Phycoerythrins in the southern tropical and equatorial Pacific Ocean: Evidence for new cyanobacterial types

Quantitative and qualitative investigations of phycoerythrins (PE) were ach ieved in the central southern tropical and equatorial Pacific during the Fl ux dans l'ouest du Pacifique equatorial (FLUPAC: September-October 1994) an d Oligotrophie en Pacifique (OLIPAC: November 1994) cruises. We observed ma inly high-phycourobilin (PUB) PE related to small Synechococcus spp. (0.6-1 .4 mu m) This PE was characterized by fluorescence excitation peaks at 496 and 550 nm in 50% glycerol. Highest concentrations (0.1-0.2 mu g L-1) were recorded either in the mixed layer (nutrient-enriched area) or at depth in the vicinity of the 0.1 mu M NO3- isopleth (oligotrophic waters). Maximum a bundance of Synechcoccus did not exceed 31 x 10(3) cell mL(-1). No subpopul ations of Synechococcus were evidenced by flow cytometry. Nevertheless, in a few samples, two new PE spectral types were observed. The first type was a high-PUB PE with two fluorescence excitation peaks at 494 and 564 nm. It appears to be attributable to nonmotile round cells, 2-3 mu m in size and e asily detected by flow cytometry, likely cyanobacteria. They were only obse rved in very oligotrophic waters south of 15 degrees 30'S along 150 degrees W in the top 60 m. In this area, nitrate, nitrite, and ammonia were undete ctable above 120 m, while phosphate was always recorded. This suggests that these larger cyanobacteria may fix dinitrogen (N-2) to supply their growth and therefore possibly play a significant role in oceanic new production. The second type, a high-phycoerythrobilin (PEB) PE was observed in three sa mples obtained at or near the equator. It displayed two fluorescence excita tion peaks at 496 and 536 nm. The organisms that contained this PE type wer e not identified. These data suggest that PE is more diverse in oceanic wat ers than was previously assumed.