Diversity and distribution of photosynthetic bacteria in the Black Sea

Using an infrared fast-repetition-rate (IRFRR) fluorometer, we measured bacteriochlorophyll a (BChl a) fluorescence kinetics to examine the spatial pattern of the distribution of aerobic and anaerobic photosynthetic bacteria in the Black Sea in June 2001. Aerobic photosynthetic bacteria containing BChl a were distributed together with chlorophyll-containing phytoplankton in the upper, aerobic portion of the water column, confirming their exclusive requirement for oxygen. The BChl a to chlorophyll a ratio averaged 10−2 within the euphotic zone. Seven strains of aerobic photosynthetic bacteria were isolated and partially characterized. They belong to two subgroups closely related to Erythrobacter and Roseobacter genera. Green sulfur photosynthetic bacteria were detected in the central part of the basin, at the boundary between suboxic and anoxic (hydrogen sulfide) zone. However, the population of green sulfur bacteria was extremely small, and confined to a very narrow density layer deep in the water column.