Invasive macroalgae shape chemical and microbial waterscapes on coral reefs.

Over the past decades, human impacts have changed the structure of tropical benthic reef communities towards coral depletion and macroalgal proliferation. However, how these changes have modified chemical and microbial waterscapes is poorly known. Here, we assessed how the experimental removal of macroalgal assemblages influences the chemical and microbial composition of two reef boundary layers, the benthic and the momentum. Chemical and microbial waterscapes were spatially structured, both horizontally and vertically, according to macroalgal dominance and boundary layers. Microbes associated with reef degradation were enriched in the boundary layers surrounding macroalgal-dominated substrata. Dominant macroalgae were surrounded by a distinct chemical pool of diverse lipid classes (e.g., diterpenoids and glycerolipids) and labile organic matter (e.g., organooxygen compounds), which diffused from algal tissues to boundary layers according to their polarity. Finally, our results highlighted strong co-variations between specific algal-derived metabolites and planktonic microbes, giving insight into their roles in coral reef functioning and resilience.