Contrasting arsenic biogeochemical cycling in two Moroccan alkaline pit lakes

Pit lakes resulting from the flooding of abandoned mines represent a valuable freshwater reserve. However, water contamination by toxic elements, including arsenic, compromises their use for freshwater supply. For a better management of these reserves, our aim was to gain insight into arsenic cycling in two Moroccan alkaline pit lakes. We first showed that dimethylarsenic dominated in stratified lake ZA whereas in lake ZL1, As(V) was prevailing. Because microbially mediated processes largely contribute to arsenic cycling, the diversity of arsenic-methylating and -oxidizing bacteria was determined through the sequencing of arsM and aioA genes. Diverse arsM-carrying bacteria were thriving in ZA while a low diversity of aioA genes was detected in ZL1. We also determined the structure of the total bacterial communities by fingerprinting (ARISA). Contrasting arsenic speciation and bacterial communities in the two lakes were associated with differences of conductivity, Total Organic Carbon and temperature. In ZA, dissolved oxygen and redox potential were the main factors driving the total bacterial community structure and the ArsM diversity. In ZL1, stable bacterial communities were associated with limited water physico-chemistry variations. Our study provides new insights into the biogeochemical behavior of arsenic and the role of arsenic transforming bacteria in alkaline pit lakes.