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Understanding the relationship between bacterial community composition and the morphology of bloom-forming Microcystis

Akins, Leighannah
http://orcid.org/0000-0001-8542-247X
http://rave.ohiolink.edu/etdc/view?acc_num=kent1543502274681124

Abstract Details

2018, PHD, Kent State University, College of Arts and Sciences / Department of Biological Sciences.
This project investigated the effects of cyanobacterial biomass on the community composition of heterotrophic bacteria and the effects of selected isolates from heterotrophic bacterial communities on the morphology of bloom-forming cyanobacteria. First, samples were collected from cyanobacterial blooms in three eutrophic lakes. High-throughput sequencing was used to characterize free-living (FL) bacterial communities and those bacterial communities that were directly associated with cyanobacterial biomass, designated as cyanobacteria-associated (CA) communities. The composition and diversity of FL and CA communities were compared within lakes and across lakes. Though diversity estimates did not vary, composition differed significantly by lake, sampling date, and whether the community was free-living or cyanobacteria-associated. The next stage of the project explored how the morphology of the common bloom-forming cyanobacterium Microcystis aeruginosa could change in response to heterotrophic bacterial neighbors. Bacteria isolated from the CA community during a Microcystis bloom in one of the lakes were co-cultured with predominantly unicellular strains of toxic and non-toxic strains of M. aeruginosa to determine whether isolates could promote colony formation in M. aeruginosa. Isolates which exhibited colony-promoting abilities were selected for further testing to investigate the mechanism by which they could influence Microcystis morphology. Toxic and non-toxic M. aeruginosa were treated with cell-free exudates of bacterial isolates. The morphology, polysaccharide content, and reflectance spectra of M. aeruginosa in each treatment group were compared to those of M. aeruginosa control cultures. Six bacterial isolates belonging to the families Pseudomonadaceae and Bacillaceae enhanced the frequency or size of M. aeruginosa colonies in cultures where a dialysis barrier prevented physical contact between heterotrophic cells and cyanobacterial cells. Toxic and non-toxic strains responded differently to all Bacillaceae isolates and one of the Pseudomonadaceae isolates. This study is the first to show that heterotrophic bacteria can promote colony formation or increase colony size in Microcystis without making physical contact with the Microcystis cells. An additional experiment was conducted to test the morphological responses of Microcystis aeruginosa to AI-2, a cross-species quorum sensing signal produced by a large number and wide taxonomic variety of bacteria that carry the luxS gene. Toxic and non-toxic cultures were treated with three different concentrations of AI-2, and the colony frequency, colony size, and polysaccharide content in different treatment groups were compared to those observed in control cultures. DNA was extracted from colony-promoting bacterial isolates, and luxS sequences were amplified by PCR. Different AI-2 concentrations spanning three orders of magnitude enhanced colony frequency in non-toxic M. aeruginosa within 24 hours. In 48 hours, non-toxic M. aeruginosa treatment and control groups exhibited similar colony frequency, but colony size was greater in cultures that received high initial doses of AI-2. An AI-2 concentration of 0.42 ¿M had a positive effect on colony size in toxic M. aeruginosa, whereas higher and lower concentrations had no effect. Of the bacterial isolates that enhanced colonial morphology of M. aeruginosa in other experiments, luxSwas detected in one. This experiment demonstrated for the first time that AI-2 has strain-specific effects on Microcystis morphology. Some heterotrophic bacteria may influence Microcystis morphology through AI-2 signaling, though this signal is not produced by all colony-promoting bacteria. Past experiments showed that heterotrophic bacteria can promote colonial morphology in Microcystis under conditions of high overall nutrient levels and high N:P ratio. CyanoHABs often occur in N-limited conditions environments, and the effects of nutrient ratios on colonial morphology are poorly understood. The final research chapter in this project examined the effects of colony-promoting bacteria on toxic and non-toxic Microcystis aeruginosa under varying nutrient conditions. Each of two M. aeruginosa strains was cultured under six different nutrient conditions with a colony-promoting bacterium, with a positive control bacterium, and with no bacterial co-culture. Colony frequency, colony size, and polysaccharide content were compared across all treatment combinations. Presence of a bacterial co-culture and N:P ratio both had significant main effects on colony size and colony frequency. Colony size was also affected by the overall level of P and identity of the M. aeruginosa strain. Two-way, three-way, and four-way interactive effects were also significant. Co-cultures with the colony-promoting bacterium had higher colony frequency than negative controls under all nutrient conditions and greater colony size than controls under high P conditions regardless of N:P ratio. Co-cultures with the positive control bacterium had higher colony frequency than negative controls under five of six nutrient conditions. M. aeruginosa strains exhibited strain-specific responses to some combinations. The study demonstrated that varying nutrient conditions modify the effects of colony-promoting bacteria on Microcystis.
Laura Leff, Ph.D. (Advisor)
142 p.
Aquatic Sciences; Biology; Ecology; Environmental Science; Microbiology
Microcystis, cyanobacteria, cyanobacterial colonies, cyanobacterial morphology, microbial interaction, cyanoHAB, microbial interactions, microbial community, AI-2, quorum sensing

Recommended Citations

Citations

  • Akins, L. (2018). Understanding the relationship between bacterial community composition and the morphology of bloom-forming Microcystis [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1543502274681124

    APA Style (7th edition)

  • Akins, Leighannah. Understanding the relationship between bacterial community composition and the morphology of bloom-forming Microcystis. 2018. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1543502274681124.

    MLA Style (8th edition)

  • Akins, Leighannah. "Understanding the relationship between bacterial community composition and the morphology of bloom-forming Microcystis." Doctoral dissertation, Kent State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=kent1543502274681124

    Chicago Manual of Style (17th edition)

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This open access ETD is published by Kent State University and OhioLINK.