Spatial and seasonal variations in abundance and spectral characteristics of phycoerythrins in the tropical northeastern Atlantic Ocean

A simple and rapid spectrofluorimetric method to quantify phycoerythrins (PEs) from the area below the fluorescence excitation spectra is proposed. In contrast to other methods which used measurements at fixed wavelengths, this method takes into account the spectral diversity of these pigments. It was applied in the tropical northeastern Atlantic during the EUMELI program at three stations which were characterized by different trophic status (eutrophic, mesotrophic and oligotrophic). Sampling was made at various periods of the year. The results showed the constant presence of phycoerythrin nearly always associated with cyanobacteria (Synechococcus). At the oligotrophic site, high phycourobilin (PUB) populations and a great stability in the abundance and spectral characteristics of PE were observed. The eutrophic and mesotrophic sites showed higher PE abundance and spectral variability, and were essentially dominated by high phycoerythrobilin (PEB) populations. A general increase with depth in the PUBPEB ratio at the eutrophic and mesotrophic sites was observed. This increase could be regular or relatively sharp through physical discontinuity (thermocline or halocline). The presence of Cryptophyceae PE was occasionally noted. The PE diversity was most probably related to the presence of different populations, but some variations could be also influenced by photoacclimation processes. Further progress in the spectrofluorimetric analysis of PE could be achieved by using three-dimensional spectra.