Relationship of steady-state photosynthesis to fluorescence in eucaryotic algae

Abstract

The change in fluorescence yield (Δφ) was measured in five species of eucaryotic algae using a ‘pump and probe’ flash technique. The half-time for the oxidation of Q, which was measured by varying the delay time between actinic (pump) and measuring (probe) flashes, averaged 400 μs and was unaffected by background irradiance between 10 12 and 10 16 quanta · cm −1 · s −1. The absorption cross-section of PS II traps was measured by varying the intensity of the actinic flash. These cross-sections did not change (± 10%) with background irradiance. The cross-section data can be fitted to a cumulative one-hit Poisson distribution. In the steady state, the relationship between δφ and photosynthetic oxygen evolution was highly nonlinear and cannot be explained by energy transfer between PS II units. Using the criteria of Δφ, about 15% of the PS II traps remain open at light saturation of O 2 evolution. Conversely, at low irradiance levels, capable of stimulating much less than 1% of the maximum steady-state photosynthetic rate, the fluorescence yield decreases by as much as 25% from the dark-adapted value. Furthermore, the data suggest that a long-lived quencher of fluorescence is formed at moderate to continuous irradiance levels, at least 10 16 quanta · cm −2 · s −1. Our results suggest that cyclic electron flow around PS II occurs under normal physiological conditions and is especially pronounced in chlorophytes.