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°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 NaH 14 CO 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 NaH 14 CO 3 resulted in labeling of both S. lividus and filaments, but the addition of nonradioactive glycolate increased the level of 14 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.