Investigating the Microbial Culprits of Taste/Odor Issues in City of Durham Drinking Water Reservoir Lake Michie and Algicidal Mitigation Tactics

Abstract

Project title: Investigating the Microbial Culprits of Taste/Odor Issues in City of Durham Drinking Water Reservoir Lake Michie and Algicidal Mitigation Tactics. Taste and odor (T&O) compounds, especially geosmin and 2-methylisoborneol (2-MIB) are of concern to water managers worldwide and those serving large cities within the Piedmont region of North Carolina (NC). Project objectives: 1) Identify the taste/odor producing populations in Lake Michie (LM) by microscopy or flow cytometry surveying, genetic data, and/or correlation between hydrologic, biochemical conditions and T&O compound concentrations; 2) Evaluate if flux of T&O compounds from cells can be minimized by reduced dosages of already used algicide EarthTec. Methods: Archived data for LM from City of Durham Water Management (CDWM) were examined to determine longer-term (6 year) trends in biochemical and physical conditions along with T&O compound concentrations, as well as contextualize higher resolution sampling in 2020. LM water was collected in 2020 from 2x the Secchi depth for multiple chemical and biological analyses - including the collection of biomass from two different size fractions (>8μm and 8-0.8μm) for DNA extraction and downstream PCR amplicon or metagenomic sequencing. Samples were also collected for microscopic analysis of plankton diversity and biovolume, as well as flow cytometry-based cell counts (phytoplankton and bacteria). Bottle incubations were used to assess the efficacy of the CuSO4 containing algaecide EarthTec in reducing concentrations of geosmin and 2-MIB. Results: Geosmin reached higher average and maximum concentrations in LM samples in the 6 years of sampling LM and is increasing over time, although datapoints supporting the trend are limited. 2-MIB also exceeded the human detection threshold of 10 ng/L in 5 of 6 years and remains a T&O concern. Geosmin often peaked earlier than 2-MIB in late spring into early summer. Dolichospermum biomass and relative abundance of geoA sequences increased in parallel with geosmin in our intensive 2020 sampling campaign. Metagenome assembled genomes (MAGs) also point to Dolichospermum populations as the dominant geosmin producers, while also uncovering Chloroflexota populations as geosmin sources which has not been previously reported. One putative 2-MIB producing Microcoleus (N-fixing cyanobacterial genus) population was uncovered based on metagenomic sequencing. Conclusions & recommendations: Filamentous N-fixing cyanobacteria (esp. Dolichospermum spp., putative Microcoleus) are key sources of T&O to LM. Previously unrecognized Chloroflexota are sources of geosmin and their contribution of the compound to the greater lake, and other lakes, requires further investigation. Increases in the relative abundance of Dolichospermum geoA sequences paralleled increases in geosmin, thus PCR-based assays hold potential as a rapid geosmin indicator tool. Overall, geosmin concentrations are on an upward trajectory and this is likely to continue with continued increases in average air temperature in the southeastern US and low total N to P ratios in LM that favor N-fixing geosmin producers. Reductions in P may help limit the growth of T&O producing N-fixers, but likely will require substantial efforts and/or financial investment (e.g. dredging). EarthTec treatment of LM water at concentrations previously used by CDWM often caused increases in geosmin (and 2-MIB at times) in our experiments. Lower EarthTec additions (ca. 0.0125 mg/L CuSO4) appear more effective to limit algal growth and T&O concentrations and are recommended.