Organic carbon cycling in a Caribbean coral reef Hidden biomass, sneezing sponges, and net heterotrophy

Coral reefs are known for their high biodiversity and productivity, but how are these productive ecosystems sustained in nutrient-poor tropical seas? This thesis investigates the organic carbon flows in a Caribbean coral reef community, shedding light on hidden biomass and the species responsible for fueling coral growth. Surprisingly, we found that half of the reef’s biomass is tucked away in shaded caves and crevices under the reef framework, largely comprising sponges. These ancient animals exhibit a new behavior, "sneezing" carbon- and nitrogen-rich organic material into the environment. Our field experiments and ecological models demonstrate that this sneezing behavior helps sustain the surrounding reef fauna and thereby the reef’s high productivity. Additionally, we found that the studied reef community consumes more organic carbon than it produces, likely by taking up carbon from water masses flowing over the reef. Our findings establish previously “invisible” ecological processes that are pivotal to understand how coral reef ecosystems function at present and in the future.