Gretchen Johnson

School of Biological Sciences

Advisor:

Dr. Julia Kubanek

Georgia Institute of Technology Schools of Biological Sciences, & Chemistry & Biochemistry

Contributions of Trophic and Defensive Symbionts to Stony Corals under Challenge

Open to the Community

Feb. 25th, 2026 

2:00 pm

Ford ES&T, Room 3235 (Ocean Room)

Zoom: https://gatech.zoom.us/j/99930798375?pwd=WJRbeh9hmnyoYbUNHZvPR7RJJWr4yq.1

Meeting ID: 999 3079 8375

Passcode: shovel

Committee Members:

Dr. Lauren Speare, Georgia Institute of Technology School of Biological Sciences

Dr. Neha Garg, Georgia Institute of Technology School of Chemistry & Biochemistry

Dr. Facundo Fernandez, Georgia Institute of Technology School of Chemistry & Biochemistry

Dr. Kostas Konstantinidis, Georgia Institute of Technology Schools of Biological Sciences, & Civil & Environmental Engineering

Dr. Joel Kostka, Georgia Institute of Technology Schools of Biological Sciences, & Earth & Atmospheric Sciences

Abstract:Microbes live in or on all multicellular organisms (hereafter, hosts). Hosts ranging from human beings to corals, which are among the most ancient animal forms that still exist, have evolved symbiotic relationships with microbes that benefit them in a variety of ways: in trophic symbioses, microbes provide nutrition to the host, while in defensive symbioses, microbes protect their hosts from predators or disease. The importance of microbial symbionts to coral survival has been emphasized as their numbers in the wild decline globally. Two leading threats to corals are thermal bleaching and disease, which are attributable to the loss and dysfunction of, respectively, trophic and defensive symbionts. This thesis will first examine the interactions between coral hosts and their trophic symbionts in the wild. A multi-omics field study was conducted to identify what patterns in gene expression and metabolism distinguish bleaching-resistant from bleaching-susceptible partners in normal, non-bleaching conditions. Results to date highlight a link between thermal tolerance and robust energy metabolism. Next, I will investigate defensive symbiosis, which is thought to be important in preventing coral diseases. Conventional methods for assessing the defensive potential of bacteria from coral microbiomes do not take place in conditions resembling the coral microbiome, which limits our understanding of ecologically relevant interactions. I will investigate how culturing conditions and the influence of other microbes may affect the ability of bacteria isolated from disease-resistant coral microbiomes to inhibit invading pathogens. Both studies in this thesis are aimed at better understanding the services trophic and defensive symbionts provide to coral hosts in realistic conditions, which yields insight applicable to coral conservation, but also broader study of microbial symbiosis.