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 irradiance requires cultures of high cell density grown in reactors with a narrow light path. The smaller the light-path, the higher the growth rate and the volume output rate (g L^-1d^-1) of cell mass. Areal productivity (g m^-2d^-1) may be inversely related to the length of light-path (e.g. Spirulina platensis) or directly related to it, as is the case with Nannochloropsis 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 Nannochloropsis became evident as cell concentration increased above a certain point. Response in cell growth to elevated irradiance was therefore possible only when the growth medium of ultrahigh cell density cultures was frequently changed. Inhibitory activity to culture growth may be directly involved in determining the optimal cell density (which results in the highest output of cell mass) and hence the optimal light-path. Under optimal growth conditions, cultures of high cell densities responded well to the rate of stirring, the relative beneficial effect of mixing increasing with the increase in cell density.