The uniqueness and biogeochemical cycling of plant root microbial communities in a floating treatment wetland

Floating treatment wetlands (FTWs) are an innovative type of phytoremediation technique being used to reduce the impact of excess nutrient loading. Plants hydroponically grown on FTWs take up nutrients from water through their roots. In general microbial communities in the rhizosphere are important for healthy growth and nutrient uptake by plants. Despite most of previous studies focused on the nutrient removal processes, very little is known about microbial communities associated with FTW plant roots. The purpose of this study was to characterize the microbiomes revolving around the submerged roots of FTW in a manmade stormwater pond and to elucidate the source of FTW plant root microbiomes. The microbial communities collected from the plant roots Canna flaccida (golden canna) and Juncus effusus (soft rush), biofilms of plant pot (polyethylene) and floating mat foam (closed-cell urethane), and surrounding water were studied using 16S rRNA gene amplicon sequencing. The FTW plant root microbiomes were dominated by Alphaproteobacteria and Cyanobacteria at the class level, and Anabaena, Rhizobium and Rhodobacter at the genus level. Microbial communities of the FTW plant roots showed unique compositions resembling most closely the surrounding water samples while being quite different from the biofilm samples, leading the conclusion that the major source of microbial populations was the surrounding water. However, the dominance of Rhizobium species was only observed in the two plant roots and not recognized in the surrounding water samples, indicating that the FTW roots may selectively shape root microbiomes. Unexpectedly, quite a few groups of microbes were associated with the sulfur cycle. This finding indicates that the oxic-anoxic gradient is formed in the FTW rhizosphere, and this environmental gradient assists to extend the phylogenetic and functional diversities of microorganisms. We anticipate the presence of intrinsic rhizosphere microbiomes and the importance of complex biogeochemical processes that include carbon, sulfur and nitrogen driven by physical activity and chemical releases of FTW plant roots. (C) 2017 Elsevier B.V. All rights reserved.