Ranunculus circinatus Sibth. is a boreal, Eurasian species. The content of heavy metals and the composition of fatty acids in R. circinatum from the Delta of the Selenga River, which is the main tributary of Lake Baikal, was studied for the first time in this paper. Plants of R. circinatum of the Selenga river delta accumulated heavy metals in significant quantities, mostly Fe and Mn, and the content of Pb and Cd was below detection limit. Palmitic C16:0, myristic C14:0 and stearic C18:0 acids were the main saturated fatty acids. The content of unsaturated acids ranged from 53.67 to 58.60%, and they were represented by acids of n3-, n6-, n7- and n9-families. Unsaturated linolenic 18:3 ω3, linoleic 18:2 ω6 and palmitoleic 16:1n7 acids were dominant regardless of the place and year of sampling. The presence of iso-, anteiso- and 2-hydroxy acids was associated with the growth of microbial communities on plants, when the presence of 17:1 n7 and 20:3 n6 acids were associated with animals in vegetation bed of B. circinatum. The obtained data confirmed that plants contribute to the operation of such a powerful natural biofilter as the delta of the Selenga River.