Combined effects of CO2 and light on the N2-fixing cyanobacterium Trichodesmium IMS101: physiological responses.

Recent studies on the diazotrophic cyanobacterium Trichodesmium erythraeum (IMS101) showed that increasing CO(2) partial pressure (pCO(2)) enhances N(2) fixation and growth. Significant uncertainties remain as to the degree of the sensitivity to pCO(2), its modification by other environmental factors, and underlying processes causing these responses. To address these questions, we examined the responses of Trichodesmium IMS101 grown under a matrix of low and high levels of pCO(2) (150 and 900 microatm) and irradiance (50 and 200 micromol photons m(-2) s(-1)). Growth rates as well as cellular carbon and nitrogen contents increased with increasing pCO(2) and light levels in the cultures. The pCO(2)-dependent stimulation in organic carbon and nitrogen production was highest under low light. High pCO(2) stimulated rates of N(2) fixation and prolonged the duration, while high light affected maximum rates only. Gross photosynthesis increased with light but did not change with pCO(2). HCO(3)(-) was identified as the predominant carbon source taken up in all treatments. Inorganic carbon uptake increased with light, but only gross CO(2) uptake was enhanced under high pCO(2). A comparison between carbon fluxes in vivo and those derived from (13)C fractionation indicates high internal carbon cycling, especially in the low-pCO(2) treatment under high light. Light-dependent oxygen uptake was only detected under low pCO(2) combined with high light or when low-light-acclimated cells were exposed to high light, indicating that the Mehler reaction functions also as a photoprotective mechanism in Trichodesmium. Our data confirm the pronounced pCO(2) effect on N(2) fixation and growth in Trichodesmium and further show a strong modulation of these effects by light intensity. We attribute these responses to changes in the allocation of photosynthetic energy between carbon acquisition and the assimilation of carbon and nitrogen under elevated pCO(2). These findings are supported by a complementary study looking at photosynthetic fluorescence parameters of photosystem II, photosynthetic unit stoichiometry (photosystem I:photosystem II), and pool sizes of key proteins in carbon and nitrogen acquisition.