Measuring photosynthetic parameters in individual algal cells by Fast Repetition Rate fluorometry

A Single Cell Fast Repetition Rate (SCFRR) fluorometer was developed to mea sure the quantum yield of photochemistry, the functional absorption cross s ection of PS II and the kinetics of electron transport on the acceptor side of PS II in individual algal cells. These parameters are used to quantify the cell-specific photosynthetic performance in natural phytoplankton assem bledges in aquatic ecosystems. The SCFRR technique measures chlorophyll flu orescence transients induced by a precisely controlled series of excitation flashlets that cumulatively saturate PS II within 120 mu s. To meet the re quirement in the analysis for single algal cells, the measurements are cond ucted in micro volumes, such that the probability of probing more than one cell at a time is vanishingly low. We designed a novel, computer-controlled hydromechanical system to deliver a portion of the sample into the measuri ng chamber and, following measurement, remove it into one of six sorting co ntainers. The fluorescence signal is induced by a series of high frequency flashlets obtained from high luminosity blue light-emitting diodes and is a cquired by a novel red-sensitive PMT-based detection system exhibiting both high sensitivity and a very wide dynamic range. The wide dynamic range of the detector allows SCFRR measurements for a wide variety of cell sizes ran ging from 1 to 100 mu m equivalent spherical diameter. The compact and ligh t-weight design makes the SCFRR Fluorometer applicable for both laboratory and field studies.