Tuesday, November 25, 2008

How Termites Get Their Nitrogen

Termites eat wood, and they require nitrogen, but wood contains very little. It turns out that a protist that lives inside the termite's gut contains a bacteria which in turn fixes atmospheric nitrogen!
How Termites Live on a Diet of Wood in the New York Times and Genome of an Endosymbiont Coupling N2 Fixation to Cellulolysis Within Protist Cells in Termite Gut in Science:
Termites harbor diverse symbiotic gut microorganisms, the majority of which are as yet uncultivable and their interrelationships unclear. Here, we present the complete genome sequence of the uncultured Bacteroidales endosymbiont of the cellulolytic protist Pseudotrichonympha grassii, which accounts for 70% of the bacterial cells in the gut of the termite Coptotermes formosanus. Functional annotation of the chromosome (1,114,206 base pairs) unveiled its ability to fix dinitrogen and recycle putative host nitrogen wastes for biosynthesis of diverse amino acids and cofactors, and import glucose and xylose as energy and carbon sources. Thus, nitrogen fixation and cellulolysis are coupled within the protist's cells. This highly evolved symbiotic system probably underlies the ability of the worldwide pest termites Coptotermes to use wood as their sole food.

In the same issue of Science there's an article about nitrogen fixation in the ocean:
Globally Distributed Uncultivated Oceanic N2-Fixing Cyanobacteria Lack Oxygenic Photosystem II
Oceanic cyanobacteria are the oxygen-producing, carbon- and nitrogen-fixing engines of our planet. Because oxygen is toxic for nitrogenase, nitrogen fixation is a bit problematic to fit into this metabolic mix. Multicellular cyanobacteria like Trichodesmium have evolved a range of temporal and compartmental strategies to protect their nitrogenase enzymes from being poisoned. A newly discovered and abundant group of unicellular cyanobacteria has dispensed with carbon fixation and oxygen production altogether and so it can run nitrogenase without hindrance. Zehr et al. (p. 1110) have extracted this group by cell sorting and, by genome analysis, show that indeed it lacks the genes both for photosystem II and for carbon fixation via the reductive pentose phosphate cycle. This finding makes it necessary to reevaluate current models of nitrogen and carbon cycling on Earth.

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