Interactions between CCM and N2 fixation in Trichodesmium

In view of the current increase in atmospheric pCO2 and concomitant changes in the marine environment, it is crucial to assess, understand, and predict future responses of ecologically relevant phytoplankton species. The diazotrophic cyanobacterium Trichodesmium erythraeum was found to respond strongly to elevated pCO2 by increasing growth, production rates, and N2 fixation. The magnitude of these CO2 effects exceeds those previously seen in other phytoplankton, raising the question about the underlying mechanisms. Here, we review recent publications on metabolic pathways of Trichodesmium from a gene transcription level to the protein activities and energy fluxes. Diurnal patterns of nitrogenase activity change markedly with CO2 availability, causing higher diel N2 fixation rates under elevated pCO2. The observed responses to elevated pCO2 could not be attributed to enhanced energy generation via gross photosynthesis, although there are indications for CO2-dependent changes in ATP/NADPH + H+ production. The CO2 concentrating mechanism (CCM) in Trichodesmium is primarily based on HCO3 − uptake. Although only little CO2 uptake was detected, the NDH complex seems to play a crucial role in internal cycling of inorganic carbon, especially under elevated pCO2. Affinities for inorganic carbon change over the day, closely following the pattern in N2 fixation, and generally decrease with increasing pCO2. This down-regulation of CCM activity and the simultaneously enhanced N2 fixation point to a shift in energy allocation from carbon acquisition to N2 fixation under elevated pCO2 levels. A strong light modulation of CO2 effects further corroborates the role of energy fluxes as a key to understand the responses of Trichodesmium.