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Photorespiratory 2-phosphoglycolate (2PG) metabolism is essential for photosynthesis in higher plants but thought to be superfluous in cyanobacteria because of their ability to concentrate C[O.sub.2] internally and thereby inhibit photorespiration. Here, we show that 3 routes for 2PG metabolism are present in the model cyanobacterium Synechocystis sp. strain PCC 6803. In addition to the photorespiratory C2 cycle characterized in plants, this cyanobacterium also possesses the bacterial glycerate pathway and is able to completely decarboxylate glyoxylate via oxalate. A triple mutant with defects in all 3 routes of 2PG metabolism exhibited a high-C[O.sub.2]-requiring (HCR) phenotype. All these catabolic routes start with glyoxylate, which can be synthesized by 2 different forms of glycolate dehydrogenase (GIcD). Mutants defective in one or both GIcD proteins accumulated glycolate under high C[O.sub.2] level and the double mutant [DELTA]glcD1/[DELTA]glcD2 was unable to grow under low C[O.sub.2]. The HCR phenotype of both the double and the triple mutant could not be attributed to a significantly reduced affinity to C[O.sub.2], such as in other cyanobacterial HCR mutants defective in the C[O.sub.2]-concentrating mechanism (CCM). These unexpected findings of an HCR phenotype in the presence of an active CCM indicate that 2PG metabolism is essential for the viability of all organisms that perform oxygenic photosynthesis, including cyanobacteria and C3 plants, at ambient C[O.sub.2] conditions. These data and phylogenetic analyses suggest cyanobacteria as the evolutionary origin not only of oxygenic photosynthesis but also of an ancient photorespiratory 2PG metabolism.

The oceanic N2-fixing cyanobacterium Trichodesmium spp. form extensive surface blooms and contribute significantly to marine carbon and nitrogen cycles in the oligotrophic subtropical and tropical oceans. Trichodesmium grows in salinities from 27 to 43 parts per thousand (ppt), yet its salt acclimation strategy remains enigmatic because the genome of Trichodesmiumerythraeum strain IMS101 lacks all genes for the biosynthesis of any known compatible solute. Using NMR and liquid chromatography coupled to mass spectroscopy, we identified the main compatible solute in T. erythraeum strain IMS101 as the quaternary ammonium compound N,N,N-trimethyl homoserine (or homoserine betaine) and elucidated its biosynthetic pathway. The identification of this compatible solute explains how Trichodesmium spp. can thrive in the marine system at varying salinities and provides further insight into the diversity of microbial salt acclimation. [ABSTRACT FROM AUTHOR]