The variable (inducible) part of fluorescence is expressed as F v = F m – F 0, and when normalized to F m (F v/F m) presents learn more a measure of the maximum quantum yield of charge separation at PSII. Under ambient light conditions, the operational quantum yield of PSII (F v′/F m′) is obtained instead. Both parameters are useful as they respond to nutrient limitation, excess light or transiently when growth conditions change. A combination of dark- and light adapted measurements can be used to determine the electron transport rate under known irradiance(s), which can in turn be used to model primary production

(Kolber and Falkowski 1993). The current work focuses on the experimental manipulation and spectral measurement of dark-adapted F v/F

m. The use of this parameter in higher level applications is discussed at length in recent reviews of literature on the subject (Suggett et al. 2004, Huot and Babin 2010). Advances in light-emitting diode (LED) manufacturing have led to the availability of Chk inhibitor narrow-band, high-power excitation light sources of high efficiency and stability. Their rapid flash capability and high output makes them the light source of choice for FRRF protocols and for PAM applications that require a small footprint. In FRRF, microsecond flashlets provide a saturating flash train within a single turnover period of PSII (<100–150 μs). PAM-type fluorometers have been developed with a combination of light sources of different colours for some time. Fossariinae FRRF instruments were until very recently limited to the use of LEDs of one colour in order to produce sufficiently bright flashlets. Blue light sources have been chosen to provide overlap with the absorption by Chla and accessory photosynthetic pigments in algae, but do not overlap with cyanobacterial phycobilipigment absorption (Johnsen and Sakshaug 2007). Recent studies have shown that blue-light equipped

FRRF instruments are relatively insensitive to the presence of cyanobacteria, if these do not possess short-wavelength forms of phycoerythrin (Raateoja et al. 2004; Suggett et al. 2004). While F v/F m can be recorded from cyanobacteria using blue excitation as long as the light source can saturate PSII, the intensity of the fluorescence is relatively low compared to algae. Variable fluorescence of cyanobacteria can alternatively be assessed from orange or red excitation sources that excite the phycobilipigments in cyanobacteria (Schubert et al. 1989). Now that LEDs are available at the brightness required by FRRF instruments, this concept stands to be adapted to the FRRF range of instrumentation. Studies on the optimization of the variable fluorescence measurement towards unbiased representation of the phytoplankton community, are therefore overdue.