Scientists at the Carnegie Institution's Department of Plant Biology discovered that as the sun goes down, the microbes essentially quit their day job of performing photosynthesis, and instead they begin to fix nitrogen, converting nitrogen gas (N2) into compounds useful for cell growth.

The research is the first to document an organism that can juggle both metabolic tasks within a single cell at high temperatures. It also helps to answer some longstanding questions about how hot-spring microbial communities get essential nitrogen compounds.

The Carnegie team, comprising Arthur Grossman, Devaki Bhaya, and Anne-Soisig Steunou, along with colleagues from four partner institutions, is studying the single-celled organism called cyanobacterium Synechococcus. Cyanobacteria, which evolved about 3 billion years ago, are thought to be the oldest organisms on the planet that can turn solar energy and carbon dioxide into sugars and oxygen via photosynthesis. Ancient cyanobacteria produced most of the oxygen that allows animals to survive on Earth.

Cyanobacteria such as Synechococcus often are found in the microbial mats that carpet hot springs, where life exists at near-boiling temperatures. The mats are highly organized communities where different organisms split up the work, with cyanobacteria serving as the main photosynthetic power plants. Microbial mats in Yellowstone National Park's Octopus Spring contain Synechococcus that can grow in waters up to around 160 degrees Fahrenheit; while other microbes in the hot spring can tolerate temperatures that exceed 175 degrees F. Until now, however, it was unclear which organisms could fix nitrogen � particularly in the hotter regions of the mat.

"The cyanobacteria are true multitaskers within the mat community," Grossman said. "We had assumed that the single-celled cyanobacteria growing at elevated temperatures were specialized for photosynthesis, but it looks like they have a more complicated metabolism than we initially suspected."

Up to now, many scientists had doubted that filamentous cyanobacteria such as Synechococcus were the major N2 fixers in microbial mats, because they specialize in photosynthesis, are not tolerant of extremely high temperatures, and only live at the cooler edges of the mat. This fact has raised questions about whether N2 fixation was critical for organisms in the hotter regions of the mat.

"Synechococcus cannot spatially separate photosynthesis and N2 fixation, as some photosynthetic organisms do," Bhaya explained. "Instead, they solve the problem by temporally separating the tasks."

Lead author Steunou and her collaborators tracked the activity of genes involved in photosynthesis and N2 fixation over a 24-hour period. They found that photosynthetic genes shut down shortly after nightfall, and N2-fixation genes switch on shortly thereafter. The nitrogenase enzyme complex snaps into action at about the same time, following the same pattern as the N2-fixation genes.

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