Fluorescence properties of the freshwater phytoplankton: Three algal classes compared

Abstract

The fluorescence properties of exponentially growing freshwater diatoms, green and blue-green algae were compared with a standard filter fluorometer and a scanning spectrofluorometer. Green algae and diatoms excited with blue light had R values (ratio of in vivo fluorescence to extractable chlorophyll a) up to 50 times higher than the cyanophytes. There were also considerable differences in R between species within each algal class—coefficients of variation were typically 35–50%—and this variation was slightly increased by treatment with the non-cyclic electron flow inhibitor, 3(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU). A major component of blue-green algal fluorescence was the red light emission from phycobilin accessory pigments. This contribution characterized their emission and excitation spectra and suggested a rapid assay for cyanophyte dominance, which was tested on natural plankton communities. Phycobilin fluorescence was insensitive to DCMU and resulted in highly wave-length-specified variations in CFC (cellular fluorescence capacity, the rise in fluorescence upon addition of DCMU). CFC values for blue-greens were consequently low when measured in the broad bandpass fluorometer, but were up to three times higher when measured with the spectrofluorometer set to the emission peak for photosystem II chlorophyll a. Similar results were obtained with natural populations of blue-green algae. The phycobilin content of blue-green species was influenced by nitrogen source and light quality during growth, and this resulted in variable R and CFC values. Fluorescence properties of diatoms and green algae were less responsive to environmental conditions during growth. Both diatoms and green algae exposed to bright light for 15 min demonstrated a strong reduction in R. Conversely blue-green algal fluorescence was depressed very little, or more commonly, was slightly enhanced by bright light. These observations underscore the importance of species composition and choice of fluorometer as critical determinants of measured algal fluorescence properties.