Microbe-Contaminant Linkages in the Upper Waters of Lakes

Abstract

The upper water column (<1 m depth) of freshwater lakes, which includes the surface microlayer (SML; <1 mm depth), is an important microbial habitat as well as an accumulation and dissemination site for chemical and microbial contaminants. This doctoral thesis reports novel insights into how the physical structure and functional capabilities of microbial communities can influence the presence of trace metals and health-relevant bacteria in the upper waters (SML and 0.5 m depth) of freshwater lakes. Two physically and geochemically contrasting lake environments, a remote sheltered boreal lake and a higher energy urban beach on Lake Ontario, were investigated to identify system-dependent physical and biogeochemical factors controlling contaminant-relevant microbial characteristics. The SML was identified as a major site for generation of contaminant-sequestering suspended flocs from a distinct biofilm-forming microbial community over diurnal timeframes via wind and sunlight exposure, with this process being enhanced at the higher energy beach site. More generally, upper waters including the SML were demonstrated to be inhabited by a diverse group of atypical facultative Fe(III)-reducing bacteria (IRB) that exhibited a SML- and lake-specific capacity for solid Fe(III) reduction directly related to floc and Fe(III) availability. Although IRB were hypothesized to be highly resistant to metals and antibiotics relative to other bacteria due to their ability to dissolve metal-rich Fe(III) minerals, this was not found to be the case. Nevertheless, IRB enriched from the SML demonstrated higher antibiotic resistance compared to those from 0.5 m depth and enriched Fe(III)-reducing communities from both depths harboured resistance-mobilizing genetic elements and included potentially pathogenic bacteria. Results of this thesis represent new knowledge concerning how microbial communities regulate the presence of contaminants in the upper waters of lakes. This has important implications for assessing the ecological and human health impacts of contaminants in freshwater systems.