This study focused on biogeochemical processes and microbial activity in sediments of a natural deep-sea CO 2 seepage area (Yonaguni Knoll IV hydrothermal system, Japan). The aim was to assess the influence of the geochemical conditions occurring in highly acidic and CO 2 saturated sediments on sulphate reduction (SR) and anaerobic 5 methane oxidation (AOM). Porewater chemistry was investigated from retrieved sedi- ment cores and in situ by microsensor profiling. The sites sampled around a sediment- hosted hydrothermal CO 2 vent were very heterogeneous in porewater chemistry, in- dicating a complex leakage pattern. Near the vents, droplets of liquid CO 2 were ob- served to emanate from the sediments, and the pH reached approximately 4.5 in a 10 sediment depth > 6cm, as determined in situ by microsensors. Methane and sulphate co-occurred in most sediment samples from the vicinity of the vents down to a depth of at least 3m. However, SR and AOM were restricted to the upper 7–15cm below seafloor, although neither temperature, low pH, nor the availability of methane and sul- phate could be limiting microbial activity. We argue that the extremely high subsurface 15 concentrations of dissolved CO 2 (1000–1700mM), through the ensuing high H 2 CO 3 levels (approx. 1–2mM) uncouples the proton-motive-force (PMF) and thus inhibits bi- ological energy conservation by ATPase-driven phosphorylation. This limits life to the surface sediment horizons above the liquid CO 2 phase, where less extreme conditions prevail. Our results may have to be taken into consideration in assessing the conse- 20 quences of deep-sea CO 2 sequestration on benthic element cycling and on the local ecosystem state