Efficient utilisation of high photon irradiance for mass production of photoautotrophic micro-organisms

Basic issues involved in effective use of a high photon irradiance for mass production of microalgae are elucidated: efficient utilisation of high irr adiance requires cultures of high cell density grown in reactors with a nar row light path. The smaller the light-path, the higher the growth rate and the volume output rate (g L(-1)d(-1)) of cell mass. Areal productivity (g m (-2)d(-1)) may be inversely related to the length of light-path (e.g. Spiru lina platensis) or directly related to it, as is the case with Nannochlorop sis sp., in which the areal output rate increased with the increase in the light-path and the areal volume (L m(-2)). Inhibition of cell growth in Nan nochloropsis became evident as cell concentration increased above a certain point. Response in cell growth to elevated irradiance was therefore possib le only when the growth medium of ultrahigh cell density cultures was frequ ently changed. Inhibitory activity to culture growth may be directly involv ed in determining the optimal cell density (which results in the highest ou tput of cell mass) and hence the optimal light-path. Under optimal growth c onditions, cultures of high cell densities responded well to the rate of st irring, the relative beneficial effect of mixing increasing with the increa se in cell density.