Fluorescence-based maximal quantum yield for PSII as a diagnostic of nutrient stress

In biological oceanography, it has been widely accepted that the maximum qu antum yield of photosynthesis is influenced by nutrient stress. A closely r elated parameter, the maximum quantum yield for stable charge separation of PSII, (phi (PSII))(m), can be estimated by measuring the increase in fluor escence yield from dark-adapted minimal fluorescence (F-o) to maximal fluor escence (F-m) associated with the closing of photosynthetic reaction center s with saturating light or with a photosynthetic inhibitor such as 3 '-(3,4 -dichlorophenyl)-1 ' ,1 ' -dimethyl urea (DCMU). The ratio F-v/F-m (= (F-m - F-o)/F-m) is thus used as a diagnostic of nutrient stress. Published resu lts indicate that F-v/F-m is depressed for nutrient-stressed phytoplankton, both during nutrient starvation (unbalanced growth) and acclimated nutrien t limitation (steady-state or balanced growth). In contrast to published re sults, fluorescence measurements from our laboratory indicate that F-v/F-m is high and insensitive to nutrient limitation for cultures in steady state under a wide range of relative growth rates and irradiance levels. This di screpancy between results could be attributed to differences in measurement systems or to differences in growth conditions. To resolve the uncertainty about F-v/F-m as a diagnostic of nutrient stress, we grew the neritic diat om Thalassiosira pseudonana (Hustedt) Hasle et Heimdal under nutrient-reple te and nutrient-stressed conditions, using replicate semicontinuous, batch, and continuous cultures. Fv/Fm was determined using a conventional fluorom eter and DCMU and with a pulse amplitude modulated (PAM) fluorometer. Reduc tion of excitation irradiance in the conventional fluorometer eliminated ov erestimation of F-o in the DCMU methodology for cultures grown at lower lig ht levels, and for a large range of growth conditions there was a strong co rrelation between the measurements of F-v/F-m with DCMU and PAM (r(2) = 0.7 7, n = 460). Consistent with the literature, nutrient-replete cultures show ed consistently high F-v/F-m (similar to0.65), independent of growth irradi ance. Under nutrient-starved (batch culture and perturbed steady state) con ditions, F-v/F-m was significantly correlated to time without the limiting nutrient and to nutrient-limited growth rate before starvation. In contrast to published results, our continuous culture experiments showed that F-v/F -m was not a good measure of nutrient limitation under balanced growth cond itions and remained constant (similar to0.65) and independent of nutrient-l imited growth rate under different irradiance levels. Because variable fluo rescence can only be used as a diagnostic for nutrient-starved unbalanced g rowth conditions, a robust measure of nutrient stressed oceanic waters is s till required.