Effects of Future Ocean Conditions on the Microbiome of Crustose Coralline Algae with Implications for Coral Settlement and Growth

Environmental stressors are known to disrupt the microbiome of reef dwelling organisms, leading to more extensive habitat damage through species loss or the alteration of reef relationships. Crustose Coralline Algae (CCA) are vital calcifiers for reef ecosystems that are known to induce the settlement and metamorphosis of many reef inhabitants, including corals. However, there is limited research on the differentiation of microbiomes between CCA species, and how environmental stressors may alter microbiome structure and in turn influence CCA interactions with the surrounding environment. We investigated how two species of CCA, Porolithon onkodes and Lithophyllum sp., with differing microbiomes may alter their microbial dynamics and settlement cues when exposed to the combined stressors of ocean acidification and sea surface temperature rise. CCA were acclimated to ambient and high temperature/pCO2 conditions in a full factorial cross, resulting in 4 treatments, for 6 months. The two species of CCA were found to have distinct bacterial assemblages and both microbiome community structure, microbiome richness, and DOM exudates in both CCA species changed in response to environmental stressors, primarily responding to temperature and most markedly under the combined treatment of ocean acidification and high temperatures. Furthermore, settlement of Pocillopora acuta coral larva increased in higher temperatures. Thus, our results suggest that both microbial dynamics and coral larval settlement cue exudates from CCA may be altered with the progression of climate change.