Catherine Polik’s research while affiliated with University of Michigan and other places

The thawing of ancient organic carbon stored in arctic permafrost soils, and its oxidation to carbon dioxide (CO2, a greenhouse gas), is predicted to amplify global warming. However, the extent to which organic carbon in thawing permafrost soils will be released as CO2 is uncertain. A critical unknown is the extent to which dissolved organic carbon (DOC) from thawing permafrost soils is respired to CO2 by microbes upon export of freshly thawed DOC to both dark bottom waters and sunlit surface waters. In this study, we quantified the radiocarbon age and ¹³C composition of CO2 produced by microbial respiration of DOC that was leached from permafrost soils and either kept in the dark or exposed to ultraviolet and visible wavelengths of light. We show that permafrost DOC most labile to microbial respiration was as old or older (ages 4,000–11,000 a BP) and more ¹³C‐depleted than the bulk DOC in both dark and light‐exposed treatments, likely indicating respiration of old, ¹³C‐depleted lignin and lipid fractions of the permafrost DOC pool. Light exposure either increased, decreased, or had no effect on the magnitude of microbial respiration of old permafrost DOC relative to respiration in the dark, depending on both the extent of DOC oxidation during exposure to light and the wavelength of light. Together, these findings suggest that photochemical changes affecting the lability of permafrost DOC during sunlight exposure are an important control on the magnitude of microbial respiration of permafrost DOC in arctic surface waters.

Cyanobacterial harmful algal blooms (cyanoHABs) dominated by Microcystis spp. have significant public health and economic implications in freshwater bodies around the world. These blooms are capable of producing a variety of cyanotoxins, including microcystins, that affect fishing and tourism industries, human and environmental health, and access to drinking water. In this study, we isolated and sequenced the genomes of 21 primarily unialgal Microcystis cultures collected from western Lake Erie between 2017 and 2019. While some cultures isolated in different years have a high degree of genetic similarity (genomic Average Nucleotide Identity >99%), genomic data show that these cultures also represent much of the breadth of known Microcystis diversity in natural populations. Only five isolates contained all the genes required for microcystin biosynthesis while two isolates contained a previously described partial mcy operon. Microcystin production within cultures was also assessed using Enzyme-Linked Immunosorbent Assay (ELISA) and supported genomic results with high concentrations (up to 900 μg L⁻¹) in cultures with complete mcy operons and no or low toxin detected otherwise. These xenic cultures also contained a substantial diversity of bacteria associated with Microcystis, which has become increasingly recognized as an essential component of cyanoHAB community dynamics. These results highlight the genomic diversity among Microcystis strains and associated bacteria in Lake Erie, and their potential impacts on bloom development, toxin production, and toxin degradation. This culture collection significantly increases the availability of environmentally relevant Microcystis strains from temperate North America.

Cyanobacteria harmful algal blooms (cyanoHABs) dominated by Microcystis spp. have significant public health and economic implications in freshwater bodies around the world. These blooms are capable of producing a variety of cyanotoxins, including microcystins, that affect fishing and tourism industries, human and environmental health, and access to drinking water. In this study, we isolated and sequenced the genomes of 21 unialgal Microcystis cultures collected from western Lake Erie between 2017-2019. While some cultures isolated in different years have a high degree of genetic similarity (Average Nucleotide Identity >99%), genomic data shows that these cultures also represent much of the breadth of known Microcystis diversity in natural populations. Only 5 isolates contained all the genes required for microcystin synthesis while 2 isolates contained a previously described partial mcy operon. Microcystin production within cultures was also assessed using Enzyme-Linked Immunosorbent Assay (ELISA) and supported genomic results with high concentrations (up to 900 μg L-1) in cultures with complete mcy operons and no or low toxin detected otherwise. These xenic cultures also contained a substantial diversity of bacteria associated with Microcystis , which has become increasingly recognized as an essential component of cyanoHAB community dynamics. These results highlight the genomic diversity among Microcystis strains and associated bacteria in Lake Erie, and their potential impacts on bloom development, toxin production, and toxin degradation. This collection significantly increases the availability of environmentally relevant Microcystis strains from temperate North America, which is changing rapidly due to climate change. Highlights Twenty one xenic Microcystis cultures were isolated from western Lake Erie and capture the diversity of Microcystis strains observed in natural populations as well as their associated bacteria Microcystis strains show variability in core and accessory gene content, and genetically similar strains produce varying concentrations and congeners of microcystins This collection is a valuable resource for studying strain diversity and interactions between Microcystis and associated bacteria Our collection increases the availability of environmentally relevant strains from temperate North America, which is historically underrepresented in culture collections.

In Lake Erie, toxin-forming harmful algal blooms (HABs) occur following high concentrations of hydrogen peroxide (H2O2). Correlation between H2O2 concentrations and HABs revealed knowledge gaps on the controls of H2O2 production in Lake Erie. One way H2O2 is produced is upon absorption of sunlight by the chromophoric fraction of dissolved organic matter (CDOM). Rates of this photochemical production of H2O2 may increase in proportion to the apparent quantum yield of H2O2 (ΦH2O2,λ) from CDOM. However, the ΦH2O2,λ for H2O2 production from CDOM remains too poorly constrained to predict the magnitude and range of photochemically produced H2O2, particularly in freshwaters like Lake Erie. To address this knowledge gap, the ΦH2O2,λ was measured approximately biweekly from June-September 2019 in the western basin of Lake Erie along with supporting analyses (e.g., CDOM concentration and composition). The average ΦH2O2,λ in Lake Erie was within previously reported ranges. However, the ΦH2O2,λ varied 5-fold in space and time. The highest ΦH2O2,λ was observed in the Maumee River, a tributary of Lake Erie. In nearshore waters of Lake Erie, the ΦH2O2,λ decreased about five-fold from June through September. Integration of the controls of photochemical production of H2O2 in Lake Erie show that the variability in rates of photochemical H2O2 production was predominantly due to the ΦH2O2,λ. In offshore waters, CDOM concentration also strongly influenced photochemical H2O2 production. Together, the results confirm prior work suggesting that photochemical production of H2O2 contributes but likely cannot account for all the H2O2 associated with HABs in Lake Erie.