Ecological diversity and low light adaptation of green sulfur bacteria Fakultät für Biologie - Digitale Hochschulschriften der LMU - Teil 02/06

This study focuses on one brown-colored representative of the green sulfur bacteria, Chlorobium sp. BS-1, that survives by means of anoxygenic photosynthesis even at very low light intensities. This unusual representative of the green sulfur bacteria lives in the chemocline of the Black Sea, which is located at 80 to 120 m depth, offering only 0.0005 % (winter) to 0.002 % (summer) of surface irradiance (0.00075 to 0.0022 µmol Quanta m-2 s-1). The Black Sea represents the world’s biggest anoxic water body, and it is permanently stratified. An overview of the habitat Black Sea and the research on phyotosynthetic bacteria in the Black Sea chemocline gives the article Anoxygenic phototrophic bacteria in the Black Sea chemocline (pages 53 ff.)
In the second article, Physiology and phylogeny of green sulfur bacteria forming a monospecific phototrophic assemblage at 100 m depth in the Black Sea (pages 75 ff.), it is shown that Chl. sp. BS-1 represents a novel phylotype in the marine cluster of green sulfur bacteria and is the only detectable phylotype of green sulfur bacteria in the Black Sea chemocline. It was shown that Chl. sp. BS-1 is the first organism known to date which fixes 14CO2 under laboratory conditions even at light intensities as low as 0.015 µmol Quanta m-2 s-1 which is much lower than the light intensity in any typical habitat of green sulfur bacteria. Therefore it is the best adapted species to extremely low light intensities documented. The major adaptive mechanism to extremely low light intensities might be a significant change in the secondary homologs of the main photosynthetic pigment, bacteriochlorophyll e (BChl e). The dominance of farnesyl esters and the presence of four unusual geranyl ester homologs of BChl e were revealed by HPLC analysis in cells shifted from 3 to 0.1 µmol Quanta m-2 s-1. Together with in situ light measurements and in situ BChl e concentrations, the growth experiments allowed for the calculation of doubling times and significance of the photosynthetic activity for the carbon and sulfur cycle in the Black Sea chemocline. With doubling times of a minimum of 3.1 years (for summer light intensity) and the contribution of only 0.002 - 0.01 % to total sulfide oxidation in the chemocline Chl. sp. BS-1 represents the slowest growing population of green sulfur bacteria known to date and does not contribute significantly to the carbon or sulfur cycle.
The article Subfossil DNA sequences of green sulfur bacteria as indicators for past water column anoxia in the Black Sea (page 119 ff.) gives insight into the history of the strain Chl. sp. BS-1 and the green sulfur bacteria in the Black Sea during the last few thousand years. The Black Sea today is considered as closest contemporary analogue to past sulfidic oceans and its biogeography over several thousand years is well documented in its stratified sedimentary record. Since the 16S rRNA gene sequence of Chl. sp. BS-1 might be a useful indicator for past photic zone anoxia, the presence of its fingerprints in past periods of the Black Sea was investigated. 16S rRNA gene sequences of green sulfur bacteria from samples of Black Sea sediments up to 7 m below seafloor were amplified and sequenced. Nine green sulfur bacterial 16S rRNA gene sequences were identified. Surprisingly, not only green sulfur bacterial fingerprints were found but also closely related species clustering at the basis of the green sulfur bacterial subtree, together with not yet cultured species detected all over the world. The new cluster was called “deep-branching green sulfur bacteria” though it was not possible to enrich live organisms with medium for photosynthetic green sulfur bacteria. The chemocline strain Chl. sp. BS-1, found in Units III, II and I (>9000 years b.p. until today), and two other sequences, found in sediments of Unit IIb (between 8200 yr. b.p. and 5000 yr.b.p) only, were the only two sequences clustering with the marine green sulfur bacteria.