The subtleties of nitrogen fixation

Science Daily reports on some suggestive research about one of the underpinnings of all ecosystems: The discovery in the last decade of new suites of microorganisms capable of using various forms of nitrogen -- discoveries that have involved a number of University of Washington researchers -- is one reason to rethink what we know about the nitrogen cycle. So say University of Washington's Claire Horner-Devine, assistant professor of aquatic and fishery sciences, and Adam Martiny of the University of California, Irvine, in a recent Science magazine opinion piece about how new insights into microbial pathways, players and population dynamics are challenging conventional models of the nitrogen cycle.

It's important to understand the environmental controls on nitrogen fixation and the nitrogen cycle so we can assess their likely response to environmental changes such as global warming, ocean acidification and dead zones in the coastal ocean, Horner-Devine says.

Nitrogen in the form of compounds such as ammonia and nitrate is necessary for building amino acids and proteins essential to all life. While nitrogen gas makes up more than three-quarters of the air we breathe, the element is unusable by life in that form. It has to be fixed, that is broken apart and latched onto other chemicals, in order to become user-friendly.

Findings described in the Science piece suggest that the ability to oxidize ammonia and fix nitrogen is more widely distributed among bacteria and archaea than researchers and modelers of the nitrogen cycle have previously taken into account. …

In addition to new pathways and players, it is becoming more apparent that understanding the global nitrogen cycle also will require consideration of ecological dynamics, something Horner-Devine studies. The Science piece cites recently published work in which the balance between the bacteria oxidizing ammonia and those oxidizing nitrite became so destabilized and chaotic that the bacteria oxidizing nitrite were exterminated and nitrification broke down. "Complexity and dynamics at these small scales of organization may require more sophisticated representations of microbial communities," Horner-Devine and Martiny write….

Claire Horner-Devine collects sediment cores in Padilla Bay mudflats to examine communities of microorganisms associated with a native and an invasive eelgrass. Photo by the University of Washington