Upper limits of photosynthetic productivity and problems of scaling

Some 1,370 W m(-2) of light energy reaches the outer atmosphere of earth and on average only 240 W m(-2) reaches the earth's surface. Only a fraction of this is used to fix CO2 through photosynthesis, and efficiencies ranging from 0.1-8% for total irradiance have been reported. The theoretical maximum quantum efficiency of carbon fixation is 0.125 mol C ( mol quanta)(-1) which relates to a maximum productivity of about 12 g C m(-2) day(-1) or 29.8 g(dw) m(-2) day(-1). This could increase to a maximum of 200 g( dw) m(-2) day(-1) in intermittent light of high frequencies, which is on average eight times higher than the average measured under field conditions where rates approaching 25 g(dw) m(-2) day(-1) are considered high. Several possibilities exist for achieving higher yields and photosynthetic efficiencies, such as limiting the antennae sizes and pulsing light at frequencies equivalent to electron turnover in the electron transport chains of photosynthesis. Scaling from laboratory experimental conditions to large commercial photobioreactors is a major stumbling block and may be the single most important factor responsible for the overall low reported areal production rates.