Cyanobacteria living in hot springs flip a metabolic switch at night, study finds

Organism in Yellowstone quits photosynthesis after sundown and begins to fix nitrogen

Courtesy: Carnegie Institution of Washington yellowstone spring

The near-boiling pools of Octopus Spring in Yellowstone National Park are ringed with microbial mats, highly organized communities where photosynthetic cyanobacteria serve as the main power plants.

Courtesy: Carnegie Institution of Washington yellowstone

Scientists Arthur Grossman (left) and Devaki Bhaya study cyanobacteria that inhabit hot springs in Yellowstone National Park. Grossman holds a courtesy appointment at Stanford.

An international team of scientists has found that photosynthetic cyanobacteria living in scalding hot springs in Yellowstone National Park have two radically different metabolic identities. As the sun goes down, the bacteria quit their day job of photosynthesis and unexpectedly begin to fix nitrogen, converting atmospheric nitrogen gas (N2) into nitrogenous compounds that are useful for cell growth.

The study, led by researchers from Stanford and the Carnegie Institution's Department of Plant Biology, is the first to identify an organism that can juggle both metabolic tasks within a single cell at high temperatures. This discovery helps answer longstanding questions about how hot spring microbial communities get nitrogen compounds, which are essential for life.

"The cyanobacteria are true multitaskers within the mat community," said Arthur Grossman, professor, by courtesy, of biological sciences at Stanford and staff scientist at Carnegie. Grossman co-authored the Yellowstone study, which was published in the Jan. 30 early online edition of the Proceedings of the National Academy of Sciences (PNAS). "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."

All organisms require nitrogen for making proteins and nucleic acids, but N2 gas from the atmosphere cannot be directly used for this purpose. It must first be reduced ("fixed") by microbes into larger, carbon-containing compounds. Cyanobacteria evolved about 3 billion years ago and are the oldest microorganisms on the planet that can turn solar energy and carbon dioxide into sugars and oxygen via photosynthesis. In fact, ancient cyanobacteria are believed to have produced most of the oxygen that allows animals to survive on Earth.

Cyanobacteria often are found in the microbial mats that carpet hot springs, where life exists at near-boiling temperatures. These mats are highly organized communities where different organisms split up the work, with cyanobacteria serving as the main photosynthetic power plants.

Grossman and his colleagues study the tiny, single-celled cyanobacterium Synechococcus. Microbial mats in Yellowstone's Octopus Spring contain Synechococcus that can grow in waters of nearly 160 F, while other microbes in the hot spring can tolerate temperatures that exceed 175 F. Until now, it was unclear which organisms could fix nitrogen, especially in the hotter regions of the mat.

N2 fixation is a problem for photosynthetic cells, since oxygen produced during photosynthesis inhibits the functionality of the nitrogenase complex—the enzyme factory that fixes N2. Other organisms have found creative solutions to this dilemma. For example, plants rely on symbiotic N2-fixing bacteria that live in their roots, far from the photosynthetic leaves, while a different type of cyanobacteria grows in multi-cellular strands and makes specialized N2-fixing cells that are walled off from the photosynthetic cells.

Many researchers believed that these filamentous cyanobacteria were the major N2 fixers in microbial mats. But they are not tolerant of extremely high temperatures and live only at the cooler edges of the mat, raising the question of whether N2 fixation was critical for organisms in the hotter regions of the mat. Because heat-tolerant cyanobacteria, such as Synechococcus, specialize in photosynthesis, many researchers had dismissed them as candidates for N2 fixation.

For the PNAS study, lead author Anne-Soisig Steunou of Carnegie and her colleagues tracked the activity of Synechococcus genes involved in photosynthesis and N2 fixation over a 24-hour period. They found that photosynthetic genes shut down shortly after nightfall, and that 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.

"Synechococcus cannot spatially separate photosynthesis and N2 fixation, as some photosynthetic organisms do," explained Carnegie scientist Devaki Bhaya, a co-author of the study. "Instead, they solve the problem by temporally separating the tasks."

Fixing N2 requires a lot of energy, which raises another problem for Synechococcus. When photosynthesis shuts down at night, the mat becomes oxygen starved, making it difficult to perform respiration—an efficient energy-generating pathway that requires oxygen to release the energy stored in sugars. Instead, the cells must rely on fermentation—a less efficient pathway that can proceed without oxygen. Steunou and colleagues found that, at night, Synechococcus turns on genes for specific fermentation pathways that release the energy that probably powers N2 fixation.

"These results add to our understanding of microbial mats as complex, integrated communities that are exquisitely adapted to life in the tough hot spring environment," Grossman said. "There may be several different organisms living in a given mat, but it seems that they are engaging in community metabolism that changes depending on the time of day. Perhaps it is more correct to consider the mat as a single functional unit rather than as a group of individual organisms."

Other co-authors of the PNAS study are Mary Bateson, Melanie Melendrez, Eric Brecht, David Ward and John Peters, all of Montana State University, and Michael Kühl of the University of Copenhagen. This work was supported by a grant from the National Science Foundation's Frontiers in Integrated Biology Program, with additional support from the Danish Natural Science Research Council, the National Institutes of Health and NASA.

Matthew Early Wright is a science writer and publications coordinator at the Carnegie Institution of Washington.