In partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Biology

In the

School of Biological Sciences

Emily Brown

Will defend her dissertation

Cellular mechanisms of ecological interactions

among marine phytoplankton

Thursday, July 22nd, 2021

1:00 PM

https://bluejeans.com/475557684/3861

Meeting ID: 475 576 684

 Thesis Advisor:

Julia Kubanek, Ph.D.

School of Biological Sciences

School of Chemistry and Biochemistry

Georgia Institute of Technology

Committee Members:

Mark Hay, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Joseph Montoya, Ph.D.

School of Biological Sciences

Georgia Institute of Technology

Pamela Peralta-Yahya, Ph.D.

School of Chemistry and Biochemistry

School of Chemical and Biomolecular Engineering

Georgia Institute of Technology

Vinayak Agarwal, Ph.D.

School of Chemistry and Biochemistry

School of Biological Sciences

Georgia Institute of Technology

ABSTRACT: The current body of work investigates the chemical mechanisms involved in predator-prey interactions using species from the chemically defended phytoplankton genus Alexandrium and cues from one type of predatory zooplankton, copepods. To explore the role of dead phytoplankton cues in predation risk assessment and chemical defense plasticity, we exposed A. minutum to chemical cues from six different lysed phytoplankton species. Chemical cues from dead conspecifics and congenerics drastically suppressed A. minutum toxin production, their chemical defense, and modestly enhanced growth regardless of their geographic co-occurrence. In contrast, exposure to cues from distantly related, but historically co-occurring phytoplankton species induced toxin production and decreased growth in A. minutum by roughly the same magnitude. This study revealed that A. minutum perceives cues from dead competitors and that phylogenetic relatedness of the competitor is important in how it trades-off utilization of resources for either growth or defense. We also investigated how A. minutum perceives copepodamides, a suite of copepod metabolites known to induce resistance against predators in diverse phytoplankton taxa, and what metabolic pathways are involved in initiating of A. minutum’s chemical defense. Recognition of copepodamides caused subtle, targeted changes in the metabolome of A. minutum including dysregulation of valine biosynthesis and enhancement of butanoate metabolism and arginine biosynthesis, as determined by nuclear magnetic resonance (NMR) and mass spectrometry (MS) based metabolomics. Additionally, inhibition experiments led to the discovery that copepodamides trigger signal transduction via disruption of serine/threonine phosphatases, which leads to increased jasmonic acid biosynthesis and signaling, and ultimately results in amplified toxin biosynthesis in A. minutum. In addition to understanding how phytoplankton recognize and respond to predators, it is also valuable to understand how predators, such as copepods, select prey. We proposed that when selecting prey copepods may detect chemical cues on phytoplankton cell surfaces that are associated with toxicity rather than directly sensing the intracellular toxins. Using MS and NMR-based metabolomics, we discovered that the non-polar metabolomes of two Alexandrium toxic species vary considerably from their non-toxic congener even though all three are very closely related. Metabolites belonging to seven different lipid classes were implicated in distinguishing the non-polar metabolomes of the Alexandrium species based on toxicity. Ultimately, we partially identified three metabolites which exhibited the greatest enrichment in both toxic species relative to the non-toxic species. Overall, my dissertation research provides insight into mechanisms that mediate ecological interactions involving marine phytoplankton. These include assessment of predation risk, physiological mechanisms behind predator cue recognition and response, and cellular traits that may enable predators to distinguish toxic from non-toxic cells, leading to increased fitness for chemically defended prey. This thesis further demonstrates the complexity of planktonic interactions at both organismal and molecular levels.