Chroococcoid cyanobacteria Synechococcus spp. in the Black Sea: pigments, size, distribution, growth and diurnal variability

Phycoerythrin-containing unicellular cyanobacteria Synechococcusspp. were studied for the first time during April–May,1994 and September–October, 1996, in the western and southernBlack Sea for pigments, size and abundance distribution viaspectrometry, epifluorescence microscopy and flow cytometry.Abundance distribution in the surface mixed layer in April–May,1994, revealed that cells were more concentrated in offshorewaters than in coastal regions under the direct influence ofthe river Danube. However, in the south, higher surface cellconcentrations were characteristic of the nearshore areas duringSeptember–October, 1996. A highly significant correlationwas observed between cell abundance and ambient physico-chemicalparameters with depth. Visual inspection of the individual cellsunder the epifluorescence microscope revealed that cells atthe subsurface, chlorophyll a maximum layer (SCML, based onin situ fluorometer readings) fluoresce more brightly and forlonger than those at the surface and at lower depths. Spectralproperties of a total of 64 Synechococcus spp. clonal isolatesfrom different depths within the euphotic layer (about the top60 m) in the southern Black Sea coast showed that all have type2 phycoerythrobilin in common, lacking phycourobilin. In vivofluorescence emission maxima for phycoerythrobilin were aboutthe same (~578 nm) for all isolates. All isolates had in vivoabsorption maxima at between 435 and 442 nm, and at about 681nm due to chlorophyll a. It was shown from the flow cytometermean forward light scatter data for size distribution that cellsat the surface mixed layer (0–10 m) were larger than cellsat lower depths (20–60 m). Based on in vivo fluorescencemeasurements, significant differences in the acclimated growthrates of clones from different depths were observed. Time versuscell count plots showed that cells of the cyanobacteria Synechococcusspp. are under grazing pressure, from midnight until noon, andslowly begin to rebuild their population in the afternoon bydividing throughout the evening.