Light attenuation by Nannochloropsis salina cultures growing in bioreactors




Sifontes, Jesse


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The availability of photosynthetic radiant energy within microalgae cultures is a primary factor determining the synthesis of biomass. As light penetrates the aqueous medium of algae cultures its flux density decreases exponentially with depth, and this decrease with depth intensifies with increasing cell density. This phenomenon is described by the Beer-Lambert’s Law, and it provides a method to calculate the availability of photosynthetic radiant fluxes within algae cultures. Estimates of photosynthetic photo flux densities within microalgae cultures can be used to estimate microalgae photosynthetic rates (PPFD) and, therefore, the potential growth rate of an algae culture. The objective of this research was to quantify the attenuation of photosynthetic photon fluxes as they penetrate cultures with a wide range of cell populations of the microalgae species Nannocholoropsis salina growing in flat bioreactors. The ultimate goal of this research was to obtain a mathematical expression of the dependency of the Beer-Lambert’s coefficient of attenuation on a wide range of incident light flux densities and cell populations of this unicellular species with potential applicability in computerized modeling tools developed for research and/or management of production systems using this microalgae species. The attenuation of a wide range of incident photosynthetic photon flux densities (PPFD) passing through 0.1016-m deep microalgae cultures with cell populations ranging from about 10 x 106 mL-1 to about 275 x 106 mL-1 was characterized using four controlled-environment flat panel bioreactors operating in the Microalgae Physiology Laboratory at the Texas A&M AgriLife Research and Extension Center in Corpus Christi, Texas. The wide range of incident PPFD levels was generated by the combination of 1) differences in spatial distribution of incident light over the lighted side of the bioreactor, 2) various distances between the light source and the bioreactor, and 3) the addition of light-attenuating shades between the light source and the bioreactor. The global equation representing the Beer Lambert’s Law coefficient of attenuation across different light intensities and bioreactors was determined as y = 2 * 10-06 x3 - 0.0019x2 + 0.5912x + 7.0564, where x, (x>0) is the culture’s cell population in millions per mL.



Beer Lambert Law, incident photosynthetic photon flux densities, Light attenuation, Nannocholoropsis salina, Physics in algae



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