Phototrophic sponge productivity may not be enhanced in a high CO2 world. (6th June 2022) - Bell, James J.
Abstract: Sponges are major components of benthic communities across the world and have been identified as potential "winners" on coral reefs in the face of global climate change as result of their tolerance to ocean warming and acidification (OA). Previous studies have also hypothesised that photosymbiont‐containing sponges might have higher productivity under future OA conditions as a result of photosymbionts having increased access to CO2 and subsequently greater carbon production. Here we test this hypothesis for a widespread and abundant photosymbiont‐containing sponge species Lamellodysidea herbacea at a CO2 seep in Papua New Guinea simulating OA conditions. We found seep sponges had relatively higher cyanobacterial abundance, chlorophyll concentrations and symbiont photosynthetic efficiency than non‐seep sponges, and a three‐fold higher sponge abundance at the seep site. However, while gross oxygen production was the same for seep and non‐seep sponges, seep sponge dark respiration rates were higher and instantaneous photosynthesis: respiration (P:R) ratios were lower. We show that while photosymbiont containing sponges may not have increased productivity under OA, they are able to show flexibility in their relationships with microbes and offset increased metabolic costs associated with climate change associated stress. Abstract : Here we test the hypothesis that the photosymbiont‐containing sponge Lamellodysidea herbacea might have higher productivity under future OA conditions as a result of photosymbionts having increased access to CO2 . We sampled sponges at a natural CO2 seep in Papua New Guinea finding seep sponges had higher abundance, photosynthetic rates, and chlorophyll concentrations than non‐seep sponges. However, while gross oxygen production was the same for seep and non‐seep sponges, seep sponge dark respiration rates were higher resulting in similar overall productivity for seep and non‐seep sponges.