Nutrient exposure alters microbial composition, structure, and mercury methylating activity in periphyton in a contaminated watershed
The conversion of mercury (Hg) to monomethylmercury (MMHg) is a critical area of concern in global Hg cycling. Periphyton biofilms may harbor significant amounts of MMHg but little is known about the community of Hg methylating microorganisms within them. Therefore, we used amplicon sequencing of the 16S rRNA gene, ITS2 region, and hgcAB gene to characterize the archaea/bacteria, fungi, and methylating microorganisms in periphyton communities grown in a contaminated watershed in East Tennessee. Further, we examined how nutrient amendments (nitrate, phosphate, and nitrate plus phosphate) altered periphyton community structure and function. We found that the richness relative to control treatments of bacteria/archaea decreased in summer and increased in autumn, while fungal diversity generally increased in summer and decreased in autumn. Interestingly, the mercury methylating communities were dominated by Proteobacteria followed by Candidatus Atribacteria across both seasons. Surprisingly, mercury methylation potential correlated with numerous bacterial families that do not contain hgcAB, suggesting that the surrounding microbiomes are important components of periphyton communities and influence rates of mercury transformation within these microbial mats. To further explore these complex community interactions, we performed a microbial network analysis and found the nitrate amended treatment resulted in the highest number of hub taxa that also corresponded with enhanced mercury methylation potential. This work provides insight into community interactions within the periphyton microbiome that may contribute to mercury cycling and will inform future research which will focus on establishing mixed microbial consortia to uncover mechanisms driving shifts in mercury cycling within periphyton habitats.