Impact of Environmental Disturbances on Aquatic Microbial Community Structure and Function

Despite their small share of Earth’s surface, freshwater ecosystems contribute significantly to regional and global carbon budgets. Bacterial communities in these ecosystems play a fundamental role in the the biogeochemical flux and food web dynamics. However, our understanding of how aquatic bacterial communities respond to changes in nutrient regimes and substrate availability arising from perturbations in their environment is limited. My research described herein focuses on investigating the impact of environmental disturbances on bacterial community dynamics in regionally important aquatic ecosystems. First, an evaluation of short-term impacts of stormflow events on the bacterial community of a highly urbanized stream of the Chicago Area Waterways (CAWS) was performed. Using 16S rRNA gene sequencing and metagenomics, assessment of the stream bacterial community composition and functional potential during dry and wet weather demonstrated the influence of increased wastewater treatment plant effluent flow following rain in shifting the stream bacterial community from abundant freshwater taxa to those more associated with anthropogenic settings. Shifts in taxonomic composition were also linked to changes in the functional gene content. Next, I evaluated bacterial community dynamics and carbon metabolism in southern Lake Michigan in light of the of the recent ecological changes in the lake due to the invasive dreissenid mussels. Bacterial community diversity, gene content and gene expression patterns in nearshore and offshore Lake Michigan and community response to a pulse of terrestrially derived DOM (t-DOM) was investigated using metagenomics and metatranscriptomics. Results highlighted overall similarities in the bacterial communities of the oligotrophic offshore and the more productive nearshore, except for differences in the relative abundance of genes/gene transcripts affiliated with Cyanobacteria and the taxonomic composition of DOM transporter gene transcripts. However, both nearshore and offshore bacterial communities responded similarly to the t-DOM pulse, in the form of increased transcription for aromatic compound metabolism. Overall, the results of these studies demonstrate sensitivity of aquatic bacterial community structure and gene content to short-term perturbations as seen in CAWS, but a potential resilience in community function (carbon metabolism) to long-term stressors such as the invasive mussels in Lake Michigan.