Phytoplankton species influence mercury cycling through bioaccumulation and Hg(II) reduction, however their contribution to oxidation of Hg(0) in aquatic systems remains largely overlooked. The present study aims at investigating the oxidation of Hg(0) by two phytoplankton species: the diatom Cyclotella meneghiniana and the green alga Chlamydomonas reinhardtii. The algae were exposed to gaseous Hg(0) at concentrations in the range of 6-12 μg m^-3 , representative for contaminated environments, under various experimental conditions (open vs. closed systems, light vs dark, and alive vs dead cells). The obtained results revealed, for a first time, that Hg(0) oxidation in C. meneghiniana cultures was light-dependent and limited to live cells, whereas C. reinhardtii maintained similar oxidation rates in both live and dead cells. C. reinhardtii cultures exhibited nearly tenfold higher Hg(0) oxidation efficiency than C. meneghiniana, demonstrating a strong species-dependent effect. Both species facilitated Hg(0) uptake from air into water, demonstrating a potential route for atmospheric Hg(0) to enter aquatic food webs. This novel evidence of phytoplankton-mediated Hg(0) oxidation highlights the importance of species identity and environmental conditions in governing Hg transformations and bioavailability. The results could have significant implications for understanding mercury bioaccumulation and toxicity in aquatic ecosystems. Further research is needed to clarify their contribution to Hg(0) oxidation in aquatic systems and to elucidate the underlying mechanisms driving the process.