Fresh stacks of muddy bacteria

Lilia Garcia, Illinois Wesleyan University

The Approach: In my last post, I wrote about Gracilaria, an invasive red seaweed on the coast of South Carolina, and its effect on Vibrio bacteria. My project aims to record the number and strains, or types, of Vibrio growing around Gracilaria and compare it to seaweed-free areas. I will also compare the Vibrio count residing on Gracilaria versus the Vibrio residing on a native seaweed called Ulva to see how an invasive species changes the bacterial community. Lastly, I want to understand how Gracilaria stops the growth of specific Vibrio strains by producing chemical compounds.

Mud samples under Gracilaria, taken by K. Coates

To begin solving my questions, I will go out to collect samples in the mudflats outside of the Grice Laboratory. I will collect tubes of water, clumps of Gracilaria and Ulva, and mud from underneath and 1.5 feet away from Gracilaria. Afterwards, I’ll spread all the samples onto dishes with nutrients specifically used to grow Vibrio. The bacteria grow in spots called colonies, and I will count each spot to see how much Vibrio there is in each sample. I am looking for a different amount of colonies in mud samples collected within or away from Gracilaria patches, and a difference in colony numbers between the Ulva and Gracilaria.

Dishes of unique Vibrio, taken by L. Garcia

A single dish from a mud sample can contain hundreds of colonies, differing in color, shape, size, and texture. Each of these colonies represent a different strain of Vibrio, uncovering the diversity of bacteria at different distances from Gracilaria. I will characterize which unique colonies are dangerous to human health, and whether they are found near or away from Gracilaria.

Zones of inhibition against Vibrio strain, taken by L. Garcia

As previously mentioned, I will also test Vibrio strains against chemical compounds made on the surface of Gracilaria. These compounds are able to control the kind of bacteria that grow around seaweed, changing the microscopic habitat. I will mix Gracilaria with chemicals to remove its surface chemistry, then spot the compounds onto dishes growing Vibrio from my mud samples. I am looking for large clear circles, called zone of inhibitions, that tell me the specific strain of Vibrio cannot grow due to the compound.

Nearly all we know about the ecological and economic impact of Gracilaria focuses on large animals, such as fish. My project zooms in on micro-organisms that have been overlooked. The information I collect will help us understand how invasive Gracilaria is changing bacterial communities not only in the Charleston Harbor, but potentially the entire coast.  Although invisible, bacteria make up the foundation of ecosystems and high Vibrio levels may be dangerous for our health. I look forward to finding the answers to my questions hiding quietly in the mud.


Thank you to my mentor Dr. Erik Sotka, and our collaborator Dr. Erin Lipp. I would also like to thank Dr. Alan Strand and Kristy Hill-Spanik for their supporting guidance. Lastly, thank you to Dr. Loralyn Cozy (IWU) for preparing me to succeed in the lab. All research is funded by Grice Marine Lab and College of Charleston through the Fort Johnson REU Program, NSF DBI-1757899