Strengthening coastal food webs by counting the microbes too
In grade school science classes, we learn about the food chain in simple terms: an insect eats some plants, a frog eats the insect, and then an owl swoops down and snatches the frog. It's usually in one direction, from smallest to biggest, but later we learn that these food chains can be more complex. A snake could also eat the frog, or both could be eaten by the same bird of prey. The food chain then becomes a food web as more connections are added between different plants and animals.
Orissa Moulton, who recently finished her PhD in the Department of Ecology and Evolution at the University of Chicago, studies more subtle aspects of food webs, specifically how chemical resources like nitrogen cycle through coastal marine ecosystems. Nitrogen is a crucial resource for plant growth-for example, seaweed consumes nitrogen excreted by animals like mussels. As Moulton has found out through her research, this nitrogen cycle is an important driver of overall productivity in an ecosystem, shaping the structure of food webs and contributing to its overall diversity.
"We think a lot in terms of the classic food chain arrow diagram, so one organism going to another through predation or parasitism," she said. "Nitrogen has the potential to change the strength of those arrows in a food web. It doesn't have to be animals eating each other, it can just be them competing for the same resource."
Now imagine the trillions of microorganisms that co-exist with the larger animals in the food chain. These bacteria, fungi, viruses, and other microbes play a crucial role in maintaining the balance of any ecosystem, and when you take them into account the food web becomes incredibly complex.
New technology like genome sequencing makes it possible to easily identify different species of microbes and study the roles they play in the ecosystem. In a recent review paper published in the journal Frontiers in Ecology and the Environment, Moulton and her colleagues from several institutions around the country examined how interactions among microbes and larger organisms affect nitrogen cycling in coastal ecosystems, such as San Juan Island in the Pacific Northwest where Moulton conducted her PhD work. Microbes can help other organisms process nitrogen, or consume it on their behalf to help maintain healthy levels.
In the paper, the authors posit several hypotheses for how and why microbes and other organisms associate with each other, whether it's because larger organisms (animals, plants, and algae) select for microbes that can best help them with nitrogen cycling, or because the two provide mutual benefits for each other, enhancing their combined ability to thrive in an environment where nitrogen is often limited. They argue for further study to deepen ecologists' understanding of these associations.
"We already know a lot about this system, but all of a sudden we've opened up new possibilities through these sequencing techniques and quick biogeochemical measurements," said Moulton. "You can really broaden your understanding of a community now."
A deeper understanding of the links in coastal ecosystems, especially the role microbes play, is crucial when human activity is changing them in profound ways. Modern agriculture increases the amount of nitrogen in the water when nitrogen-rich fertilizer runoff washes downstream, and increased fishing alters the balance of animals that consume or produce nitrogen naturally. Moulton says she hopes the baseline data she has collected in a system with relatively low human impact like San Juan Island can help develop tools and models that can be applied in more complex, rapidly changing systems as well.
"My questions are really simple, like what is driving microbial diversity in different places? That's not going be the same answer every place I go," she said. "That's really what drives a lot of ecological curiosity in interactions. It's not just one thing living alone in a lab."
The review paper was the product of a working group held at the Marine Biological Laboratory in Woods Hole, Mass., as part of its ongoing affiliation with UChicago. Moulton says that it shows the possibilities of expanding the scope of research beyond one scientist's lab and collaborating with others.
"That was so exciting for me as a grad student to have that opportunity and broaden the scope of my thesis," she said. "My thesis is this very small thing I did on San Juan Island, but the MBL working group allowed me to be a better communicator of why that's an important question to ask, and work with other really smart, successful people who have been asking similar questions their whole careers."