Native bacteria mitigate diuron and copper toxicity to microalgae isolated from a chronically contaminated coastal lagoon.

In natural environments, microalgae are invariably associated with bacteria, yet the role of these microbial consortia in modulating microalgal physiological responses to chemical stress remains largely unexplored. This study investigates whether native bacterial communities enhance the tolerance of field-isolated microalgae to toxic compounds. Microalgae-bacteria consortia were isolated from the Mediterranean coastal Or Lagoon (South of France), chronically impacted by herbicides and trace metals. The chlorophyte Chlamydomonas sp. Or2023a and the diatom Entomoneis sp. Or2023b were isolated and axenised. Both axenic and xenic cultures were exposed to the herbicide diuron and the trace metal copper in 72-hr dose-response assessments, following the evaluation of the bacterial diversity of their native phycosphere. Both isolates displayed elevated intrinsic tolerance, particularly Entomoneis sp., whose effective concentration inhibiting 50% of growth (Half-maximal Effective Concentration-EC50) values (77 µg L-1 for diuron; 109 µg L-1for free ion copper) exceeded most published benchmarks for marine diatoms, a pattern consistent with possible prior selection or acclimation to chronic chemical exposure. Beyond this baseline, native bacteria further modulated sensitivity in a species- and compound-specific manner. In Chlamydomonas sp., bacteria mitigated diuron toxicity (EC50 from 6 to 11 µg L-1), though at the cost of reduced growth under non-stressed conditions, suggesting a growth-defence trade-off. In Entomoneis sp., bacteria alleviated copper toxicity (∼30% EC50 increase) and induced hormetic growth responses, consistent with copper bioavailability regulation by metal-tolerant Rhodobacteraceae dominating its phycosphere. These findings suggest that microalgal sensitivity to contaminants is jointly shaped by environmental history and native microbial consortia, underscoring the need to incorporate field-isolated algal-bacterial assemblages into ecotoxicological frameworks.