Fluorescence as a diagnostic of nutrient stress

Fluorescence measurements have been used for decades for determination of algal biomass and assessment of physiological status both in the laboratory and the field. They can provide rapid non-invasive indicators of algal physiological stress. Fluorescence-based studies of nutrient stress in phytoplankton have been predominately limited to the measurement of maximum quantum yield for photosystem II (F_v/F_m). Reported results for cultures grown under acclimated N-limited growth conditions showed a decrease in F _v/F_m as a function of N-limited growth rate. My research reports contradictory results using two independent measurement systems showing consistently high F_v/F_m (∼0.65) independent of irradiance and N-limited growth rate. Experiments on nutrient-replete and N-starved cultures contribute to the growing body of evidence for using F_v/F _m as a proxy for N-stress, and show that a decrease in F_v/F _m under N-starved conditions is also a function of pre-conditioned N-limited growth rate. The two independent measurement systems, Turner Designs and Pulse Amplitude Modulation (PAM) Fluorometer, showed strong correlation, although reduction of the actinic measuring beam in the Turner Designs system was required for cultures grown under low-light. Since the basic relationship between N-stress and F_v/F_m breaks down under acclimated balanced N-limited growth, a complementary or more sensitive diagnostic is required.; Active fluorometers (i.e. PAM) have the capability to resolve fluorescence measurements under actinic light providing an opportunity to investigate other fluorescence metrics as a function of irradiance to provide proxies for quantifying physiological stress. I interfaced a light incubation chamber and a PAM fluorometer to allow parallel measurements of fluorescence parameters such as calculated electron transport rates and maximum non-photochemical quenching (NPQ _max) over time as a function of irradiance. This new measurement system eliminates the confounding effects of varying irradiance during measurement that influence results of the established rapid-light-curve method. The new measurement system allows for determination of the parameter of light saturation (E_KF) in F vs. E curves, as well as NPQ_max . Both parameters were a function of growth irradiance. (Abstract shortened by UMI.)...