In the worst future scenario, the uptake of anthropogenic CO₂ by seawater will reduce the ocean pH for up to 0.4 units by 2100. Nonetheless, the effects of ocean acidification (OA) on sediment biogeochemistry remain poorly understood. We investigated the effects of OA on sedimentary organic matter (OM) characteristics along a natural pH gradient at the Vulcano Island (Italy). Sediment samples were collected during summer 2024 in the Levante Bay at three stations characterized by natural (CpH = 8.1), moderately acidified (MpH = 7.9), and low (LpH = 7.7) pH values, resembling current, moderate-case and worst-case scenarios of OA, respectively. Sediments were analyzed to assess protein, carbohydrate, lipid, and phytopigment contents, and estimate OM ageing (in terms of the protein to carbohydrate ratio), nutritional quality (algal fraction of biopolymeric C), and OM turnover time (from extracellular enzymatic activities). MpH sediments showed higher protein contents, but lower lipid and carbohydrate loads compared to CpH and LpH. Sediments in MpH showed higher values of the protein to carbohydrate ratio, but the lowest values of the algal fraction of biopolymeric C, indicating a fresher OM origin but a nutritional quality lower than that in CpH and LpH. In LpH sediments C turnover time (20±1 d) was higher than in CpH (81 d) and MpH (35 d), suggesting that a stronger OA could accelerate sedimentary C cycling. Furthermore, OM biochemical composition differed significantly among the three pH conditions. Though these results are limited to the summer season, comparing these results with those obtained from other naturally acidified sites, suggest that OA can strongly influence the biochemical composition and degradation rates of sedimentary OM, but with variable and not consistent responses at different pH values. These preliminary results highlight the complex and non-linear effects of OA on sedimentary biogeochemistry.