Petroleum hydrocarbon attenuation and biogeochemical responses in mangrove sediments during Rhizophora mangle acclimation

Mangrove sediments are important reservoirs of organic carbon, but petroleum contamination can alter their biogeochemical functioning by introducing large amounts of petrogenic organic matter and modifying carbon dynamics. Understanding these transformations and the role of mangrove vegetation in hydrocarbon attenuation is essential for nature-based recovery of oil-impacted coastal environments. This study evaluated changes in petroleum hydrocarbons and sediment biogeochemical properties during a 180-day acclimation experiment using Rhizophora mangle and mangrove sediment exposed to crude oil from the Campos Basin, Brazil, at 0, 0.5, 1.0, and 2.0% (v/w), including planted and unplanted conditions. Total petroleum hydrocarbons (TPHs) and unresolved complex mixture (UCM) were quantified by GC-FID, and sediment biogeochemical characterization included total organic carbon (TOC), organic matter, total sulfur (TS), total nitrogen, phosphorus, pH, and particle size. The original sediment presented low concentrations of TPHs (9.36 ± 2.07 mg kg⁻¹), UCM (2.42 ± 0.16 mg kg⁻¹), and TOC (0.09 ± 0.01%). Petroleum addition increased the sediment organic fraction, with initial TOC reaching 0.56, 1.20, and 2.17% at 0.5, 1.0, and 2.0% oil, respectively. TPHs reached 2659.43 ± 7.16 mg kg⁻¹ at 1.0% and 5424.28 ± 68.71 mg kg⁻¹ at 2.0% oil. After 180 days, TPH and UCM concentrations decreased across contaminated treatments, with maximum TPH reductions of 1017, 1843, and 2685 mg kg⁻¹ at 0.5, 1.0, and 2.0% oil, respectively. TOC also decreased, particularly at intermediate contamination levels, indicating transformation of petroleum-derived organic carbon during the experiment. TPHs, UCM, and TOC were strongly associated, supporting petroleum hydrocarbons as the main source of organic carbon in the experimental system. At the highest contamination level, TS showed a distinct increase after 180 days, suggesting that high petroleum loading may also affect sediment sulfur cycling. TPH attenuation occurred across all contamination levels, including the highest oil concentration. The acclimation of R. mangle under these conditions, together with the observed hydrocarbon attenuation and changes in carbon and sulfur dynamics, reinforces its potential for phytoremediation of petroleum-contaminated sediments. These findings support acclimation as a relevant step toward the application of R. mangle in nature-based solutions for the recovery of oil-impacted mangrove environments. The authors acknowledge the financial support of the Human Resources Program of the Brazilian National Agency of Petroleum, Natural Gas and Biofuels (PRH-ANP/PRH36-UFBA), under the management of the São Paulo Research Foundation (FAPESP), Process No. 2025/03612-4; the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001; and the REMAREC Project, funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Process No. 420945/2025-3.