CO₂ leakage alters biogeochemical and ecological functions of submarine sands

Subseabed CO₂ storage is considered a future climate change mitigation technology. We investigated the ecological consequences of CO₂ leakage for a marine benthic ecosystem. For the first time with a multidisciplinary integrated study, we tested hypotheses derived from a meta-analysis of previous experimental and in situ high-CO₂ impact studies. For this, we compared ecological functions of naturally CO₂-vented seafloor off the Mediterranean island Panarea (Tyrrhenian Sea, Italy) to those of nonvented sands, with a focus on biogeochemical processes and microbial and faunal community composition. High CO₂ fluxes (up to 4 to 7 mol CO₂ m^-2 hour^-1) dissolved all sedimentary carbonate, and comigration of silicate and iron led to local increases of microphytobenthos productivity (+450%) and standing stocks (+300%). Despite the higher food availability, faunal biomass (-80%) and trophic diversity were substantially lower compared to those at the reference site. Bacterial communities were also structurally and functionally affected, most notably in the composition of heterotrophs and microbial sulfate reduction rates (-90%). The observed ecological effects of CO₂ leakage on submarine sands were reproduced with medium-term transplant experiments. This study assesses indicators of environmental impact by CO₂ leakage and finds that community compositions and important ecological functions are permanently altered under high CO₂.