The influence of water quality on wetland-associated microbial communities

Within a wetland environment, bacteria in association with plant roots play a vital role in maintaining the health of freshwater ecosystems. In order to gain insight into the stability and processes occurring within natural and constructed wetland environments we need to develop a better understanding of the relationship between wetland plants, root-associated microbial communities and environmental factors. Human population growth and urbanization have resulted in greater contaminant loads (inorganic nutrients, fecal contamination etc.) entering our waterways. As such, we need a better understanding of how anthropogenic impacts influence the structure and function of the wetland-associated microbial communities that we rely on to maintain the integrity of our freshwater ecosystems. To meet this need we designed a series of experiments to investigate the hypothesis that wetland-associated microbial communities highly impacted by anthropogenic activities subjected to poor water quality inputs (high inorganic nutrient load) would differ from less impacted communities in terms of community structure, function, remedial capabilities and resilience. Furthermore, we hypothesized that plant species would play a role in how the associated microbial community would respond to these differences in water quality. To investigate these hypotheses we used a multi-faceted approach involving both in situ field-based studies (Grand River, ON) and ex situ lab-scale wetland mesocosm studies. We examined microbial communities in association with several different plant species (Phalaris arundinacea, Iris versicolor, Potamogeton natans and Veronica spicata) across field sampling locations (Grand River, ON). Lab-scale mesocosm studies involved sub-surface flow wetland mesocosms planted with either P. arundinacea or V. anagallis-aquatica receiving water from sites with contrasting water quality charactersitics. To ascertain the ability of the microbial communities associated with these mesocosm treatments to resist environmental perturbations, mesocosms were exposed to 5mg/L of inorganic phosphorus to simulate runoff from a rain event. We used PCR in combination with denaturing gradient gel electrophoresis (DGGE) to examine the structure of microbial communities in association with wetland plant roots and water-associated communities. Functional community characteristics were examined by obtaining community-level carbon source utilization patterns with BiologTM EcoPlates. We examined the influence of water quality and plant species on fecal contamination associated microbial pathogens by enumerating fecal coliforms as well as Salmonella spp., Escherichia coli and Enterococcus spp. specifically, from water and root-associated microbial communities using the membrane fecal coliform method and quantitative real-time PCR. The remediation potential of ex situ mesocosm-based microbial communities experiencing different water quality treatments in association with our study plant species were determined by quantifying inorganic nitrogen and phosphorus concentrations from mesocosm outflow water. From our field-based studies we found that the structure and function of microbial communities in association with wetland plant roots was affected by sampling location, however this effect was dependent on the plant species in question as well as the root-associated community type (rhizoplane or rhizosphere). Furthermore, plant species differed in their retention of microbial DNA from fecal contamination associated microorganisms. Our ex situ mesocosm-based wetland studies yielded comparable results. We found that the root-associated microbial communities from P. arundinacea and V. anagallis-aquatica were altered structurally and functionally by the different water quality treatments. However, functional characteristics of P. arundinacea-associated communities were affected by water quality treatment to a greater extent than those communities associated with V. anagallis-aquatica. Furthermore, the influence of water quality treatment on microbial community structure and function differed by community type. Rhizoplane-associated microbial communities exhibited the most dramatic structural and functional changes when challenged with varying water quality treatments. Exposure to short-term phosphorus loading as 5 mg/L of inorganic phosphorus resulted in changes to microbial community structure and function in both plant species-associated microbial communities, most notably within the rhizoplane. Structural and functional community diversity was reduced following the inorganic phosphorus treatment for rhizoplane-associated microbial communities. Both mesocosm-based wetland communities performed equally well at removing inorganic nutrient loads from the various water quality sources.

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