Bridging marine microbiota ecology and IMO policy: rethinking open-loop scrubber environmental impacts

Laboratory single-species ecotoxicity tests provide early-warning indicators of hazards from shipping open-loop scrubber effluent. However, such tests, conducted under highly controlled, microbiota-free conditions, neglect contaminant interactions with diverse microbial communities (billions of cells L⁻¹) prior to encountering larger marine organisms. Extrapolating results of single-species experiments and their modeling outcomes to marine ecosystems can overestimate ecological risk and can result in policies such as banning open-loop scrubbers. In fact, scrubbers may not only result in reduced SOx emissions but may also lower atmospheric Polycyclic Aromatic Hydrocarbons (PAHs) emissions, especially in port areas, thus contributing to improved air quality. Obviously, such trade-offs between cleaner air and cleaner seas present challenges to policymakers. Mesocosm ecotoxicity experiments show bacterioplankton, phytoplankton, and metazoan plankton communities' resilience to diluted effluent in seawater and the increase of bacterial xenobiotic biodegradation gene machinery. These experiments indicate a fast bacterial biodegradation of abundant PAHs affecting the exposure levels to the rest of marine organisms. Another key factor mitigating PAH impacts in mesocosms is diatom-dominated phytoplankton as their enlarged phycosphere is a microhabitat for PAH-degrading bacteria. Marine microbes can quickly break down pollutants and sustainable environmental risk assessment policies must weigh this natural resilience alongside the air-quality gains from scrubbers.