Combined effects of CO 2 and light on the N 2 fixing cyanobacterium Trichodesmium IMS101: Physiological responses

49 Recent studies on the (IMS101) 50 showed that increasing pCO 2 availability enhances N 2 fixation and growth. Significant 51 uncertainties remain as to the degree of the sensitivity to pCO 2 , its modification by other 52 environmental factors, and underlying processes causing these responses. To address these 53 questions we examined the responses of Trichodesmium IMS101 grown under a matrix of low 54 and high levels of pCO 2 (150 and 900 µatm) and irradiance (50 and 200 µmol photons m -2 s - 55 1 ). Growth rates as well as cellular C and N content increased with increasing pCO 2 and light 56 levels in the cultures. The pCO 2 -dependent stimulation in organic C and N production was 57 highest under low-light. High pCO 2 stimulated rates of N 2 fixation and prolonged the duration 58 while high-light affected maximum rates only. Gross photosynthesis increased with light, but 59 did not change with pCO 2 . HCO 3- was identified as the predominant carbon source taken up in 60 all treatments. Inorganic carbon uptake increased with light, but only gross CO 2 uptake was 61 enhanced under high pCO 2 . A comparison between carbon fluxes in vivo and those derived 62 from 13 C fractionation indicate a high internal carbon cycling, especially in the low pCO 2 63 treatment under high-light. Light-dependent O 2 uptake was only detected under low pCO 2 64 combined with high-light or when low-light-acclimated cells were exposed to high-light, 65 indicating that Mehler reaction functions also as a photo-protective mechanism in 66 Trichodesmium . Our data confirm the pronounced pCO 2 effect on N 2 fixation and growth in 67 Trichodesmium and further show a strong modulation of these effects by light intensity. We 68 attribute these responses to changes in the allocation of photosynthetic energy between carbon 69 acquisition and the assimilation of C and N under elevated pCO 2 . These findings are 70 supported by a complementary study (Levitan et al, same volume), looking at photosynthetic 71 fluorescence parameters of PSII, photosynthetic unit stoichiometry (PSI:PSII) and of key proteins in the carbon and nitrogen acquisition.

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