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

Abstract

Quantitative and qualitative investigations of phycoerythrins (PE) were achieved in the central southern tropical and equatorial Pacific during the Flux dans l'ouest du Pacifique equatorial (FLUPAC: September‐October 1994) and Oligotrophie en Pacifique (OLIPAC: November 1994) cruises. We observed mainly high‐phycourobilin (PUB) PE related to small Synechococcus spp. (0.6–1.4 μm). This PE was characterized by fluorescence excitation peaks at 496 and 550 nm in 50% glycerol. Highest concentrations (0.1–0.2 μg L−1) were recorded either in the mixed layer (nutrient‐enriched area) or at depth in the vicinity of the 0.1 μM NO3− isopleth (oligotrophic waters). Maximum abundance of Synechococcus did not exceed 31 × 103 cell mL−1. No subpopulations 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 μm in size and easily detected by flow cytometry, likely cyanobacteria. They were only observed in very oligotrophic waters south of 15°30′S along 150°W in the top 60 m. In this area, nitrate, nitrite, and ammonia were undetectable above 120 m, while phosphate was always recorded. This suggests that these larger cyanobacteria may fix dinitrogen (N2) 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 samples obtained at or near the equator. It displayed two fluorescence excitation peaks at 496 and 536 nm. The organisms that contained this PE type were not identified. These data suggest that PE is more diverse in oceanic waters than was previously assumed.