Seawater carbonate chemistry and carbon and nitrogen fixation in the hermatypic coral Galaxea fascicularis

The supply of metabolites from symbionts to scleractinian corals is crucial to coral health. Members of the Symbiodiniaceae can enhance coral calcification by providing photosynthetically fixed carbon (PFC) and energy, whereas dinitrogen (N2)-fixing bacteria can provide additional nutrients such as diazotrophically-derived nitrogen (DDN) that sustain coral productivity especially when alternative external nitrogen sources are scarce. How these mutualistic associations benefit corals in the future acidifying ocean is not well understood. In this study, we investigated the possible effects of ocean acidification (OA; pHs 7.7 and 7.4 vs. 8.1) on calcification in the hermatypic coral Galaxea fascicularis with respect to PFC and DDN assimilation. Our measurements based on isotopic tracing showed no significant differences in the assimilation of PFC among different pH treatments, but the assimilation of DDN decreased significantly after 28 days of stress at pH 7.4. The decreased DDN assimilation suggests a nitrogenous nutrient deficiency in the coral holotiont, potentially leading to reduced coral calcification and resilience to bleaching and other stressful events. This contrasting impact of OA on carbon and N flux demonstrates the flexibility of G. fascicularis in coping with OA, apparently by sustaining a largely undamaged photosystem at the expense of N2 fixation machinery, which competes with coral calcification for energy from photosynthesis. These findings shed new light on the critically important but more vulnerable N cycling in hospite, and on the trade-off between coral hosts and symbionts in response to future climate change.

共生体向石珊瑚供应代谢物，对于珊瑚健康至关重要。虫黄藻科（Symbiodiniaceae）成员可通过提供光合固定碳（PFC）与能量，增强珊瑚的钙化作用；而固氮细菌则可提供额外营养物质，例如固氮来源氮（DDN），在外界可替代氮源匮乏时维持珊瑚的生产力。目前学界对于这类互利共生关系在未来酸化海洋中如何使珊瑚获益的机制仍不甚明晰。
本研究针对海洋酸化（OA；设置pH 7.7、7.4与对照组pH 8.1）对造礁珊瑚丛生盔形珊瑚（Galaxea fascicularis）钙化作用的潜在影响展开探究，重点关注其光合固定碳与固氮来源氮的同化过程。基于同位素示踪的检测结果显示，不同pH处理组间光合固定碳的同化水平无显著差异；但在pH 7.4的胁迫条件下培养28天后，固氮来源氮的同化水平显著下降。
固氮来源氮同化水平的下降表明珊瑚共生体存在氮营养缺乏的情况，或可导致珊瑚钙化作用减弱，且其应对白化及其他胁迫事件的恢复力下降。海洋酸化对碳与氮通量的这种差异化影响，体现了丛生盔形珊瑚应对海洋酸化的灵活性：该珊瑚可维持基本未受损伤的光合系统，却以固氮机制为代价——后者会与珊瑚钙化作用竞争光合来源的能量。
本研究结果为揭示宿主体内至关重要却更为脆弱的氮循环机制，以及珊瑚宿主与共生体在应对未来气候变化时的能量权衡策略，提供了新的见解。