Reduced seawater pH alters marine biofilms with impacts for marine polychaete larval settlement Metagenome

Ocean acidification (OA) can negatively affect early-life stages of marine organisms, with the key processes of larval settlement and metamorphosis potentially vulnerable to reduced seawater pH. Settlement success depends strongly on suitable substrates and environmental cues, with marine biofilms being key settlement inducers for a range of marine invertebrate larvae. This study experimentally investigated (1) how seawater pH determines growth and community composition of marine biofilms, and (2) whether marine biofilms developed under different pH conditions can alter settlement success in the New Zealand serpulid polychaete Galeolaria hystrix. Biofilms were developed under six pH treatments (spanning from 7.0 to ambient [8.1]) in a flow-through system for up to 14 months. Biofilms of different ages (7, 10 and 14 months) were used to assay settlement success rates of competent G. hystrix larvae. Biofilm microbiomes were characterized through amplicon sequencing of the small subunit ribosomal rRNA gene (16S and 18S). Biofilm community composition was stable over time within each pH treatment and biofilm age did not affect larval settlement selectivity. Seawater pH treatment strongly influenced biofilm community composition, as well as subsequent settlement success when biofilms were presented to competent Galeolaria larvae. Exposure to biofilms incubated under OA-treatments caused a decrease in larval settlement success of up to 40% compared to the ambient treatments. We observed a tipping point in the settlement success between biofilms incubated under predicted average and extreme end-of-century levels of OA. Settlement selectivity decreased with pH on biofilms incubated at pH 7.9 and 7.7. This trend was reversed at pH 7.4, resulting in high settlement success, comparable to ambient biofilms. For the first time, we show that long-term incubation of marine biofilms under a wide range of reduced seawater pH treatments can alter marine biofilms in such a way that settlement success in marine invertebrates can be compromised.