Manipulation of photon absorption cross-section areas and biomass productivity of unicellular algal bioreactors under continuous illumination

Present knowledge regarding manipulation of photon absorption cross-sectional areas of unicellular algal cells and its effect on bioproductivity is limited and cannot be applied to large-scale biomass production. Expecting that in the future such knowledge will come forward, this paper discusses the effect of manipulation of the photon absorption cross-sectional area of the PS II chlorophyll antenna on bioproductivity of flat-plate bioreactors under continuous illumination. A simple model for biomass generation in flat-plate bioreactors is developed. Two cross-sectional manipulation procedures aimed at optimizing reactor productivity are discussed: (1) finding an optimal constant cross-sectional area and (2) finding an optimal cross-sectional area profile that varies with depth in the reactor. It is well known that at low culture-density, photon exploitation efficiency is high at low photon flux densities (linear part of a biomass P-I curve) and diminishes in inverse proportion to flux density at high fluxes. Consequently, if instead of irradiating a given area of a low-culture density by a high photon flux density, the total flux is spread over a larger reactor surface-area at low flux densities, productivity per 1 m2 of reactor surface increases. Here, it is shown that the same idea also applies to high-culture density reactors and that the effect can be amplified significantly through judicious manipulation of the photon absorption cross-sectional area of the antenna. Compared to usual "natural" reactors (photon absorption cross sections are ≈1 nm2), bioproductivity of reactors operating under optimized photon absorption cross-sectional area may be 2-4 times higher.