CO₂ leakage from carbon dioxide capture and storage (CCS) systems affects organic matter cycling in surface marine...

Carbon dioxide capture and storage (CCS), involving the injection of CO₂ into the sub-seabed, is being promoted worldwide as a feasible option for reducing the anthropogenic CO₂ emissions into the atmosphere. However, the effects on the marine ecosystems of potential CO₂ leakages originating from these storage sites have only recently received scientific attention, and little information is available on the possible impacts of the resulting CO₂-enriched seawater plumes on the surrounding benthic ecosystem. In the present study, we conducted a 20-weeks mesocosm experiment exposing coastal sediments to CO₂-enriched seawater (at 5000 or 20,000 ppm), to test the effects on the microbial enzymatic activities responsible for the decomposition and turnover of the sedimentary organic matter in surface sediments down to 15 cm depth. Our results indicate that the exposure to high-CO₂ concentrations reduced significantly the enzymatic activities in the top 5 cm of sediments, but had no effects on subsurface sediment horizons (from 5 to 15 cm depth). In the surface sediments, both 5000 and 20,000 ppm CO₂ treatments determined a progressive decrease over time in the protein degradation (up to 80%). Conversely, the degradation rates of carbohydrates and organic phosphorous remained unaltered in the first 2 weeks, but decreased significantly (up to 50%) in the longer term when exposed at 20,000 ppm of CO₂. Such effects were associated with a significant change in the composition of the biopolymeric carbon (due to the accumulation of proteins over time in sediments exposed to high-pCO₂ treatments), and a significant decrease (∼20-50% at 5000 and 20,000 ppm respectively) in nitrogen regeneration. We conclude that in areas immediately surrounding an active and long-lasting leak of CO₂ from CCS reservoirs, organic matter cycling would be significantly impacted in the surface sediment layers. The evidence of negligible impacts on the deeper sediments should be considered with caution and further investigated simulating the intrusion of CO₂ from a subsurface source, as occurring during real CO₂ leakages from CCS sites.