CO leakage can cause loss of benthic biodiversity in submarine sands.

One of the options to mitigate atmospheric CO increase is CO Capture and Storage in sub-seabed geological formations. Since predicting long-term storage security is difficult, different CO leakage scenarios and impacts on marine ecosystems require evaluation. Submarine CO vents may serve as natural analogues and allow studying the effects of CO leakage in a holistic approach. At the study site east of Basiluzzo Islet off Panarea Island (Italy), gas emissions (90-99% CO) occur at moderate flows (80-120 L m h). We investigated the effects of acidified porewater conditions (pH range: 5.5-7.7) on the diversity of benthic bacteria and invertebrates by sampling natural sediments in three subsequent years and by performing a transplantation experiment with a duration of one year, respectively. Both multiple years and one year of exposure to acidified porewater conditions reduced the number of benthic bacterial operational taxonomic units and invertebrate species diversity by 30-80%. Reduced biodiversity at the vent sites increased the temporal variability in bacterial and nematode community biomass, abundance and composition. While the release from CO exposure resulted in a full recovery of nematode species diversity within one year, bacterial diversity remained affected. Overall our findings showed that seawater acidification, induced by seafloor CO emissions, was responsible for loss of diversity across different size-classes of benthic organisms, which reduced community stability with potential relapses on ecosystem resilience.