The Impact of Microbial Interactions and Hydrogen Peroxide on Western Lake Erie Cyanobacterial Blooms

Abstract

Cyanobacterial harmful algal blooms (CHABs) are threats to freshwaters globally. Some strains of the harmful cyanobacterium, Microcystis, produce microcystins, toxins that sicken animals and people. The proportions of microcystin-producing (toxic) and non-microcystin-producing (nontoxic) strains of Microcystis are an important determinant of bloom microcystin concentrations. Microcystins may protect cyanobacteria from hydrogen peroxide (H2O2), which is ubiquitous in surface waters and can stress microbes. Therefore, H2O2 concentrations may impact the proportions of toxic and nontoxic Microcystis during blooms by favoring toxic strains, but the current literature conflicts in supporting this hypothesis. Sources and sinks of H2O2 during CHABs are not well characterized. Some microorganisms produce enzymes to decompose H2O2, and microbial decomposition is the dominant sink for H2O2 in surface waters. However, H2O2 decomposition varies across microbial taxa. Some microbes lack enzymes for H2O2 decomposition and rely on other community members for H2O2 decomposition. In addition, microbial production is an important source of H2O2 in surface waters and may be greater than known chemical sources of H2O2. Thus, impacts from H2O2 depend on community wide H2O2 production. Microorganisms likely affect Microcystis growth, as CHABs contain diverse communities of microbes, some of which physically attach to Microcystis colonies. However, which organisms degrade and produce H2O2 during CHABs and the microbial communities that specifically associate with Microcystis are unknown. To address the above knowledge gaps, a combination of cultivation experiments, field incubations, and cultivation-independent approaches were used. Microcystis strains were cultured with an exogenous H2O2 scavenger to investigate how H2O2 impacts Microcystis growth. Several toxic and nontoxic strains were unaffected, but one toxic strain had improved growth rates with the H2O2 scavenger, which suggests that microcystin production alone does not determine the impact of H2O2 exposure on Microcystis strains. The microbes that contain and express genes for H2O2 decomposition during western Lake Erie CHABs were identified using multi-omics approaches. Key genes for H2O2 decomposition were absent in many Microcystis strains, and the expression of these genes in phytoplankton seston was dominated by particle-attached bacteria, implicating the bacteria as major H2O2 sinks. These results suggest that bacterial decomposition of H2O2 affects growth rates of some Microcystis strains. To characterize the importance of colony-attached bacteria for H2O2 dynamics in CHABs, H2O2 production and decay rates in western Lake Erie were measured with and without filtering out all microbes or phytoplankton aggregates >105 μm diameter. Biotic H2O2 production was the dominant source of H2O2 on average and was related to photosynthesis and microbial community composition. H2O2 production and decay were not affected by Microcystis colonies, implicating other free-living microbes as main sources and sinks. Colony-attached bacteria may protect Microcystis when H2O2 production outpaces decay in free-living communities. Bacterial communities associated with Microcystis were characterized with 16S rRNA amplicon sequencing of individual colonies. Microcystis microbiomes lacked universal members, yet colonies with shared Microcystis oligotype and sampling date had more similar microbiomes. Therefore, H2O2 decomposition may vary across strains and colonies over time. Genomes of bacteria identified as key catalase producers were further analyzed to investigate potential metabolic interactions with phytoplankton. Evidence for uptake of vitamins, peptides, and algal exudates suggests that the bacteria use organic nitrogen and carbon in phytoplankton exudates for growth. Furthermore, use of oligopeptide exudates may be associated with efflux and deamination of amino acids by the bacteria, which perhaps regenerates nitrogen for phytoplankton growth.