Cyanobacteria

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JOURNAL OF PLANKTON RESEARCH

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Chroococcoid cyanobacteria Synechococcus spp. in the Black Sea: pigments, size, distribution, growth and diurnal variability
ZAHIT UYSAL
INSTITUTE OF MARINE SCIENCES, MIDDLE EAST TECHNICAL UNIVERSITY, P.B.

, , ERDEMLI-·ÇEL, TURKEY I

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Phycoerythrin-containing unicellular cyanobacteria Synechococcus spp. were studied for the first time during April–May, 1994 and September–October, 1996, in the western and southern Black Sea for pigments, size and abundance distribution via spectrometry, epifluorescence microscopy and flow cytometry. Abundance distribution in the surface mixed layer in April–May, 1994, revealed thatcells were more concentrated in offshore waters than in coastal regions under the direct influence of the river Danube. However, in the south, higher surface cell concentrations were characteristic of the nearshore areas during September–October, 1996. A highly significant correlation was observed between cell abundance and ambient physico-chemical parameters with depth. Visual inspection of theindividual cells under the epifluorescence microscope revealed that cells at the subsurface, chlorophyll a maximum layer (SCML, based on in situ fluorometer readings) fluoresce more brightly and for longer than those at the surface and at lower depths. Spectral properties of a total of 64 Synechococcus spp. clonal isolates from different depths within the euphotic layer (about the top 60 m) in thesouthern Black Sea coast showed that all have type 2 phycoerythrobilin in common, lacking phycourobilin. In vivo fluorescence emission maxima for phycoerythrobilin were about the same (~578 nm) for all isolates. All isolates had in vivo absorption maxima at between 435 and 442 nm, and at about 681 nm due to chlorophyll a. It was shown from the flow cytometer mean forward light scatter data for sizedistribution that cells at the surface mixed layer (0–10 m) were larger than cells at lower depths (20–60 m). Based on in vivo fluorescence measurements, significant differences in the acclimated growth rates of clones from different depths were observed. Time versus cell count plots showed that cells of the cyanobacteria Synechococcus spp. are under grazing pressure, from midnight until noon, and slowlybegin to rebuild their population in the afternoon by dividing throughout the evening.

I N T RO D U C T I O N
The picoplanktonic, chroococcoid cyanobacteria, Synechococcus spp., are known to be major contributors to the total photosynthetic biomass in the oceans (Berman, 1975; Waterbury et al., 1979; Johnson and Sieburth, 1979; Li et al., 1983; Platt et al., 1983; Takahashi and Bienfang, 1983;Iturriaga and Mitchell, 1986; Glover et al., 1986; Booth, 1988; Li et al., 1992), especially in the more oligotrophic regions such as the Mediterranean Sea (Li et al., 1993; Magazzu and Decembrini, 1995; Agawin and Agusti, 1997). The group also possesses high specific growth rates (Bienfang and Takahashi, 1983; Douglas,

1984; Landry et al., 1984). The first members of the picoplankton to bediscovered were the phycoerythrincontaining, unicellular cyanobacteria, Synechococcus (Waterbury et al., 1979; Johnson and Sieburth, 1979). In oligotrophic oceans, this group contributes up to an estimated 25% of photosynthetic carbon fixation (Waterbury et al., 1986) and accounts for 64% of the total photosynthesis in the North Pacific Ocean (Iturriaga and Mitchell, 1986). The in vivo spectralproperties of clonal isolates of marine Synechococcus have revealed three major pigment groups: clones lacking phycoerythrin (PE), clones containing PE composed of phycoerythrobilin and phycourobilin

© Oxford University Press 2001

JOURNAL OF PLANKTON RESEARCH

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chromophores, and clones containing PE composed of only phycoerythrobilin...
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