Creative Commons License

Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

ORCID 0000-0003-3363-0191

Date of Graduation


Document Type


Degree Name

Master of Science (MS)


Department of Biology


Morgan Steffen


Microcystis aeruginosa is a freshwater cyanobacterial species that degrades freshwater and brackish ecosystems due to its capacity to form toxic cyanobacterial harmful algal blooms (cyanoHABs). There both abiotic and the biotic factors that exacerbate the presence of cyanoHABs. Current research is looking at the microbial interactions that occur between microbes and phytoplankton. Microbes interact with cyanobacteria in the phycosphere, where nutrients are exchanged between the two. Understanding the chemical currencies exchanged can help to show interactions that are beneficial for cyanoHAB formation A key player in the growth promotion of cyanoHABs is hypothesized to be auxins, which are synthesized by bacterial symbionts, though this has yet to be tested. To understand the mechanism behind this interaction, M. aeruginosa-dominated cHABs in Alabama were exposed to both the auxin Indole-3-acetic acid (IAA) and tryptophan, a precursor for bacterial production of IAA. Metatranscriptomes generated across a time series were paired with growth data (i.e., cell counts, Fv/Fm) to determine the impact of IAA on Microcystis cell physiology and the constituents of its phycosphere microbiome. Field experiments were paired with lab experiments using non-axenic M. aeruginosa strains as well as a field experiment within a JMU retention pond. Community and functional analysis showed that bacteria within the Microcystis phycosphere were able to produce genes related to IAA synthesis from tryptophan and metabolism of algal derived carbon. These interactions indicate that there is a bidirectional exchange of nutrients in the phycosphere during the field experiment. One microbe of importance in all of these metabolic processes belongs to the genus Cupriavidus. Differential expression of Microcystis in response to exogenous IAA and tryptophan input showed that there was an increase in expression for metabolic genes responsible for Microcystis growth and cell division at each timepoint compared to the control. A network analysis for each treatment showed that that there was a more complex microbial network for the control compared to the tryptophan and the IAA treatments. This result could potentially be due to the exogenous input of IAA and tryptophan, decreasing the need for microbial-derived nutrients.

Available for download on Friday, April 12, 2024