A peculiar strain of Green sulfur bacteria (GSB), especially adapted to low light intensities and producing a unique bacteriochlorophyll (BChl) homologue, has been found, to date, only in the Black Sea and in Faro Lake. GSBs are anoxygenic photoautotrophic prokaryotes which form a distinct and coherent phylogenetic lineage and are commonly found in euxinic (anoxic and sulfidic) aquatic habitats at depths where light is very faint.
All known green sulfur bacteria are obligate anaerobic photolithoautotrophs which can only grow in the presence of light and reduced sulfur compounds as photosynthetic electron donors. Since its discovery, this group of prokaryotes has gained a lot of interest because of a number of unique features. In fact, GSBs perform anoxygenic photosynthesis through specific antenna pigments (bacteriochlorophylls c, d and e) and particular carotenoids (isorenieratene, β-isorenieratene and chlorobactene) which are packed in peculiar organelles, the chlorosomes.
Chlorosomes and the associated photosynthetic pigments constitute very sensitive light receptors that can capture even minute amounts of light and thus enable the green sulfur bacteria to photosynthesize and to grow at very low light intensities. The efficient light harvesting of GSBs has important ecological consequences because it allows these bacteria to occupy such deep water layers in stratified environments that the reduced sulfur compounds they use as electron donors are in great supply.
The analysis of 16S rRNA gene sequence of PSBs found in the chemocline of the Black Sea revealed the presence of a single phylotype, Chlorobium sp. strain BS-1, which, after selective enrichment, was found to be halophilic and to exhibit extraordinary low-light adaptation. The same analyses performed on samples from the “red water layer” of Faro Lake in collaboration with Dr. Ovidiu Ruecker (formerly at the DSMZ), showed exactly the same phylotype. Furthermore, a distinctive bacteriochlorophyll e homologue, previously detected only in the Black Sea, has been found in Faro Lake by me and Prof. Carles Borrego (University of Girona).