Comparison of microbial communities inside and outside of a denitrification hotspot in confined groundwater

A denitrification hotspot was detected in Kumamoto groundwater in a previous study, little information is concerned with the microbial community profile including denitrifier. Seven samples from inside and outside of the hotspot were collected to analyze the bacterial, archaeal, and denitrifier community using a cloning library approach. The results showed that the microbial diversity and distribution were distinct among the zones. Major operational taxonomic units closely related to Proteobacteria, Firmicutes, and Actinobacteria were found in the denitrification hotspot. Additionally, type I methylotrophic bacteria, such as Methylobacter and Methylomonas, only inhabited the hotspot zone. A positive association between methanogenic archaea and methylotrophic bacteria was observed. Interestingly, acidic ammonia-oxidizing archaea were detected in the neutral groundwater environment. In addition, gene-specific analysis targeting nirS indicated that the majority of denitrifiers belonged to the Proteobacteria including Sulfuritalea hydrogenivorans, Pseudomonas balearica and Sulfuricella denitrificans. Based on the comparative analysis of microbial communities and physiochemical parameters inside and outside of the denitrification hotspot, the anaerobic environment with organic compounds and nitrate could support the biotic reduction of nitrate in confined groundwater.