MEPS 283:73-80 (2004)  -  doi:10.3354/meps283073

ABSTRACT: Based on previous single turnover, pulse amplitude modulated, and fast repetition rate fluorometers, I describe a novel, bench top, single turnover fluorometer (BIG-STf) that quantifies multiple biophysical properties of Photosystem II (PSII) of phytoplankton over a programmable range of background light levels. The instrument measures the photochemical conversion efficiency (variable fluorescence/maximal fluorescence yield, F_v/F_m) and functional cross-sectional area of PSII (σ_PSII) over a background light gradient, and generates light-response curves of the biophysical properties of PSII. These curves can be used to assess variability in PSII structure and function or, in conjunction with oxygen- or carbon-derived photosynthesis-irradiance (P-E) curves, to evaluate how the properties of PSII may influence total photosynthetic rate and efficiency. Nitrogen-starved batch cultures of Skeletonema costatum are used to demonstrate the utility of these measurements by comparing the quantum yield of carbon uptake as a function of light (φ_C-E) and its saturation intensity index (E_k,φ) to F_v/F_m and σ_PSII as a function of light (F_v/F_m-E and σ_PSII-E) and their saturation intensity indices (E_k,PSII). It is shown that in addition to changes in dark-measured values (φ_C,max, F_v/F_m(0) and σ_PSII(0)) there are also significant changes in the shapes of φ_C-E, F_v/F_m-E and σ_PSII-E curves in response to N starvation. Changes in the shape of the curves (summarized by approximate 2-fold decreases in both E_k,φ and E_k,PSII) are consistent with the observed ~2-fold increase in σ_PSII (r² = 0.74). These results suggest that in response to N starvation (1) most of the decrease in φ_C can be explained by PSII-dependent processes, (2) there are decreased saturation intensities of F_v/F_m and φ_C with concomitant increases in σ_PSII(0), and (3) the turnover rate (1/τ) does not change significantly. The PSII light-response curves, which can be measured quickly by the BIG-STf instrument, provide a direct means of evaluating the role that PSII plays in regulating photosynthetic rates and efficiency in aquatic environments.

KEY WORDS: Fluorescence · Photosystem II · Instrumentation · Fluorometer · Nitrogen starvation · Skeletonema costatum