It has been experimentally shown that the green algae Acrosiphonia arcta, habitats the littoral of the Barents Sea, remains viable for 10 days in case of explosion of crude oil (concentration of 5 mg/l in the marine water). This concentration corresponds to a weak oil spill in marine water. Morphological and functional changes in the symbiotic association of acrosiphonia and epiphytic bacteria on its surface were traced by the methods of microbiology, light and electron microscopy, and physiology. The preservation of a high level of photosynthesis, intact ultrastructure in most algae cells till the end of the experiment was determined. At the same time, by the end of the experiment, under the influence of oil in the cells of algae, the proportion of chloroplasts decreased, the pyrenoid and starch granules disappeared. The dynamics of the number of epiphytic bacteria in the experiment, the proportion of hydrocarbon-oxidizing bacteria in their number (number) was traced. The ability of acrosiphonia to absorb and transform oil products has been demonstrated. The ability of acrosiphonia to develop in oil-polluted waters on any substrate, preparing it for colonization by larger perennial macrophyte algae, determines the importance of A. arcta in the restoration of coastal phytocenoses.