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 bio-geochemical cycling. In this study, we established the relationship between gross photosynthetic O-2 evolution and light-dependent O-2 consumption in Trichodesmium erythraeum IMS101 acclimated to three targeted pCO(2) concentrations (180 mu mol mol(-1)=low-CO2, 380 mu mol mol(-1)=mid-CO2, and 720 mu mol 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 N-2 fixation. Oxygen uptake increased linearly with gross O-2 evolution across light intensities ranging from darkness to 1100 mu mol 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.