Deoxygenation enhances photosynthetic performance and increases N2 fixation in the marine cyanobacterium Trichodesmium under elevated pCO2

Effects of changed levels of dissolved O-2 and CO2 on marine primary producers are of general concern with respect to ecological effects of ongoing ocean deoxygenation and acidification as well as upwelled seawaters. We investigated the response of the diazotroph Trichodesmium erythraeum IMS 101 after it had acclimated to lowered pO(2) (similar to 60 mu M O-2) and/or elevated pCO(2) levels (HC, similar to 32 mu M CO2) for about 20 generations. Our results showed that reduced O-2 levels decreased dark respiration significantly, and increased the net photosynthetic rate by 66 and 89% under the ambient (AC, similar to 13 mu M CO2) and the HC, respectively. The reduced pO(2) enhanced the N-2 fixation rate by similar to 139% under AC and only by 44% under HC, respectively. The N-2 fixation quotient, the ratio of N-2 fixed per O-2 evolved, increased by 143% when pO(2) decreased by 75% under the elevated pCO(2). Meanwhile, particulate organic carbon and nitrogen quota increased simultaneously under reduced O-2 levels, regardless of the pCO(2) treatments. Nevertheless, changed levels of O-2 and CO2 did not bring about significant changes in the specific growth rate of the diazotroph. Such inconsistency was attributed to the daytime positive and nighttime negative effects of both lowered pO(2) and elevated pCO(2) on the energy supply for growth. Our results suggest that Trichodesmium decrease its dark respiration by 5% and increase its N-2-fixation by 49% and N-2-fixation quotient by 30% under future ocean deoxygenation and acidification with 16% decline of pO(2) and 138% rise of pCO(2) by the end of this century.