Antibiotic resistance genes correlate with metal resistances and accumulate in the deep water layers of the Black Sea.

Seas and oceans are a global reservoir of antibiotic resistance genes (ARGs). Only a few studies investigated the dynamics of ARGs along the water column of the Black Sea, a unique environment, with a peculiar geology, biology and history of anthropogenic pollution. In this study, we analyzed metagenomic data from two sampling campaigns (2013 and 2019) collected across three different sites in the Western Black Sea at depths ranging from 5 to 2000 m. The data were processed to annotate ARGs, metal resistance genes (MRGs) and integron integrase genes. The ARG abundance was significantly higher in the deep water layers and depth was the main driver of beta-diversity both for ARGs and MRGs. Moreover, ARG and MRG abundances strongly correlated (r = 0.95). The integron integrase gene abundances and composition were not influenced by the water depth and did not correlate with ARGs. The analysis of the obtained MAGs showed that some of them harbored intI gene together with several ARGs and MRGs, suggesting the presence of multidrug resistant bacteria and that MRGs and integrons could be involved in the selection of ARGs. These results demonstrate that the Black Sea is not only an important reservoir of ARGs, but also that they accumulate in the deep water layers where co-selection with MRGs could be assumed as a relevant mechanism of their persistence.