Diagnostics of photosynthesizing organisms by the method of non-linear fluorimetry

Use of pulsed lasers opens new opportunities in diagnostics of photosynthetic organisms (PSO). Due to use of pico- and femtosecond laser spectroscopy [1,2] - absorption spectroscopy and fluorescence spectroscopy - was achieved large progress in study of primary processes of photosynthesis. Orders of magnitudes of many photophysical parameters were determined. In this paper it is proposed for PSO diagnostics one more laser method - method of nonlinear fluorimetry (saturation fluorimetry). This method is based on measuring of nonlinear dependence of fluorescence intensity In on exciting laser radiation intensity L,,c. Because of high local concentration of pigment molecules in PSO chloroplasts dependence I-fl(I-exc) deviates from linear one at sufficiently low values of intensity I(exc)approximate to 1 kW/cm(2). This effect on the one hand complicates the determination of chlorophyll a concentration from the fluorescence intensity; but on the other hand it opens up the possibility of determination of photophysical parameters. The procedure for the determination of the non-saturated fluorescence parameter (Do which is proportional to concentration of chlorophyll a molecules and the photophysical parameter A has been elaborated. Parameter A is given by A=sigma iota(2)gamma n(0), where a is the excitation cross-section of chlorophyll a (Chl a) molecules, T is the lifetime of the excited chlorophyll a molecules taking into account all processes of a deactivation of excitation except the singlet-singlet annihilation, and gamma n(0) is the maximum rate of the singlet-singlet annihilation. In the paper investigations were carried out with aquatic PSO -phytoplankton (PP).