ANOXYGENIC PHOTOTROPHIC BACTERIA IN THE BLACK SEA CHEMOCLINE

摘要

Currently, the Black Sea is the largest anoxic water body on the planet and represents the closest contemporary analogue to past sulfidic oceans. For the understanding of such oxic/anoxic environments, green sulfur bacteria are significant, since they can substantially alter the carbon and sulfur cycles and may be used as indicator organisms for the reconstruction of past photic zone anoxia. In the chemocline of the Black Sea, brown-colored green sulfur bacteria form an extremely dilute, but detectable population. Based on analysis of their 16S rRNA gene sequences, these bacteria represent a single and novel phylotype. Measurements of light intensities in the chemocline and experiments with laboratory cultures revealed that the strain from the Black Sea is extraordinarily low-light adapted and grows with doubling times of several years. Fossil 16S rRNA gene sequences of the chemocline green sulfur bacterium have been detected in different sediment layers, indicating past photic zone anoxia during which the chemocline must have been positioned at a depth of up to 150 m. Although not significant in the carbon and sulfur cycles of the Black Sea, the particular type of green sulfur bacterium present in the chemocline thus represents a valuable indicator organism for the reconstruction of the paleoenvironmental conditions of the Black Sea.