Sydney Greenlee: Unlocking Microbial Mysteries in the Gulf of Maine

I’ve always loved the ocean since I was a little kid. Though I grew up in the suburbs of Washington, D.C., my earliest memories are of my parents taking me tidepooling in Northern California and watching the sea otters play at Monterey Bay Aquarium. I’ve always been captivated by the exotic and mysterious forms that marine life takes and the strategies these critters use to survive and thrive in the sea. I had wanted to be a marine scientist from a young age and get paid to ask ‘why?’ all the time, but I hadn’t expected to study microbial ecology, harmful algal blooms, and environmental DNA! I’m currently a 5th year PhD student in biological oceanography, advised by Damian Brady and Pete Countway.

I came to Maine nearly 10 years ago as an undergraduate student at Colby College, where I majored in biology and environmental science. I hadn’t thought about microbial life very much until I participated in the Sea Change semester program at Bigelow Laboratory for Ocean Sciences, where I fell in love with the wonders of algae, environmental DNA (eDNA), and the oceanography of the Gulf of Maine. This is also where I met my PhD co-advisor, SRS Pete Countway, who I’ve worked with since 2018! My undergraduate research with Pete focused on eDNA, and he invited me to apply to the Maine eDNA PhD program. Right after graduating, I was invited on a 6-week oceanographic research cruise in the Southern Ocean with Dr. Barney Balch at Bigelow. The cruise track was from Cape Town, South Africa to 60° South up to Mauritius; studying the ways water masses and eddies affect and transport microbial communities. Participating in global-scale oceanography research led me to pursue oceanography as my PhD major and to think about the way, on both a global and microscopic scale, that the physical, chemical, and biological aspects of the ocean are all connected.

One of the things that consistently amazes me about the ocean is how interconnected everything is. Phytoplankton are single-celled organisms that produce 50% of the world’s oxygen and form the base of the marine food web. Every commercially-harvested marine species relies directly (like oysters or scallops) or indirectly (like lobster or tuna) on phytoplankton for food. Despite their microscopic size (about the width of a human hair, or less!), there are hundreds of different microalgae species in the Gulf of Maine alone with diverse shapes, sizes, and behaviors suited to different seasons and environments. Some of these single-cells can swim, hunt, and produce toxic compounds; and not all algae are nutritionally equal. Microbes are so small and so responsive to change in the ocean that it is important to understand how their communities might shift in response to oceanographic changes in the Gulf of Maine.

But since microbes are so small, they can be tricky to study. From 2020 – 2024, my research was funded by the statewide, multi-institution Maine EPSCoR Environmental DNA (eDNA) project. All organisms shed DNA- their unique genetic signature- into the environment. By taking seawater samples and filtering out scales, hair, or entire microbial cells, you can use the DNA to detect any organism you’re looking for in the sample! Some phytoplankton can produce widespread, toxic bloom events, known as harmful algal blooms or ‘HABs’. These HAB species can look just like non-toxic species of the same genus, so part of my research as a Maine eDNA graduate student was to develop and implement a qPCR technique that can detect as few as 1 cell of the highly toxic diatom Pseudo-nitzschia australis that can produce amnesic shellfish poisoning toxins. Eventually, agencies like the DMR could incorporate this into their existing shellfish monitoring programs to minimize precautionary closures due to look-alike non-toxic diatoms. I also use eDNA to understand the ecology of phytoplankton and bacteria- did you know that they have seasonal turnover and respond to light, temperature, and salinity? Since they’re so small and grow quickly, they can be important indicators for ocean change. With eDNA, we can see if warming waters shift the seasonality of these microbes or if harmful species are favored by certain conditions.

Currently, I am funded by NOAA’s NERACOOS NHABON program to deploy a phytoplankton-detecting device called the Imaging Flow Cytobot (IFCB). The IFCB can image 5 mL of seawater every 20 minutes- an impossible task for a human, given that 5 mL of seawater can contain tens or even hundreds of thousands of phytoplankton cells! This frequency is amazing to have, since the ocean is such a dynamic environment, and phytoplankton communities can change hourly with the height of the tide, or daily in response to light availability. The ability to sample coastal water with such high frequency is a fantastic opportunity to quickly detect harmful species and understand these finer-scale temporal dynamics in phytoplankton communities. After each sample, the IFCB uploads the image set to a dashboard. Currently, the IFCB is set up at the Darling Marine Center’s Marine Culture Lab and is sampling the seawater that is coming into the hatchery, where researchers are culturing oysters, scallops, and more (linked here). This gives us more information on the nutritional quality of what the shellfish are feeding on and serves as a nearly real-time detection method for potentially harmful species! We can then use a machine learning algorithm to classify each image to a phytoplankton species, providing us with very high resolution data on phytoplankton ecology. 

I’m excited to be doing research on phytoplankton and HAB species because of the aquaculture applications of these tools. I’m not originally from Maine, but I’ve lived here for nearly a decade and am really proud to do research that can help my community!