Variation in autotrophic and heterotrophic sponge abundance in a shallow water seagrass system

Sponges are well known to feed heterotrophically through suspension feeding, but their relationships with photosynthetic symbionts means they also have the potential to utilise or release photosynthetically-derived carbon. Here we determined the nutritional mode of abundant seagrass sponge species in the Wakatobi National Park, Indonesia from the near-reef flat zone to the high shore to assess the role they might play in nutrient fluxes. We measured in situ gross primary production (GPP) to respiration (P:R) ratios, photosynthetic pigment concentrations, and light compensation and saturation points of eight sponge species representing almost 100% of the total sponge assemblage biomass. We found that all sponge species contained photosynthetic pigments and had positive gross primary production, indicating the presence of photosynthetic symbionts. However, based on their daily oxygen budget not all sponge holobionts were autotrophic (P:R>1); six species were autotrophic, with five of them being net oxygen producers. Based on biomass, autotrophic sponges only dominated (98% of biomass) the sponge assemblage in the near-reef-flat zone. We suggest that the higher light intensity in the high- and middle-shore zones of the meadow triggers photoinhibition, limiting autotrophic sponge distribution further up the shore. We propose that the considerable biomass of autotrophic sponges in the lower parts of the seagrass meadow challenges our current view that sponges are generally consumers of large amounts of carbon from the water column, as photosynthetic symbionts have the potential to meet at least some of their sponge hosts carbon requirements or may release dissolved organic carbon to the environment.