Creative Commons License

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

Date of Graduation


Document Type


Degree Name

Master of Science (MS)


Department of Biology


Morgan Steffen

Louie Wurch

Steven Cresawn


Cyanobacterial harmful algal blooms (cHABs) plague freshwater systems worldwide and are projected to increase in intensity in the coming decades. cHABs damage aquatic ecosystems by blocking light penetration into the water column, creating hypoxic conditions, and releasing toxins. One of the most prolific cHAB formers is the cosmopolitan genus of cyanobacteria Microcystis. Global climate change and anthropogenic loading of nutrients such as nitrogen (N) and phosphorus (P) fuel Microcystis bloom formation. Increasing global temperatures favor Microcystis because of its high optimal growth temperature. N input is of particular importance for Microcystis because it is unable to fix atmospheric N, unlike other co-occurring genera of cyanobacteria such as Dolichospermum. Understanding these abiotic factors is essential for managing bloom formation, but the lesser understood biotic factors, including microbial interactions, may also be critical to bloom formation and success. Microcystis spp. co-occur with a consortium of microorganisms in the microenvironment surrounding the phytoplankton cells, called the phycosphere. This close association with microbes gives way for interactions between Microcystis and the constituents of its phycosphere. The Microcystis phycosphere is rich in nutrients that are produced by its members, and other members benefit from the provision of these nutrients. To better understand bidirectional nutrient exchange between Microcystis and heterotrophic bacteria in the phycosphere, two stable isotope (SIP) experiments were performed to trace the movement of nutrients within two lab strains of M. aeruginosa. DNA-SIP was used with 15N-nitrate and 15N-urea to track the N utilization capabilities of bacteria within the Microcystis cultures and identify bacteria that could use these N sources and those that could not. These results give insight into how N is used in the phycosphere and the potential for N exchange between the phycosphere partners. To trace carbon (C) exchange in the Microcystis phycosphere, 36 heterotrophic bacteria were isolated from a 2018 Microcystis bloom in Lake Tai (Taihu) to be used in an RNA-SIP experiment. This consortium of 36 bacterial isolates promoted the growth of axenic Microcystis aeruginosa NIES 843 during a batch culture experiment. M. aeruginosa NIES 843 was then labeled with 13C to trace C exchange from Microcystis to the Taihu bacterial consortium. Analysis of the transcriptomes from this experiment will identify the bacteria capable of utilizing Microcystis-derived C and give insight to the gene actively being used during this exchange of C. These SIP experiments give direct evidence of bidirectional nutrient exchange in the Microcystis phycosphere and further provide evidence of the important role heterotrophic bacteria play in cHAB formation.



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