Cellular inorganic carbon fluxes in Trichodesmium: a combined approach using measurements and modelling

To predict effects of climate change on phytoplankton, it is crucial to understand how their mechanisms for carbon acquisition respond to environmental conditions. Aiming to shed light on the responses of extra- and intracellular inorganic C (C(i)) fluxes, the cyanobacterium Trichodesmium erythraeum IMS101 was grown with different nitrogen sources (N(2) vs NO(3) (–)) and pCO(2) levels (380 vs 1400 µatm). Cellular C(i) fluxes were assessed by combining membrane inlet mass spectrometry (MIMS), (13)C fractionation measurements, and modelling. Aside from a significant decrease in C(i) affinity at elevated pCO(2) and changes in CO(2) efflux with different N sources, extracellular C(i) fluxes estimated by MIMS were largely unaffected by the treatments. (13)C fractionation during biomass production, however, increased with pCO(2), irrespective of the N source. Strong discrepancies were observed in CO(2) leakage estimates obtained by MIMS and a (13)C-based approach, which further increased under elevated pCO(2). These offsets could be explained by applying a model that comprises extracellular CO(2) and HCO(3) (–) fluxes as well as internal C(i) cycling around the carboxysome via the CO(2) uptake facilitator NDH-1(4). Assuming unidirectional, kinetic fractionation between CO(2) and HCO(3) (–) in the cytosol or enzymatic fractionation by NDH-1(4), both significantly improved the comparability of leakage estimates. Our results highlight the importance of internal C(i) cycling for (13)C composition as well as cellular energy budgets of Trichodesmium, which ought to be considered in process studies on climate change effects.