Seawater carbonate chemistry and photosystem II (PSII) function, bleaching of coralline alga Amphiroa gracilis and larval settlement ofthe sea urchin Heliocidaris erythrogramma

Coralline algae are a crucial component of reef systems, stabilising reef substrate, providing habitat and contributing to accretion. Coralline algae and their surface microbial biofilms are also important as settlement cues for marine invertebrates, yet few studies address the impact of future environmental conditions on interactions between coralline algae, reef microbes and settlement by larvae of marine invertebrates. We exposed the temperate coralline algal species Amphiroa gracilis to warming and/or acidification scenarios for 21 days. Algae became bleached but photosystem II (PSII) function was not measurably impacted. Settlement by larvae of the sea urchin Heliocidaris erythrogramma was reduced and the structure of the prokaryotic community associated with A. gracilis was altered. Coralline algae in ambient conditions were dominated by Alphaproteobacteria from the Rhodobacteraceae including Loktonella; those under warming were dominated by Bacteroidetes and Verrucomicrobia; acidification resulted in less Loktonella and more Planctomycetes; and a combination of warming and acidification caused increases in Bacteroidetes, Verrucomicrobia and the Alphaproteobacteria family Hyphomonadaceae. These experiments indicate that predicted future environmental change may reduce the ability of some temperate reef coralline algae and associated reef microbes to facilitate settlement of invertebrate larvae as well as having a direct impact to algae via bleaching.

珊瑚藻（coralline algae）是珊瑚礁生态系统的关键组成部分，可稳定礁体基质、提供栖息生境并促进礁体加积。珊瑚藻及其表面微生物生物膜同时也是海洋无脊椎动物附着的重要诱导因子，然而目前鲜有研究探讨未来环境变化对珊瑚藻、礁体微生物与海洋无脊椎动物幼虫附着过程之间互作关系的影响。本研究将温带珊瑚藻物种细枝珊瑚藻（Amphiroa gracilis）暴露于增温、酸化及复合胁迫场景下，处理时长为21天。受试藻体出现白化现象，但光系统II（PSII）的功能未出现可检测到的损伤。红斑海胆（Heliocidaris erythrogramma）幼虫的附着率显著降低，且细枝珊瑚藻表面附着的原核生物群落结构发生改变。处于正常环境条件下的珊瑚藻表面群落以红杆菌科（Rhodobacteraceae）下的α-变形菌纲（Alphaproteobacteria）为主，其中包含洛顿菌属（Loktonella）；增温组的群落则以拟杆菌门（Bacteroidetes）和疣微菌门（Verrucomicrobia）占主导；酸化组的洛顿菌属丰度下降，浮霉菌门（Planctomycetes）丰度上升；而增温与酸化复合胁迫组的拟杆菌门、疣微菌门以及α-变形菌纲的生丝微菌科（Hyphomonadaceae）丰度均出现升高。本实验结果表明，未来预测的环境变化不仅会通过白化作用直接对温带珊瑚藻造成损伤，还可能削弱部分温带礁体珊瑚藻及其共生礁体微生物促进无脊椎动物幼虫附着的能力。