Microbiomes of algae-dominated coral reefs are more susceptible to combined than individual effects of ocean acidification and dissolved organic carbon

Coral reef-associated microorganisms respond fast to changing environmental conditions. Global and local stressors can result in rapid shifts in bacterial community structure, having direct consequences on the ecological function and biogeochemical transformations. Here we evaluated the effect of elevated dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) on bacterial diversity in coral and macroalgal mesocosms. 16S-based diversity was compared between water and sediments in coral and algal mesocosms over a 42-day incubation period. DOC acted as the main driver of changes in bacterial beta-diversity, resulting in the concomitant proliferation of Enterobacteriaceae and Rhodobacteraceae in water samples from coral and algal mesocosms. DIC alone had a minor impact on community composition, but when combined with increased DOC concentrations, strong bacterial community shifts were observed in algal mesocosms. Furthermore, shifts in the bacterial community associated with increasing DOC are predicted to displace important functional groups such as diazotrophs and nitrite reducers in the water column. Our results suggest that increasing benthic algal coverage is an additional factor contributing to coral reef vulnerability to DIC and DOC enrichments via its modifying effects on bacterial diversity and functioning