Ammonium loading drives bacterial community shifts in biofilms attached to the submerged macrophyte Hydrilla verticillata

The effects of ammonium on biofilm attached to submerged macrophytes remain unclear. We studied the microbial communities of epiphytic biofilms on the leaf surface of Hydrilla verticillata under increasing ammonium loadings using 16S rRNA gene sequencing and RT-PCR. The study showed that the microbial community structures were influenced by increased ammonium loadings. N-cycling bacteria including the nitrifier Nitrospira and the denitrifiers Rhodobacter, Hyphomicrobium, and Flavobacterium were found to be predominant. Copy numbers of Ncycling functional genes (including amoA, nxrA, narG, napA, nirK, nirS, qnorB, and nosZ) were stimulated by increased ammonium loadings, showing the high potential of attached biofilms in removing ammonium-N through coupled nitrification-denitrification. Our results indicated that submerged macrophytes facilitated the ammonium removal process not only by providing favorable surface niches for ammonium oxidizers, nitrifiers, and denitrifiers in the attached biofilms but also by mediating dissolved oxygen concentrations and alkalinity condition in the water.