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 therefore offer a unique opportunity to investigate potential long-term effects of ocean acidification on marine life. To test whether sediment-associated microbial communities in their natural habitats are affected by prolonged exposure to pCO2 levels of 100-1100 µatm above today’s concentrations, we examined the composition and diversity of bacterial and archaeal communities within the top three centimetres of oxic sandy sediment along a natural CO2 gradient at 1-3 m water depth. Increasing pCO2 was accompanied by higher bacterial richness, combined with a strong increase in rare members for both bacterial and archaeal communities. The structure of microbial communities from sites with CO2 concentrations close to today’s conditions (400-650 µatm pCO2) were significantly different from those with elevated CO2 levels (700-1500 µatm pCO2). With increasing pCO2, we also observed a significant increase in sequence abundance related to several organic matter degrading types of Flavobacteriaceae and Rhodobacteraceae, which might be linked to concurrent shifts in benthic cover from corals to seagrasses and benthic algae, and to enhanced primary productivity. A strong decrease in sequences related to bacterial nitrifying organisms such as Nitrosococcus and Nitrospirales accompanied an increase in sequences affiliated to the archaeal ammonia oxidising Thaumarchaeota Nitrosopumilus maritimus. Our study suggests that microbial community structure and diversity, and likely key ecosystem functions and element cycles, may be altered in coastal sediments by long-term CO2 exposure to levels predicted for the end of the century.