Deoxygenation enhances photosynthetic performance and increases N(2) fixation in the marine cyanobacterium Trichodesmium under elevated pCO(2)

Effects of changed levels of dissolved O(2) and CO(2) 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) (~60 μM O(2)) and/or elevated pCO(2) levels (HC, ~32 μM CO(2)) 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, ~13 μM CO(2)) and the HC, respectively. The reduced pO(2) enhanced the N(2) fixation rate by ~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 CO(2) 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.