Climate change stressors destabilize the microbiome of the Caribbean barrel sponge, Xestospongia muta

• Ocean acidification and thermal stress did not have any effects on the microbiome of the sponge, Xestospongia muta.
• Ocean acidification and thermal stress did effect the photosynthetic potential of the cyanobacterial symbionts of X. muta.
• Ocean acidification destabilizes the microbiome while thermal stress stabilizes the microbiome of X. muta.

The effect of climate change, both thermal stress and ocean acidification, on coral reefs is of increasing concern with the effects on calcification at the organismal level, and changes in the ratio of accretion to erosion on larger spatial scales of particular interest. But far fewer studies have been done on non-calcifying organisms, such as sponges, that have important ecological roles on coral reefs. Here we report the results of a combined thermal stress and ocean acidification experiment on the ecologically dominant barrel sponge, Xestospongia muta, found on coral reefs throughout the Caribbean basin. The results show that ocean acidification alone, as well as its interaction with elevated seawater temperature, has significant effects on the sponge microbiome. Specifically, the significant interactive effects of thermal stress and ocean acidification led to a decline in the productivity potential of the symbiotic cyanobacteria in these sponges with a subsequent impact on nutrient transfer, as carbohydrate, between symbiont and host. Additionally, while neither environmental stressor predictably changed sponge microbiome community composition, ocean acidification alone reduced the stability of sponge microbiomes and their predicted functions. Future changes in ocean acidification and thermal stress predicted by current climate models could negatively impact the microbiomes of coral reef organisms and therefore also affect their organismal performance and fitness in the future.