A marine virus controls the green algae Ulva lactuca proliferation in the bay of Marseille

Background. Ulva lactuca, a green macro algae, is a worldwide problem due to green tides observed in Western Europe (particularly in Brittany, west of France) and in Eastern China. It is a nitrophilic algae and surprisingly Ulva proliferation is not observed in the bay of Marseille (Provence, south of France) despite abundant sources of nitrate due to the Rhône River. Methods and Results. In this study we analyzed possible reasons why Ulva proliferation stops rapidly at the end of winter in the bay of Marseille. We show that nitrate concentration at Marseille is an average of 6 + 2 µM and this concentration is similar to what is observed in Brittany. Ulva collected in Brittany can grow and change its phenotype with sea water samples collected in Provence in winter but Breton Ulva become white (bleaching) with sea water samples collected in Provence in June 2018, 2019 and 2020. Breton Ulva bleaching is still observed after 0.2 µm sea water filtration and DAD HPLC shows a peak with a viral particle UV spectrum increasing with Ulva bleaching. Furthermore, epifluorescence microscopy analysis of SYBR-Gold pigment stain with 0.2 µm filtered sea water shows that virus-like particles increase to 6.5 million per ml with Ulva bleaching. Conclusion. This study suggests that a virus in bay of Marseille sea water induces Ulva bleaching and controls its proliferation. Sea water containing this virus, which is not harmful for mammals or other species including algae that are not Ulva, may be used to control Ulva proliferation.