Microorganisms for ammonia/ammonium-oxidization and anammox in high arsenic Holocene-Pleistocene aquifers

Abstract

High arsenic (As) groundwaters are generally associated with high concentration of ammonium in Pleistocene-Holocene aquifers. Ammonia/ammonium-oxidizing microorganisms (AOM) play an important role in the nitrogen cycle. To investigate AOM distribution and their diversity in the high As aquifers, 12 samples from deep wells (80–120 m below surface level) and another 12 samples from shallow wells (15–30 m) in the Hetao Plain of Inner Mongolia were taken for this investigation. Anammox bacteria were mainly detected in deep wells while ammonia-oxidizing archaea (AOA) were detected only in shallow wells. Ammonia-oxidizing bacteria (AOB) were detected in both shallow wells and deep wells. Anammox bacteria were diverse, but primarily composed of Candidatus Brocadia, Ca. Kuenenia, Ca. Scalindua, Ca. Jettenia, and unidentified groups. Only two genera were detected in both AOA and AOB, which were dominated by Nitrosopumilus (79%–100%) and Nitrosomonas (83.3%–100%), respectively. The distribution of AOM in high arsenic groundwater was associated with hydrogeological conditions and geochemical factors including oxidation-reduction potential (ORP), dissolved oxygen (DO), total organic carbon (TOC), and concentrations of As, NH4+-N, NO2−-N, and Fe. The qPCR results showed ratios of hzo/hzsB and amoA genes to 16S rRNA genes in shallow and deep wells varied widely (0.12%–3.04%). Positive correlations were observed between hzo/Arch-amoA gene abundances and Fe(II) as well as Fe(II) and As, implying microbial ammonia/ammonium oxidizing might be linked to iron reduction and As release. This work improves our understanding of the nitrogen cycle in high As aquifer systems.