Pollution-induced tolerance to copper (Cu) in microorganisms under oxic and hypoxic conditions in Baltic Sea sediments: Sub-lethal sensitivity thresholds measured on community structure and gene expression responses

Microbial communities in coastal sediments sustain key biogeochemical processes but face growing pressures from metal contamination and oxygen depletion. In this study, we derived the quantitative thresholds for effects of sediment pollutants on microbial diversity and their functions using the pollution-induced community tolerance (PICT) concept. Ecotoxicological EC10 thresholds for sediment microbiomes were calculated based on both biodiversity loss and functional gene expression measured through metatranscriptomics. A 10-day mesocosm experiment was conducted with sediments from a historically Cu-contaminated harbour (Oskarshamn) and a pristine reference site (Askö) in the Baltic Sea, exposed to five Cu doses (50-700 mg L⁻¹) under oxic and hypoxic conditions. Communities from Oskarshamn showed consistently higher EC10 threshold values for both diversity and Cu resistance/efflux gene expression, indicating functional pre-adaptation to long-term contamination. Under hypoxic condition, gene-based EC10 values were up to five times higher than under oxic conditions. Random Forest modelling further identified porewater Cu and dissolved oxygen as dominant predictors of Cu partitioning and microbial sensitivity. This study establishes, for the first time, a microbial species sensitvity distribution (SSD) framework linking geochemical drivers with molecular and community responses to derive site-specific EC10 thresholds for improved sediment risk assessment.