Abstract
Different methods for the determination of the maximum and the effective quantum efficiency of photochemistry using variable chlorophyll a (Chl-a) fluorescence (pump and probe [P&P], pulse amplitude modulation [PAM], and fast repetition rate [FRRF]) were compared using natural samples. The methods differ in the applied light pulse used to saturate PSII photochemistry, regarded as single (STF) and multiple (MTF) turnover flashes. Measurements were conducted in situ and on natural phytoplankton samples during two cruises, one to the North Sea and one to the South Atlantic. In both cases in situ STF measurements using FRRF were identical to measurements on discrete samples using P&P, confirming that different instrumentation and methods agreed well for the determination of STF parameters. On the other hand, estimates of the quantum efficiencies calculated from MTF (PAM) measurements were higher than quantum efficiencies calculated from STF (P&P) measurements. However, the relative differences in the quantum efficiencies determined with MTF and STF measurements were the same in both cruises and agreed well with formerly reported differences determined on algal cultures. A simple factor can be used to convert the maximum quantum efficiency determined either by MTF or STF measurement, at least for diatom dominated phytoplankton. Similarly a non-linear function can be used for the conversion of the effective quantum efficiency. Finally, photosynthesis (P) vs. irradiance (E0) relationships calculated from the effective quantum efficiency differed between MTF and STF measurement, but the basic parameters of the P vs. E0 curve, i.e. the maximum electron transport rate, ETRm, and the light efficiency factor, α, showed a linear relationship and can be converted by a linear function. These relationships solve some problems of converting results from STF measurements into those of MTF measurements or vice versa.
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