Site-specific differences in microbial community structure and function within the rhizosphere and rhizoplane of wetland plants is plant species dependent

Two types of organisms that are vital to the ecosystem services provided by wetlands are plants and soil microorganisms, more specifically, the bacteria associated with the roots of wetland plants. In order to better understand the relationships between wetland plants and their associated microorganisms, we tested the hypothesis that the microbial rhizosphere and rhizoplane communities in wetland systems highly impacted by anthropogenic activities subjected to poor water quality inputs would differ from those communities in areas less impacted by anthropogenic activity receiving higher water quality inputs. To test this hypothesis we compared structural and functional characteristics of microbial communities from water, rhizosphere and rhizoplane microbial communities associated with three wetland plant species (Iris versicolor, Potamogeton natans and Veronica spicata) at two sites along the Grand River (Ontario Canada) experiencing different levels of anthropogenic impact. West Montrose (WM) is a northern site which has historically been classified as having "good" water quality, while Rare (RA) is a site located in a highly urbanized area receiving effluent from a municipal waste water treatment plant (WWTP). Microbial community composition, assessed using PCR-DGGE, differed between sites, community types and plant species, however, the magnitude of variance between sites was dependent on the associated plant species and the community type. Rhizoplane microbial communities associated with I. versicolor, P. natans, and V. spicata shared 8.1%, 19.9% and 23.9% structural similarity between sites. Rhizosphere microbial communities associated with I. versicolor, P. natans, and V. spicata exhibited 33%, 11.2% and 15.6% structural similarity between sampling locations. Functional community characteristics, determined by Biolog (TM) Ecoplate carbon source utilization profiles exhibited by microbial communities from the rhizoplane and rhizosphere, were unique to each site for I. versicolor and P. natans (rhizoplane community only), but not for V. spicata. Assessment of fecal coliforms and fecal indicator microorganisms (Salmonella, Enterococcus and Escherichia coll.) also revealed site-specific differences as well as differences among plant species, most notably the absence of Enterococcus spp. from the rhizosphere of P. natans at both sampling sites. These findings suggest that plant species differ in their ability to recruit root-associated microorganisms, and may be affected differently by variability in environmental conditions, such as site-specific differences in water quality or water chemistry.