Bioremediation of oil-contaminated beach sediments of Pari Islands, Thousands Archipelago, Indonesia: Evaluating microalgal toxicity and bioremediation effectiveness

Oil-contaminated beach sediments (OCBS) are a persistent environmental issue that threatens coastal biodiversity, ecosystem functioning, and sediment quality in tropical marine environments. This study evaluated both the bioremediation performance and ecotoxicological responses of OCBS subjected to various treatments over 60 days, using tropical marine microalgae (Tetraselmis sp. and Isochrysis sp.) as sensitive bioindicators. Five treatments were tested, including natural attenuation, a single bacterial inoculation (Alcanovorax sp. TE-9), a bacterial consortium (Alcanivorax sp., Pseudomonas balearica, and Bordetella sp.), and combinations with osmocote fertilizer. Microbial growth, oil and grease concentrations, and microalgal responses were also monitored. Microbial activity fluctuated during incubation but declined after prolonged exposure. The bacterial consortium combined with Osmocote (Treatment E) achieved the greatest oil and grease reduction (45 %) after 60 days, which was attributed to nutrient-driven stimulation of bacterial proliferation and cooperative hydrocarbon catabolism among the consortium members, which enhanced biosurfactant production and hydrocarbon breakdown. Tetraselmis sp. exhibited significant growth inhibition, whereas Isochrysis sp. showed greater tolerance, emphasizing the importance of species-specific bioassays in post-bioremediation assessments. This study presents the first comprehensive field-scale evaluation of OCBS bioremediation in Indonesia, integrating chemical, microbial, and ecotoxicological endpoints and provides an applicable framework for assessing sediment health recovery in tropical coastal ecosystems.