Fast repetition rate fluorometry is not applicable to studies of filamentous cyanobacteria from the Baltic Sea

Abstract

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 light harvesting 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.