Elevated CO2 Has Little Influence on the Bacterial Communities Associated With the pH-Tolerant Coral, Massive Porites spp.

As anthropogenic CO2 emissions increase, ocean acidification is becoming a critical threat to coral reef ecosystems. While many coral species suffer from degraded health or shifting microbial communities under high pCO2 and low pH, some coral taxa demonstrate higher resilience. This study investigates whether the tissue-associated bacterial communities of the pH-tolerant coral, massive Porites spp., remain stable under the stressful chemical conditions found at natural volcanic CO2 seeps in Papua New Guinea.

Researchers sampled massive Porites corals from two volcanic CO2 seep sites (pH 7.44–7.85) and nearby ambient control sites (pH 8.0–8.1). By utilizing 16S rRNA gene amplicon sequencing, the team profiled the microbial community composition of the coral tissues. They compared the community structures between seep and control sites, examined for core and indicator taxa, and correlated community findings with reef location to determine the primary drivers of microbial variation.

Contrary to previous findings in acid-sensitive corals—where CO2 exposure often leads to significant microbial dysbiosis—massive Porites corals maintained remarkably stable microbial community structures across all sites regardless of pH levels. The most influential factor driving variation in the microbial communities was the geographical location of the reef, rather than the CO2 treatment. The genus Endozoicomonas emerged as the dominant core symbiont across all samples. The researchers conclude that the ability of massive Porites to regulate its internal pH likely contributes to the maintenance of a consistent and stable microbial assembly.

This research challenges the assumption that elevated CO2 will universally disrupt coral-associated microbiomes. By identifying massive Porites as a resilient holobiont that thrives in conditions exceeding those predicted for the coming century, the study suggests that such robust species may come to dominate future reef assemblages as more sensitive corals decline due to ocean acidification.