Description and application of the background irradiance gradient-single turnover fluorometer (BIG-STf)
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, Fv/Fm) and functional cross-sectional area of PSII (σPSII) 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 (φC-E) and its saturation intensity index (E k,φ) to Fv/Fm and σPSII as a function of light (Fv/Fm-E and σPSII- E) and their saturation intensity indices (Fk,PSII). It is shown that in addition to changes in dark-measured values (φC,max, F v/Fm(0) and σPSII(0)) there are also significant changes in the shapes of φC-E, Fv/F m-E and σPSII-E curves in response to N starvation. Changes in the shape of the curves (summarized by approximate 2-fold decreases in both Ek,φ and Ek,PSII) are consistent with the observed ∼2-fold increase in σPSII (r2 = 0.74). These results suggest that in response to N starvation (1) most of the decrease in φC can be explained by PSII-dependent processes, (2) there are decreased saturation intensities of Fv/Fm and φC with concomitant increases in σPSII(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.
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