Principles for attaining maximal microalgal productivity in photobioreactors: an overview

Efficient management of mass algal cultures requires appreciation of the most important factors governing the light regime of the average cell, i.e. the interrelationships between the intensity of the light source - never the sole factor involved in mass culture productivity - and the optimal cell density affected by the optical path. The latter is a dominant factor in photosynthetic productivity of ultra high cell density cultures (UHDC) cultured in flat plate reactors. Indeed, a very short optical path (5-10 mm) permits a most efficient use of strong light by facilitating ultra-high cell densities (ca. 10-20 g dry cell mass I-I), in which the condensed cells are exposed to very high frequency light/dark cycles. Another important feature of dense cultures concerns the very small but highly efficient light dose available to cells under extreme mutual shading. The low productivity of the single cell in the culture is well compensated, in terms of culture productivity, by the high culture cell mass exposed to very high frequency light/dark cycles. The combined effects of all these factors result in high efficiency of strong light-use for photosynthesis. UHDC are associated with growth inhibition which represents a severe production obstacle. Once this aspect is better understood and managed, UHDC in ultra short optical path reactors may become a useful production mode of photoautotrophic cell mass and secondary metabolites.