Diurnal rhythm and vertical distribution of chlorophyll fluorescence as evidence of bacterial photosynthetic activity in the Black Sea

Abstract

Abstract Variability of light scattering (turbidity) and chlorophyll fluorescence in the Black Sea was studied with a submersible fluorometer in 1989 and 1991. Diurnal changes in fluorescence occurred within the layer of oxygen and hydrogen sulfite co-existence (C-layer) at the depth of the daily mean secondary fluorescence maximum, far below the light intensity level necessary for phytoplankton photosynthesis. A harmonic function maximal at night approximated these changes. There was no diurnal rhythm of fluorescence at depth levels adjacent to the secondary maximum, or in the variability of turbidity measured together with fluorescence. The maxima of chlorophyll fluorescence and water turbidity differed from each other in depth, shape, and fine structure within the C-layer. These findings support the idea that the phototrophs Chlorobium phaeobacteroides and C. phaeovibriaidesare carriers of fluorescing bacteriochlorophyll e in the Black Sea and indicate that the bacteria might be photosynthetically acti...