Contradictory Impacts of Nitrate on the Dissimilatory Arsenate-Respiring Prokaryotes-Induced Reductive Mobilization of Arsenic from Contaminated Sediments: Mechanism Insight from Metagenomic and Functional Analyses

Dissimilatory arsenate-respiring prokaryotes (DARPs) are considered to be a key impetus of the reductive dissolution of solid-phase arsenic. However, little is known about the interaction between nitrate and DARPs so far. In this study, we showed that nitrate either inhibited or promoted the DARP population-catalyzed reductive mobilization of As in sediments. Metagenomic analysis of the microbial communities in the microcosms after seven days of As release assays suggested that microbes mainly consisted of: Type-I DARPs having potential to reduce NO3- into NO2- and Type-II DARPs having potential to reduce NO3- to NH4+. We further isolated two cultivable DARPs, Neobacillus sp. A01 and Paenibacillus sp. A02, which represent Type-I and -II DARPs, respectively. We observed that nitrate suppressed A01-mediated release of As(III) but promoted A02-mediated release of As(III). Furthermore, we demonstrated that this observation was due to the fact that nitrite, the end product of incomplete denitrification by Type-I DARPs, suppressed the arrA gene expression per cell and growth of all DARPs, whereas ammonium, the end product of dissimilatory nitrate reduction to ammonium (DNRA) by Type-II DARPs, enhanced the arrA gene expression per cell and significantly promoted the growth of all DARPs. These findings suggest that the actual effects of nitrate on DARP population-catalyzed reductive mobilization of arsenic, largely depend on the ratio of Type-I to Type-II DARPs in sediments.