Changes in microbial communities in coastal sediments along natural CO₂ gradients at a volcanic vent in Papua New Guinea

Natural CO₂ venting systems can mimic conditions that resemble intermediate to high pCO₂ levels as predicted for our future oceans. They represent ideal sites to investigate potential long-term effects of ocean acidification on marine life. To test whether microbes are affected by prolonged exposure to pCO₂ levels, we examined the composition and diversity of microbial communities in oxic sandy sediments along a natural CO₂ gradient. Increasing pCO₂ was accompanied by higher bacterial richness and by a strong increase in rare members in both bacterial and archaeal communities. Microbial communities from sites with CO₂ concentrations close to today's conditions had different structures than those of sites with elevated CO₂ levels. We also observed increasing sequence abundance of several organic matter degrading types of Flavobacteriaceae and Rhodobacteraceae, which paralleled concurrent shifts in benthic cover and enhanced primary productivity. With increasing pCO₂, sequences related to bacterial nitrifying organisms such as Nitrosococcus and Nitrospirales decreased, and sequences affiliated to the archaeal ammonia-oxidizing Thaumarchaeota Nitrosopumilus maritimus increased. Our study suggests that microbial community structure and diversity, and likely key ecosystem functions, may be altered in coastal sediments by long-term CO₂ exposure to levels predicted for the end of the century.