Microbial influnece on stream nutrient dynamic
The role of sediment microbial communities in regulating the flux of nutrients in aquatic ecosystems has been increasingly recognised. However, in the Great Lakes, where nutrient mitigation focuses on harmful algal blooms, there are limited studies examining the fundamental role of water/sediment microbes in nutrient biogeochemical cycling. Little is understood in this regard considering the increase in anthropogenic pressure on in-stream biological processes impacting nutrient flux to lakes. In this study, metabarcoding and metatranscriptomics approaches were used to investigate the microbial community and gene regulation. The study focused on nitrogen (N) metabolism in a nutrient-polluted watershed of Lake Erie in southwestern Ontario, Canada. Nutrients and microbial analyses of water and sediments were collected in 2020 and 2021 from Sturgeon Creek headwaters to the nearshore of Lake Erie. Results showed no significant shifts in community structure with nutrient concentrations or land use. Metabolically, active genes involved in denitrification (consisting of 32-53% of N metabolic transcripts) show the highest expression within agricultural and wetland dominant locations. Based on active gene expression patterns, the urbanised location coinciding with peak nitrate (NO3-N) concentrations showed the greatest potential for nitrous oxide (N2O) emission along this transect. In contrast to denitrification, direct nitrification potential (5-21% of N metabolic transcripts) increases two-fold approaching downstream and nearshore lake locations. Across this river-lake corridor, expression of key functional genes associated with N transformation shows strong correlation with the change in concentrations of aqueous NO3-N and nitrite (NO2-N) and the ratio of NO2/ NO3. Our findings demonstrate a clear link between sediment microbial metabolism and overlying water chemistry in this lotic system. We suggest that future studies assessing nutrient mitigation consider sediment biogeochemical processes and their fundamental role of N-metabolising bacteria in association with the nutrient and hydrological dynamics of overlying waters.