Sponges are celebrated heterotrophs but also key primary producers on changing coral reefs

Trophic interactions and nutrient cycling lay at the heart of ecosystem
health and biodiversity. In recent years, our understanding of these
drivers has been repeatedly challenged by rapid and unanticipated climatic
effects, combined with an increasing awareness that carbon acquisition by
living organisms often does not meet the textbook duality of autotrophy
versus heterotrophy. On coral reefs, mixotrophic feeding that combines
these two strategies is widespread. Mixotrophy has been largely overlooked
in sponges, which are ecologically important and highly abundant animals
that are commonly celebrated both as efficient heterotrophic feeders as
well as climate-change winners in these rapidly declining ecosystems. Many
Caribbean sponges associate with photosynthetic symbionts, and we here
combine oxygen flux measurements with chlorophyll fluorometry in 24
abundant species to show that ─in contrast to presumed strict
heterotrophy─ large portions of their metabolic needs can be covered
through symbiont-supplied autotrophic inputs, even when these are in low
abundance and when net photosynthesis remains negative. At the ecosystem
level, we find that half of the sponge species on the reefs of Curaçao
contribute to 11% of gross primary productivity of the entire benthic
ecosystem, ranking them the 4th most important producers after macroalgae,
hard corals, and gorgonians, and higher than crustose-coralline algae,
which are well-known phototrophs. Together with their heterotrophic carbon
capturing, we argue that the widespread presence and anticipated
contribution of photosymbiotic sponges to coastal ecosystem productivity
call for further investigation and for revision of benthic food web models
and carbon budgets.