Identification of Synechococcus spp. in the Sargasso Sea by immunofluorescence and fluorescence excitation spectroscopy performed on individual cells

Using fluorescence excitation spectra and immunofluorescence, we examined individual cells from depth profiles collected from two stations in the Sargasso Sea in July and August 1986. All cells examined showed excitation spectra typical of Synechococcus strains with a phycoerythrin containing a high ratio of phycourobilin (PUB) to phycoerythrobilin (PEB) chromophores (WH8103‐type). The WH8103‐type phycoerythrin is a variant of the pigment found in the Sargasso Sea isolate WH7803 (type 1 phycoerythrin), and the ratio of PUB to PEB fluorescence in WH8103 is five times greater than it is in the more often studied WH7803. In immunofluorescence assays, antisera directed against WH7803 (type 1) failed to label most cells throughout the photic zone at both stations, which is indirect support for the dominance of the high PUB : PEB ratio pigment types. We suggest that in the Sargasso Sea Synechococcus strains with phycoerythrins containing high ratios of PUB: PEB (WH8103‐type) may be more common than type 1 strains such as WH7803.