( yanobacteria growing at elevated temper...)Hot-spring bacteria flip a metabolic switch

All cells 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 or fixed into larger, carbon-containing compounds. N2 fixation is a problem for photosynthetic cells, since the oxygen produced during photosynthesis inhibits 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 away from the photosynthetic leaves. A different type of cyanobacteria grows in multicellular strands and makes specialized N2-fixing cells called heterocysts that are walled off from the photosynthetic cells.

Many researchers believed these filamentous cyanobacteria were the major N2 fixers in microbial mats. But they are not tolerant of extremely high temperatures, and only live 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, single-celled cyanobacteria like Synechococcus specialize in photosynthesis, many researchers had dismissed them as candidates for N2 fixation.

"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-fixatio
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Source:Carnegie Institution