Identification of the types of photosynthetic organisms (PSO) by their fluorescence spectra cannot always be performed. Most types of sea and fresh water algae feature one fluorescence band centred at 685 nm attributed to chlorophyll “a” of the photosystem II (with an exception of blue-green algae, whose spectra exhibit also fluorescence bands of phycobelin pigments). Fluorescence bands of higher plants have more complex structure and, thus, carry more information. However, their spectra depend, for example, on the position of the pump spot on a leaf, so the identification problem arises in this case also. Therefore, the problem of identification of PSO types seems to be very urgent.

In this paper, we investigate the applicability of the fluorescence saturation technique to solving the specified problem. Among the parameters that can be extracted from the fluorescence saturation curves, the most informative for the purposes of the PSO type identification are the effective excitation cross section of the chlorophyll-a molecules and the singlet-singlet annihilation constant. Both these parameters depend on the process of the excitation migration over the photosynthetic unit (PSU) and, therefore, on the structure and the state of PSO. In this paper, we present the results of the numeric solution of direct and inverse problems of the fluorescence saturation spectroscopy based on the models (with different detalization extent) of the photophysical processes in PSU. Different algorithms were tested for solving two- and three-parametric problems. The accuracy of the proposed methods was estimated. The values of these parameters were determined by means of processing the real saturation curves recorded for some types of algae. In combination with the pump-and-probe technique, the fluorescence saturation spectroscopy allows us to identify PSO types and determine its state, which are important both in themselves and for the correct determination of the chlorophyll-a concentration by the fluorescent method.