Microbial composition of biofilms associated with lithifying rubble of Acropora palmata branches

Coral reefs are amongst the most productive ecosystems on the planet, but are rapidly declining due to global warming-mediated changes in the oceans. Particularly for the Caribbean region, Acropora sp. stony corals have lost ~80% of their original coverage, resulting in vast extensions of dead coral rubble. We analysed the microbial composition of biofilms that colonize and lithify dead A. palmata rubble in the Mexican Caribbean and identified the microbial assemblages that can persist under scenarios of global change, including high temperature and low pH. Lithifying biofilms have a mineral composition that includes aragonite and magnesium calcite (16 mole% MgCO₃) and calcite, while the mineral phase corresponding to coral skeleton is basically aragonite. Microbial composition of the lithifying biofilms are different in comparison to surrounding biotopes, including a microbial mat, water column, sediments and live A. palmata microbiome. Significant shifts in biofilm composition were detected in samples incubated in mesocosms. The combined effect of low pH and increased temperature showed a strong effect after 2-week incubations for biofilm composition. Findings suggest that lithifying biofilms could remain as a secondary structure on reef rubble possibly impacting the functional role of coral reefs.