Warm‐adapted sponges resist thermal stress by reallocating carbon and nitrogen resources from cell turnover to somatic growth

Abstract

Ocean warming will affect the functioning of coral reef ecosystems with unknown cascading effects. Any perturbation in the ability of sponges to recycle the dissolved organic matter released by primary producers and make it available to higher trophic levels, might have unknown consequences for the reef trophic chain. Biogeochemical processes were measured in the sponge Rhabdastrella globostellata from the semi‐enclosed lagoon of Bouraké, where temperatures reach 33.8°C and fluctuates by 6.5°C on a daily basis, and from a control reef (28°C). Using 13C‐ and 15N‐labeled coral mucus, we experimentally investigated to what extent high temperature affected the carbon (C) and nitrogen (N) resources allocation in the sponge tissue and detritus. Sponges from Bouraké maintained at 32°C incorporated more 13C‐ and 15N‐labeled coral mucus in the tissue and showed less detritus release when compared with sponges maintained at 28°C. In contrast, at 32°C control sponges showed lower 13C‐ and 15N‐labeled coral mucus incorporation in tissue and higher release of detritus. Our results suggest that sponges adapted to extreme temperatures of Bouraké were able to reallocate C and N resources from cell turnover to somatic growth and reduce tissue damage. In contrast, non‐adapted sponges at the control reef lack this mechanism and underwent tissue disintegration, highlighting the lethal effect of future warming. The change in C and N allocation in adapted sponges suggests a potential adaptation mechanism that allows R. globostellata to survive under thermal stress, but it could alter the availability of essential sources of energy with unknown consequences on the future reef trophic interactions.