Abstract 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. Read the free Plain Language Summary for this article on the Journal blog.
Achlatis et al. (Tue,) studied this question.