Photosymbionts contribute to niche partitioning in coral reef sponges

Photosymbionts expand the metabolic capabilities of host sponges, but their potential role in mediating niche partitioning on crowded and oligotrophic coral reefs is unknown. To address this question, we conducted 2 ex situ isotope tracer experiments with 10 of the most abundant sponge species in the Caribbean. To target autotrophic and heterotrophic nutrient acquisition by microbial symbionts, we incubated sponges in seawater laced with the inorganic compounds NaH¹³CO₃ and Na¹⁵NO₃ under both light and dark conditions. We also measured host sponge heterotrophic feeding rates by incubating the same species with ¹³C- and ¹⁵N-labeled bacterial cells. In all experiments, 7 replicates of each sponge species were used. Following incubations with inorganic sources, the d¹³C and d¹⁵N values of sponge cells isolated from photosymbiont-hosting sponge species increased more than those of species lacking photosymbionts. In addition, 72% of the variation in d¹³C and d¹⁵N values across experimental samples was explained by sponge species identity. Sponges rapidly consumed bacterial cells, but there was substantial variation in heterotrophic feeding rates among sponge species. When considering all 3 resource pools (symbiont autotrophy, symbiont heterotrophy, and sponge heterotrophy) and both elements, sponge species identity accounted for over 80% of variation among specimens. In addition, we observed a clear separation of sponge species along a continuum of heterotrophic feeding on particulate organic matter to autotrophic metabolism via photosymbionts. These data demonstrate that the combined influence of sponge and photosymbiont metabolism enables coexisting sponge species to exploit unique resource pools on Caribbean reefs.