Development 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 (óPSII) 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 (öC-E) and its saturation intensity index (Ek,ö) to Fv/Fm and óPSII as a function of light (Fv/Fm-E and óPSII-E) and their saturation intensity indices (Ek,PSII). It is shown that in addition to changes in dark-measured values (öC,max, Fv/Fm(0) and óPSII(0)) there are also significant changes in the shapes of öC-E, Fv/Fm-E and óPSII-E curves in response to N starvation. Changes in the shape of the curves (summarized by approximate 2-fold decreases in both Ek,ö and Ek,PSII) are consistent with the observed ~2-fold increase in óPSII (r2 = 0.74). These results suggest that in response to N starvation (1) most of the decrease in öC can be explained by PSIIdependent processes, (2) there are decreased saturation intensities of Fv/Fm and öC with concomitant increases in óPSII(0), and (3) the turnover rate (1/ô) does not change significantly. The PSII lightresponse 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.

Duke Scholars

Author Zackary Johnson Marine Science and Conservation