted pCO₂ concentrations (180 μmol mol–1=low-CO₂, 380 μmol mol–1=mid-CO₂, and 720 μmol mol–1=high-CO₂). We found that biomass- (carbon) specific, light-saturated maximum net O₂ evolution rates (PnC,max) and acclimated growth rates increased from low- to mid-CO₂, but did not differ significantly between mid- and high-CO₂. Dark respiration rates were five times higher than required to maintain cellular metabolism, suggesting that respiration provides a substantial proportion of the ATP and reductant for N₂ fixation. Oxygen uptake increased linearly with gross O₂ evolution across light intensities ranging from darkness to 1100 μmol photons m–2 s–1. The slope of this relationship decreased with increasing CO₂, which we attribute to the increased energetic cost of operating the carbon-concentrating mechanism at lower CO₂ concentrations. Our results indicate that net photosynthesis and growth of T. erythraeum IMS101 would have been severely CO₂ limited at the last glacial maximum, but that the direct effect of future increases of CO₂ may only cause marginal increases in growth.