By Erica Goldman

Nitrogen. In the Chesapeake, it has become the element we love to hate. But this leading cause of the Chesapeake's problems also serves as an essential nutrient for the growth of the plants and algae that form the base of Bay's food web.

In a delicate balancing act, different chemical processes enable nitrogen in the Bay to change form as it moves through the ecosystem. Understanding the nature of these transformations can help clarify how and why too much nitrogen can become a problem. Measuring different forms of nitrogen in the environment has provided key information about when, where, and how fast these different processes occur.

Without microbes, key chemical transformations of nitrogen would not be possible. Different communities of bacteria shepherd different stages of biochemical processes, using nitrogen to meet their metabolic needs. Of these bugs, of their ecological habits and their genetic makeup, scientists still know very little.

Making connections across scales of observation — between genetic function, microbial ecology, nutrient transformation — clearly requires expertise from multiple disciplines.

The time for such an effort seemed right in 1998, when molecular biologists Bess Ward from Princeton University, Mary Voytek from the U.S. Geological Survey in Reston, Virginia, and Jon Zehr at the University of California Santa Cruz, approached sediment biogeochemist Jeff Cornwell and physiological ecologists Pat Glibert and Todd Kana at Horn Point Laboratory, part of the University of Maryland Center for Environmental Science. The ambitious "Biocomplexity of aquatic microbial systems" project was born.

The goal: to use molecular tools to assess the diversity of microbes associated with the Bay's nitrogen cycle. The team would target the genetic makeup of these bugs across a broad geographic gradient, from the Bay's Choptank River all the way to the Sargasso Sea in the open Atlantic. That gradient moves from waters with too much nitrogen to waters with very little. Along the way researchers would ask questions about the relationship between complexity in microbial communities and the physical and chemical factors in the environment around them.

Funding for this effort came from the National Science Foundation's Biocomplexity in the Environment program in 2000. This funding initiative, launched by Rita Colwell at the beginning of her tenure as NSF director in 1998, was designed specifically to bring together scientists from different disciplines to address complex environmental problems. Armed with new molecular tools and a new framework, the team set out to make cross-scale connections about the role that microbes play in the Chesapeake's nitrogen cycle. It was not always smooth sailing. The molecular biologists and the estuarine ecologists had to learn each other's languages — at times a real communication challenge. And the original molecular approach proposed to evaluate microbial diversity, using gene chips (also called DNA microarrays), turned out to be less of a "wonder technique" than expected for this application, says Voytek.

But some tantalizing results have begun to emerge from the project, which is just wrapping up this year. For example, the team found across the board that the genetic diversity of the microbes involved in the nitrogen cycle was much higher than expected. "We were really stunned at the microdiversity," says Voytek.

That high genetic diversity showed up in the gene responsible for nitrogen fixation, she says. Nitrogen fixation, which takes nitrogen gas from the environment, is a metabolically expensive way for bacteria to get nitrogen, she explains. Nitrification, which uses nitrogen in the form of ammonia, and denitrification, which uses it in the form of nitrate, are less costly. If these other forms of nitrogen are readily available, which they almost always are in the Bay, any process besides nitrogen fixation would be cheaper from an energetic standpoint. "We hypothesized that we would only find the ability to fix nitrogen in environments that were poor in nitrogen, not in places like the nitrogen-rich Choptank River," says Voytek. The idea that microbes were "efficient and frugal" with their genetic material — that if they didn't need to perform a process, they would lose the ability to do it — proved wrong in this case.

Another surprise came from the nitrifying bacteria, says Voytek. The only thing these bugs do is oxidize ammonia to make nitrate, she says. One would expect to find high diversity in the Choptank River, which did prove to be the case, because ammonia levels fluctuate dramatically due to fertilizer input. But genetic diversity also measured quite high in the Sargasso Sea, where the environment is stable with respect to ammonia concentration.

From the other side of the equation, some interesting patterns are also beginning to materialize, says researcher Jeff Cornwell. When we measure high rates of denitrification in the sediment, we almost always find a rich and diverse community of microbes, he says. These are the early stages of trying to connect "who is out there to what is going on."

This synthesis of molecular and biogeochemical techniques applied to study microbes in their environment is still in its infancy, agrees Voytek. "I can't tell you yet how to make the Bay healthier from what we've done." Right now, she says, more traditional approaches such as measuring rates of sediment erosion will tell managers more about the potential success of restoration efforts than evaluating the structure of microbial communities. But understanding how microbes function in the ecosystem to modulate the Bay's water quality could prove important in the future, she says, especially as we anticipate changes in the Bay's water cycle in response to climate change. This approach to biological complexity, Voytek says, linking microbial genetic diversity to ecosystem function, is definitely where research needs to go.

Design by Sara Raimo, National Science Foundation

What does biocomplexity as a concept really mean? According to researcher Todd Kana, a scientist at Horn Point Laboratory, it means that there's something about a biological system, like the ecology of the Bay, which cannot be explained by a simple sum of its parts. "It's what happens when you add biology with its unpredictable nature to a physical system," he says.

And the complexity part comes in, adds researcher Pat Glibert, in the range from molecules, to elemental cycling, to the shape of an ecosystem's structure. "It is a scaling issue — from molecular to community-level scaling," she says. Shown at left is a part of the image developed to provide a graphic identity for the biocomplexity initiative at NSF launched by Rita Colwell.

Is biocomplexity something new? "In a sense, we have always studied biological interactions and questions of scale," says Kana. "But understanding how biology drives the complexity of a system is something new."

This page was last modified September 15, 2018