铁限制条件下束毛藻对海洋酸化的响应及其机理

The growth of Trichodesmium, probably the most important nitrogen fixer in the oceans, is often limited by the low availability of the trace metal iron in surface waters. Previous studies have shown that under iron rich conditions increasing CO2 down-regulated the carbon concentrating mechanisms in Trichodesmium, the energy saved through which was reallocated and used by other cellular processes including nitrogen fixation, therefore stimulating the nitrogen fixation and growth. Under iron limiting conditions, however, preliminary study found that decreasing pH inhibited nitrogen fixation in the cyanobacterium, consequently resulting in a decrease in carbon fixation and growth. To mechanistically understand the different effects of ocean acidification on Trichodesmium under different iron nutrient statuses, by experimentally distinguishing the CO2 effects from the pH effects, this proposed project will try to reveal whether the positive effect of high CO2 observed under iron replete conditions exists under iron deplete conditions as well, how it would compare with the negative effect of low pH, and if iron nutrient status influences the respective effects of CO2 and pH. In addition, proteomic study will be conducted to explore how Trichodesmium regulates the expression of iron-containing proteins under the coupling effects of iron limitation and ocean acidification, with the goal of elucidating how the organism utilizes the limited iron resource under the environmental stress. Based on the laboratory studies, field work will be carried out at the Northern South China Sea basin and Kuroshio Current to investigate the responses of Trichodesmiumto ocean acidification under respective local conditions. Together, the laboratory and field experiment will comprehensively provide an insight into the influence of ocean acidification on Trichodesmium.

痕量金属铁是海洋生态系统中重要固氮者束毛藻的主要生长限制因子之一。在铁充分的条件下，海水酸化CO2升高引起束毛藻二氧化碳浓缩机制下调，由此节省的细胞能量被利用在固氮作用等细胞过程中，从而促进了固氮和生长。在铁限制的条件下，海水pH下降抑制了束毛藻的固氮作用，结果导致其固碳和生长速率的下降。为了阐明上述不同铁营养盐条件下酸化效应的差异，本申请项目将从实验手段上入手、对酸化的CO2效应和pH效应进行区分比较，揭示与各效应相关联的机制，并评估铁营养盐水平的改变对上述效应的影响。此外，本申请项目将运用蛋白质组学技术探讨铁限制和酸化联合胁迫下束毛藻如何系统性的调控含铁蛋白的表达，从而揭示环境压力下细胞对铁的分配和利用。在实验室机制性探索的基础上，进一步在南海北部海盆和黑潮水系开展现场原位条件下束毛藻对酸化响应的研究，通过与实验室结果的比较与结合，旨在系统性的揭示海洋酸化和铁的耦合作用对束毛藻的效应。

工业革命以来海洋吸收了近三分之一人为活动排放的CO2，导致海水CO2上升、pH下降这一“海洋酸化”现象。固氮蓝藻束毛藻不仅是海洋生态系统中重要的初级生产者，而且是主要的“新氮”输入者，可贡献高达50%的全球海洋固氮量。因此，阐明海洋酸化对束毛藻的影响及其机理，对了解全球变化下的海洋生态系统以及碳、氮生物地球化学循环具有重要意义。本资助项目采用海洋化学与生物学交叉的理念和研究手段，系统解析了海水CO2上升和pH下降对束毛藻分别的效应及其机理，探讨了痕量金属铁对酸化效应的调控作用，并研究了南海北部海域天然束毛藻群落对铁限制与酸化联合胁迫的响应。我们的研究发现，由于海水CO2上升的正效应小于pH 下降的负效应，使得海洋酸化总体上抑制束毛藻的生长和固氮作用。系统的生理、生化分析表明，海洋酸化导致胞质内pH下降，束毛藻一方面通过上调固氮酶蛋白的表达，以缓解受此影响而降低的固氮酶效率；另一方面为了维持胞质内pH稳态，上调多种或含铁、或需耗能的跨内囊体膜的质子泵和跨质膜的转运体蛋白的表达。然而，在铁限制条件下，为满足酸化导致的固氮酶上调对铁的需求，一系列内囊体膜上含铁的质子泵被下调，这同时造成了能量合成的受阻，使得海洋酸化的抑制作用加强。在室内探究的基础上，我们在南海北部开展了现场原位条件下束毛藻对酸化响应的研究，获得了与室内实验发现相吻合的结果。本资助项目的研究成果为认识和预测全球变化对海洋生态系统的影响及其可能带来的气候反馈提供了重要的科学依据。