A numerical study is presented aiming to maximize the solar to hydrogen energy conversion efficiency of a mixed culture containing microorganisms with different radiation characteristics. The green algae Chlamydomonas reinhardtii CC125 and the purple non-sulfur bacteria Rhodobacter sphearoides ATCC 49419 are chosen for illustration purposes. The previously measured radiation characteristics of each microorganism are used as input parameters in the radiative transport equation for calculating the local spectral incident radiation within a flat panel photobioreactor. The specific hydrogen production rate for each microorganism as a function of the available incident radiation is recovered from data reported in the literature.The results show that for mono-cultures, the solar to H2 energy conversion efficiency, for all combinations of microorganism concentrations and photobioreactor thicknesses, fall on a single line with respect to the optical thickness of the system. The maximum solar energy conversion efficiency of mono-cultures of C. reinhardtii and R. spaheroides are 0.061 and 0.054%, respectively, corresponding to optical thicknesses of 200 and 16, respectively. Using mixed cultures, a total conversion efficiency of about 0.075% can be achieved corresponding to an increase of about 23% with respect to that of a mono-culture of C. reinhardtii. It has been shown that the choice of microorganism concentrations for maximum solar energy conversion efficiency in mixed cultures is non-trivial and requires careful radiation transfer analysis coupled with H2 production kinetics taking into account the photobioreactor thickness.