Natural volcanic CO ₂ seeps reveal future trajectories for host–microbial associations in corals and sponges

Abstract

Atmospheric carbon dioxide (CO ₂) levels are rapidly rising causing an increase in the partial pressure of CO ₂ ( pCO ₂) in the ocean and a reduction in pH known as ocean acidification (OA). Natural volcanic seeps in Papua New Guinea expel 99% pure CO ₂ and thereby offer a unique opportunity to explore the effects of OA in situ. The corals Acropora millepora and Porites cylindrica were less abundant and hosted significantly different microbial communities at the CO ₂ seep than at nearby control sites <500 m away. A primary driver of microbial differences in A. millepora was a 50% reduction of symbiotic Endozoicomonas. This loss of symbiotic taxa from corals at the CO ₂ seep highlights a potential hurdle for corals to overcome if they are to adapt to and survive OA. In contrast, the two sponges Coelocarteria singaporensis and Cinachyra sp. were ∼40-fold more abundant at the seep and hosted a significantly higher relative abundance of Synechococcus than sponges at control sites. The increase in photosynthetic microbes at the seep potentially provides these species with a nutritional benefit and enhanced scope for growth under future climate scenarios (thus, flexibility in symbiosis may lead to a larger niche breadth). The microbial community in the apparently pCO ₂-sensitive sponge species S. massa was not significantly different between sites. These data show that responses to elevated pCO ₂ are species-specific and that the stability and flexibility of microbial partnerships may have an important role in shaping and contributing to the fitness and success of some hosts.