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.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2022) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2023-05-15.