Spectral diversity of phycoerythrins and diazotroph abundance in tropical waters

Phycoerythrin (PE) spectral diversity was investigated in eastern tropical Australian waters and around New Caledonian and Fijian archipelagos. Colony sorting of filamentous cyanobacteria revealed slight differences in the PE excitation spectrum of Trichodesmium thiebautii and T. erythraeum. Spectra of PE from Katagnymene spiralis and Richelia intracellularis were examined for the first time. PE spectra of filamentous cyanobacteria (Trichodesmium, Katagnymene, and Richelia) showed a broader phycoerythrobilin (PEB) band than those of Synechococcus. The influence of PE Trichodesmium on the global spectrum of PE in natural waters was clearly visible at various stations. The PEB band was large at the surface and narrower at increased depth, suggesting a shift of the cyanobacterial community from a dominance of diazotrophic filamentous cyanobacteria to small Synechococcus. Size fractionation of water samples confirmed this. A good linear relationship was observed between PE concentration in the .10-mm cellular-size fraction and the abundance of filamentous cyanobacteria expressed by either trichome numbers, total trichome surface area, or total trichome volume. PE in the .10-mm fraction is a useful tool for rapidly quantifying filamentous cyanobacteria. Neither diel variations nor photoacclimation significantly influenced the PE fluorescence excitation spectra in T. thiebautii and T. erythraeum. Using this method, we identified green colonies of filamentous cyanobacteria in deep waters (50– 120 m) of the Coral Sea with a novel high-phycourobilin PE. While morphologically similar to Trichodesmium, it possesses distinctive photosynthetic responses and could be a new species. Phycobiliproteins (PBPs) play an important role in lightharvesting pigment complexes of three groups of photosynthetic organisms: cyanobacteria, Rhodophyceae, and Cryptophyceae. They are also of significant ecological interest in providing information on biomass and taxonomic composition of these groups in the photosynthetic community (Glazer et al. 1982; Lantoine and Neveux 1997; Neveux et al. 1999; Wood et al. 1999). However, contrary to liposoluble algal pigments, which are extensively measured in the field to assess the community structure, PBPs are rarely used and data on them are still scarce. In subtropical and tropical oceanic waters, the major PBPs are phycoerythrins (PEs) associated with cyanobacteria. Two functional groups can be distinguished within these cyanobacteria according to their capability to fix dissolved N2 (diazotrophy). The smallest unicellular species (Prochlorococcus, cluster A Synechococcus: Waterbury and Rippka 1989) are unable to fix N2, while other species, unicellular or colonial, free-living or symbiotic, are diazotrophic (Capone et al. 1997; Zehr et al. 2001; Falcón et al. 2002). Diazotrophy represents a useful adaptation to oligotrophic conditions where other N sources are exhausted (Karl et al. 2002), provided that supplies of other macro(PO 22 4 ) and micronutrients (Fe) are sufficient to sustain diazotroph growth (Mills et al. 2004; Moutin et al. 2005). The importance of oceanic diazotrophs in N cycling and N global budget has resulted in new interest in measuring both rates and abundance of diazotrophs (La Roche and Breitbarth 2005). N2 fixation may supply up to half of the new particulate N in the subtropical north Pacific (Karl et al. 1997) and increases the sequestration of CO2 via particulate export to deeper waters (Gruber and Sarmiento 1997). Trichodesmium spp. are one of the dominant diazotrophs in tropical and subtropical seas (Capone et al. 1997). Their abundance is generally assessed by time-consuming counting of filaments (or trichomes) using microscopy (Carpenter et al. 2004). Spectrofluorometric measurement of PE concentration has promised to be a more rapid way to assess their abundance; however, there are relatively few field data on the fluorescence characteristic of PE and its Acknowledgments We thank D. Capone and E. J. Carpenter for their invitation to participate on the Revelle 98 and Ewing 99 cruises and A. Le Bouteiller for organizing the Diapazon program. Financial support from CNRS-INSU (PNTS 98), IRD (UR099 and UR-086), PROOF and the U.S. National Science Foundation (OCE 99-71778) is acknowledged. We thank the Santa Úrsula University (Rio de Janeiro) for supporting the participation of M. Tenório on the Revelle cruise and the Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico (CNPq) of the Brazilian Government for the 2001–2005 award of a Ph.D. scholarship at M. Tenório. 2 Present address: Institut de Recherche pour le Développement (UR 103 CAMELIA), Station Marine d’Endoume, 13007 Marseille, France. 1 Corresponding author ([email protected]). Limnol. Oceanogr., 51(4), 2006, 1689–1698 E 2006, by the American Society of Limnology and Oceanography, Inc.