Fluorescence-based characterization of phycoerythrin-containing cyanobacterial communities in the Arabian Sea during the Northeast and early Southwest Monsoon (1994-1995)

Scanning fluorescence spectroscopy was used to investigate the spatial and temporal variability in the fluorescence signature of phycoerythrin-contain ing organisms in the Arabian Sea during the early Northeast and early South west Monsoon (1994-1995), Phycoerythrin (PE) emission spectra were relative ly invariant among all the samples collected on either cruise; the relative ly symmetrical PE emission peaks showed maxima at wavelengths ranging from 563-572 nm. PE excitation spectra always showed either a strong shoulder or a peak at wavelengths absorbed maximally by phycourobilin (PUB) chromophor es as well as a peak at wavelengths absorbed maximally by phycoerythrobilin (PEB) chromophores. Thus, the Arabian Sea appears to be different from the Black Sea or Gulf of Maine in that PUB-lacking forms of PE rarely, if ever , dominate the PE signal. Fluorescence excitation signatures differed in th e relative excitation of PE emission by wavelengths absorbed by PUB (simila r to 495 nm, Ex(PUB)) and by wavelengths absorbed by PEB (similar to 550 nm , Ex(PEB)); these were distinguished by having either very low (similar to 0.6), very high (similar to 1.8), or intermediate Ex(PUB):Ex(PEB) ratios. T he distribution of samples with different PE fluorescence signatures was in vestigated extensively during the early Southwest Monsoon, and communities characterized by the low Ex(PUB):Ex(PEB) ratios were closely associated wit h cooler (24-27 degrees C), fresher (35.7-36.25 psu) water influenced by co astal upwelling. In general, "ambient" surface water of the Arabian Sea dur ing the early Southwest Monsoon was of intermediate temperature (27-29 degr ees C) and salinity (36.15-36.4 psu) and showed intermediate or high values for Ex(PUB):Ex(PEB). This suggests that the PE fluorescence signature can be used to follow the fate of upwelling-influenced water masses and the pop ulations they transport. (C) 1999 Elsevier Science Ltd. All rights reserved .