Fast repetition rate (FRR) fluorometry has been used successfully to investigate the variable fluorescence characteristics of cyanobacteria in oceanic Case 1 waters. In these waters, the effective absorption bands of the lightharvesting pigments for photosystem II (PSII) of the dominant cyanobacterial taxa overlap with the peak of the FRR excitation at 470 nm. The effective PSII absorption of the ecologically-significant filamentous cyanophytes in the Baltic Sea-Nodularia spumigena (Mertens ex Bornet and Flahault 1886) and Aphanizomenon sp. ((L.) Ralfs ex Bornet and Flahault 1886)-is, however, restricted to beyond 550 nm. We tested the applicability of a currently commercially available FRR fluorometer to studies of these two cyanobacterial taxa. We propose that the FRR technique should not be used in studies on these taxa, or on any cyanophyte containing phycoerythrocyanin instead of phycoerythrin, the former having inefficient PSII light harvesting in the wave band of the FRR excitation. This issue should be taken into account whenever field studies utilizing the FRR system are planned in those Case 2 water bodies, in which cyanobacteria lacking phycoerythin are among the dominant phytoplankton groups.Since the introduction of its commercial version in the mid-1990s, the fast repetition rate (FRR) fluorometer (Kolber and Falkowski 1992) has been rapidly adopted in the field oceanographic research. Being capable of nearly instantaneous high-frequency in situ determinations of a suite of algal photophysiological parameters, this technique has helped us to comprehend, for example, the extent of iron limitations on phytoplankton growth in the world's oceans (Boyd et al. 2000).Up to the present time, FRR-based estimates of the photosynthetic competence (F v /F m ) of cyanobacteria (Cyanophyceae) at the species level are scarce, as the bulk of the published FRR datasets originate from field studies conducted in the world's oceans. So far, the FRR technique has mainly probed the photophysiology of natural phytoplankton populations containing the ubiquitous oceanic picocyanobacterial genera Synechococcus spp. and Prochlorococcus spp. Physiologically reasonable values of F v /F m , falling within the range of 0.20 to 0.55, have been observed in the context of Fe-enrichment experiments (Behrenfeld et al. 1996), and diurnal studies (Behrenfeld and Kolber 1999) on communities codominated by these two genera, as well as in water-col-1 Corresponding author (mika.raateoja@fimr.fi)
AcknowledgmentsWe thank the Tvärminne Zoological Station, University of Helsinki, for offering the laboratory facilities and time; S. Laney for providing the v5LITE software; the working groups of K. Sivonen, University of Helsinki and of E. Granéli, University of Kalmar, as well as S. and G. Hällfors of the Finnish Institute of Marine Research, for providing the algal strains; J. Dunning and J. Atkins of Chelsea Technologies Group for providing information about the optical set-up of FAST tracka , R. King for improving the language, and tw...
