Antibiotic stress–induced hormesis in phytoplankton and bacteria through their mutual cooperation

Phytoplankton-bacteria interactions are critical but often overlooked in assessing the impacts of pollutants on ecosystems. Herein, we used a coculture consisting of the green alga <i>Chlamydomonas reinhardtii</i> and river bacteria to investigate their responses to antibiotic stress. Both partners exhibited hormesis in cocultures but were inhibited in monocultures under exposure to 10 different antibiotics, especially azithromycin (AZM). Notably, mutualistic cooperation between the partners shifted the effect of AZM from inhibition in monocultures to promotion in cocultures. <i>C. reinhardtii</i> alleviates AZM stress on bacteria by providing organic carbon and efficiently removing antibiotics. In turn, the altered phycospheric bacteriome supplied ammonia, phosphate, vitamin B₁₂, and indole-3-acetic acid to promote <i>C. reinhardtii</i> growth. The antibiotic-induced growth promotion was also observed in natural phytoplankton-bacteria communities. Our findings challenge the reliability of ecotoxicity assessment that is typically based on single-species tests, emphasizing the importance of cross-kingdom interactions in assessing pollutant effects.