Removing the light history signal from normalized variable fluorescence (Fv/Fm) measurements on marine phytoplankton

Variable fluorescence normalized to maximal fluorescence, Fv/Fm, determined by Fast Repetition Rate Fluorometry (FRRF) is being increasingly used to compare photosynthetic electron transport capacity in natural phytoplankton communities. Interpreting results of such studies is, however, complicated by the fact that both nutrient status and light history (photoinhibition under in situ conditions) are known to influence Fv/Fm. Thus, the value of Fv/Fm measurements in the field would be greatly enhanced if the light history signal could be separated from other influences. Here, both field and laboratory studies demonstrate that dark treatment (30 min–4 h) before FRRF measurement is not sufficient to remove a light history signal in Fv/Fm. The signal could, however, be essentially eliminated by incubation of samples in low light prior to Fv/Fm determination. For the study conditions tested, the most effective treatment for removal of the signal was 4 h at 50 µmol m–2 s–1. However, the effectiveness of the light treatment in removing the light history was influenced by temperature. Therefore, no universal protocol for eliminating the light history signal can be developed, but recommendations are given for developing site-specific approaches for separating the light history signal from other factors influencing Fv/Fm. Carrying out light incubations before determining Fv/Fm not only provides the possibility for eliminating a light history signal in the measurements but the difference between Fv/Fm measured after light and dark incubations appears also to be a potentially useful indicator of the degree of photoinhibition experienced by phytoplankton under natural conditions.