Seawall structures are an important ecological environment in any coastal setting. The protection of the land is a key role for these engineered structures, although they serve an alternate role as a habitat for many organisms from the marine environment. Therefore, it is vital to understand what features of these materials, typically natural stone and concrete, encourage or discourage settlement. Settlement may also be influenced by microbial communities, biofilms, that form on these structures. Thus, settlement may be a consequence of selection for a particular microbial community, driven by the physico-chemical attributes of the materials used. Alternatively, the microbial community may play little or no obvious role in larval settlement.
In this study, we monitored the initial settlement behaviour of the coral larvae Pocillopora acuta on substrates such as glass, ceramic tile, concrete and various stones (limestone, granite, marble and sandstone). We further compared settlement after first forming monospecies biofilms on these surfaces. Finally, we formed biofilm communities on these surfaces and correlated settlement efficiency with community and functional gene composition. The goal of these experiments was to determine what role the microbial inhabitants play in shaping the macro-inhabitants of the seawall substrates.
In line with previously published work, we showed that biofilms are a key agent in the induction of coral larval settlement. This was particularly evident for monospecies biofilms where the species used were genetically similar, but had striking differences in settlement efficiency. Our findings also suggest that the substrate is an important factor in coral larval settlement, where some substrates supported settlement and others either had no measurable effect or were toxic to larvae.
Thus, the settlement decision of larvae is likely to involve a complex process that accounts for the substratum properties as well as the microbial community that forms on that substratum. This reinforces the need for further investigations to fully understand the relationships between substrate, biofilm and macro-settling organisms in a coastal environment.