Congratulations to Derek Smith who defended his dissertation on Friday, January 22, 2021
Advisor: Greg Dick
Cyanobacterial harmful algal blooms (CHABs) are global threats to freshwater ecosystems. Species of the harmful cyanobacterium, Microcystis, produce microcystins, liver toxins that sicken animals and people when ingested. Not all Microcystis strains produce microcystins, and the relative proportions of microcystin-producing (toxic) and non-microcystin-producing (nontoxic) strains of Microcystis is an important determinant of toxin concentrations during blooms. However, the ecological factors that determine the relative abundance of Microcystis strains is unknown. Some evidence suggests that microcystins protect cyanobacteria from hydrogen peroxide (H2O2), which is ubiquitous in surface waters and can damage cellular structures. Therefore, H2O2 concentrations may impact the proportions of Microcystis during blooms by favoring toxic strains, but evidence in the current literature conflicts in supporting this hypothesis.
Despite its potential importance, sources and sinks of H2O2 in CHABs are not well characterized. Microorganisms produce catalase and peroxidase enzymes to decompose H2O2 to harmless products and are the dominant sink for H2O2 in surface waters. However, H2O2 degradation capabilities vary widely across microbial taxa. Some microbes are poor degraders, are thus sensitive to H2O2 and rely on other community members for H2O2 decomposition. In addition, microbial H2O2 production is becoming understood as an important source of H2O2 in surface waters. Particularly during CHABs, known chemical sources of H2O2 cannot always explain observed H2O2 concentrations. Thus, impacts from H2O2 depend on community-wide H2O2 production and decomposition. Microcystis is likely impacted by other microorganisms, as CHABs contain diverse communities of co-occurring microbes, some of which physically attach to Microcystis colonies. However, which organisms degrade and produce H2O2 during cyanobacteria blooms and the microbial communities that specifically associate with Microcystis are unknown.
In Chapter 2, microbes that express genes for H2O2 decomposition in western Lake Erie CHABs were identified using metagenomic and metatranscriptomic approaches. Key genes for exogenous H2O2 decomposition were absent in many Microcystis strains and expression of these genes in phytoplankton seston was dominated by attached bacteria, implicating the bacteria as major H2O2 sinks. To investigate how H2O2 decomposition impacts Microcystis growth, several toxic and nontoxic Microcystis strains were cultured with an exogenous H2O2 scavenger. Growth of several toxic and nontoxic strains was unaffected by the H2O2 scavenger, but one toxic strain had improved growth rates with the H2O2 scavenger. This result suggests that H2O2 impacts Microcystis strains differently, but not in a toxic vs. nontoxic manner.
In Chapter 3, Microcystis colony associated microbiomes were characterized with 16S rRNA amplicon sequencing of individual colonies. Microcystis microbiomes lacked universal members, yet some taxa occurred more regularly than others, and colonies with shared 16S oligotype and sampling date had more similar microbiomes.
Chapter 5 characterized biotic H2O2 production and decay rates in western Lake Erie. Biotic H2O2 production was the dominant source of H2O2 on average and increased with chlorophyll a concentration. H2O2 production was related to photosynthesis and microbial community composition, yet H2O2 production and decay were not impacted by Microcystis colonies. Therefore, H2O2 production and decay from other community members may impact Microcystis growth. Overall, this dissertation shows that microbial community-wide processes and interactions play critical roles in H2O2 fluxes and likely impact CHAB development.