On the role of DOM under varying climate change conditions in Crocosphaera

Diazotrophs provide the main source of reactive nitrogen to the ocean, sustaining primary productivity and CO2 uptake. Climate change is raising temperatures, decreasing pH and reducing nutrient availability. How microbes respond to these changes is largely unexplained. Similarly, the role of DOM in the growth and survival of certain diazotrophic organisms is poorly understood. Moreover, growing evidence indicates some diazotrophs are capable of utilizing distinct DOM compounds via osmotrophy providing them with additional metabolic plasticity and ecological advantages compared to other non-diazotrophic microbes. We aimed to understand how osmotrophy could modify carbon uptake and alleviate energy stress in diazotrophs under ongoing climate change perturbations. We hypothesized that Crocosphaera preferentially uses DOM when labile as a carbon source in present pH conditions, as compared to future more acidic scenarios with higher access to inorganic carbon. Alternatively, the lower pH may cause Crocosphaera to be energy limited when trying to maintain intracellular homeostasis which would favour DOM uptake as an extra source of energy. We exposed cultures of the unicellular cyanobacterium Crocosphaera watsonii WH8501 to current and future climate change scenarios (RCP 4.5 and 6.0 from IPCC 2014) combining temperature and pH levels (26ºC and pH 8.1, 28ºC and pH 8.0 and 30ºC and pH 7.9). We followed their growth, nitrogen and dissolved inroganic carbon fixation rates, DOM specific uptake rates and gene expression responses along 8 days comparing DOM amended samples with not amended controls. We discuss how the different treatments alter Crocosphaera metabolism, allowing or not to cope with warmer and more acidic ocean conditions, while providing an improved mechanistic understanding of the role of osmotrophy in diazotrophs and allowing for a better comprehension of the global nitrogen cycle in the future ocean.

固氮微生物（diazotrophs）是海洋活性氮的主要来源，维系着海洋初级生产力与二氧化碳吸收。气候变化正导致海水温度升高、酸碱度下降，同时降低养分可利用性。目前学界对微生物如何响应这些环境变化的机制尚未得到充分阐释。同样，溶解有机物（Dissolved Organic Matter, DOM）在特定固氮微生物生长与存活中所发挥的作用，也鲜有研究探明。此外，越来越多的证据表明，部分固氮微生物可通过渗透营养（osmotrophy）利用不同种类的溶解有机物，这使其相较于其他非固氮微生物，拥有了更强的代谢可塑性与生态竞争优势。本研究旨在明晰在持续的气候变化扰动下，渗透营养如何改变固氮微生物的碳吸收过程，并缓解其能量胁迫。我们提出如下假说：相较于未来高无机碳可利用性的酸性更强的场景，在当前的酸碱度条件下，束毛藻属（Crocosphaera）会优先将易降解溶解有机物作为碳源加以利用。反之，当海水酸碱度降低时，束毛藻在维持细胞内稳态的过程中可能会面临能量限制，此时其会更倾向于通过摄取溶解有机物来获取额外的能量来源。我们将单细胞蓝细菌沃森束毛藻（Crocosphaera watsonii WH8501）的培养物置于当前与未来气候变化场景下培养——参考政府间气候变化专门委员会（Intergovernmental Panel on Climate Change, IPCC）2014年报告中的典型浓度路径（Representative Concentration Pathway, RCP）4.5与6.0，设置了三组温度与酸碱度组合：26℃、pH 8.1；28℃、pH 8.0；以及30℃、pH 7.9。在为期8天的培养过程中，我们监测了菌株的生长情况、固氮与溶解无机碳固定速率、溶解有机物特异性摄取速率，以及基因表达响应，并对比了添加溶解有机物的实验组与未添加的对照组。本研究探讨了不同处理方式如何改变束毛藻的代谢过程，进而影响其应对暖化与酸化海洋环境的能力；同时，本研究深化了学界对渗透营养在固氮微生物中作用机制的理解，有助于未来更精准地预测未来海洋全球氮循环的变化趋势。