Seawater carbonate chemistry and photosynthetic performance and N2 fixation in the marine cyanobacterium Trichodesmium

Effects of changed levels of dissolved O2 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 pO2 (~60 μM O2) and/or elevated pCO2 levels (HC, ~32 μM CO2) for about 20 generations. Our results showed that reduced O2 levels decreased dark respiration significantly, and increased the net photosynthetic rate by 66 and 89% under the ambient (AC, ~13 μM CO2) and the HC, respectively. The reduced pO2 enhanced the N2 fixation rate by ~139% under AC and only by 44% under HC, respectively. The N2 fixation quotient, the ratio of N2 fixed per O2 evolved, increased by 143% when pO2 decreased by 75% under the elevated pCO2. Meanwhile, particulate organic carbon and nitrogen quota increased simultaneously under reduced O2 levels, regardless of the pCO2 treatments. Nevertheless, changed levels of O2 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 pO2 and elevated pCO2 on the energy supply for growth. Our results suggest that Trichodesmium decrease its dark respiration by 5% and increase its N2-fixation by 49% and N2-fixation quotient by 30% under future ocean deoxygenation and acidification with 16% decline of pO2 and 138% rise of pCO2 by the end of this century.

溶解氧（dissolved O₂）与溶解二氧化碳（dissolved CO₂）水平变化对海洋初级生产者（marine primary producers）的影响，是当前持续发生的海洋脱氧（ocean deoxygenation）、海洋酸化（ocean acidification）及上升流海水（upwelled seawaters）所引发生态效应研究中的普遍关注焦点。本研究针对固氮生物（diazotroph）红海束毛藻（Trichodesmium erythraeum）IMS 101展开探究，将其在低氧分压（pO₂，约60 μM O₂）和/或高二氧化碳分压（HC，约32 μM CO₂）条件下驯化约20个世代后，分析其响应特征。结果显示，低氧水平可显著降低暗呼吸速率；在环境CO₂组（AC，约13 μM CO₂）与高CO₂组中，其净光合速率分别提升66%与89%。在环境CO₂组中，低氧分压可使固氮作用（N₂ fixation）速率提升约139%，而在高CO₂组中仅提升44%。固氮商（N₂ fixation quotient，即每释放1分子O₂所固定的N₂的比值）在高CO₂条件下，当氧分压降低75%时提升了143%。与此同时，无论CO₂分压处理条件如何，低氧水平下颗粒有机碳（particulate organic carbon）与颗粒有机氮（particulate organic nitrogen）的细胞含量均同步升高。尽管如此，氧与二氧化碳水平的变化并未对该固氮生物的比生长速率（specific growth rate）产生显著影响。这种不一致性可归因于：低氧分压与高CO₂分压对生长所需能量供应的影响在日间呈正向、夜间呈负向。本研究结果表明，在本世纪末海洋脱氧（氧分压下降16%）与酸化（CO₂分压上升138%）的未来海洋环境中，红海束毛藻的暗呼吸速率将降低5%，固氮速率提升49%，固氮商提升30%。