Sponge-derived matter is assimilated by coral holobionts

Abstract

Abstract Coral reef biodiversity is maintained by a complex network of nutrient recycling among organisms. Sponges contribute by assimilating nutrients produced by other organisms like coral and algae, and releasing them as particulate and dissolved matter, but to date, only a single trophic link (to a macroalgae) fueled by sponge-derived matter has been identified. We sought to determine if sponge-coral nutrient exchange is reciprocal using a stable isotope ‘pulse-chase’ experiment to trace the uptake of sponge-derived matter by the coral holobiont. Caribbean sponges were incubated with inorganic 13C and 15N tracer compounds, then placed into aquaria containing fragments from three coral species (Acropora cervicornis, Orbicella faveolata, and Eunicea flexuosa). Coral holobionts incorporated 2.3-26.8x more 15N than 13C from sponge-derived matter, and A. cervicornis and its symbiotic micro-algae (Symbiodiniaceae) incorporated more of both elements than the other corals. E. flexuosa host and Symbiodiniaceae fractions incorporated comparable 13C and 15N, suggesting an equal propensity to assimilate sponge-derived matter. Variable isotopic incorporation among coral species aligns with their ecophysiological characteristics. Our results confirm the reciprocal nature of the sponge-coral nutrient exchange and elucidate a novel trophic pathway that has implications for improving coral aquaculture and management approaches.