Photoexcretion and fate of glycolate in a hot spring cyanobacterial mat.

Photosynthesis by Synechococcus lividus, the sole oxygenic phototroph inhabiting the surface of the 55 degrees C cyanobacterial mat in Mushroom Spring, Yellowstone National Park, causes superoxic and alkaline conditions which promote glycolate photoexcretion. At O(2) concentrations characteristic of the top 2 mm of mat during the day, up to 11.8% of NaHCO(3) fixed in the light was excreted, and glycolate accounted for up to 58% of the excreted photosynthate. Glycolate was neither incorporated nor metabolized by S. lividus, but it was incorporated by filamentous microorganisms in the mat. Incubation of mat samples with NaHCO(3) resulted in labeling of both S. lividus and filaments, but the addition of nonradioactive glycolate increased the level of C in the aqueous phase and decreased the extent of labeling of filaments. This suggests that cross-feeding of glycolate from S. lividus to filamentous heterotrophs occurs and that underestimation of the extent of photoexcretion is probable.