Microalgal Photosynthesis and Growth in Mass Culture

Abstract

Abstract The development of large-scale outdoor microalgae production requires a thorough understanding of microalgal growth which should be encompassed in a mathematical model. The model should be as simple as possible allowing use in outdoor practice by persons with varying backgrounds. This chapter provides a basis for such a model connecting microalgal growth and photobioreactor productivity to light exposure. Only light exposure is included as an environmental variable because sunlight irradiance will ultimately limit areal productivity and all other cultivation parameters must be balanced to that number. Within microalgal mass cultures inside photobioreactors a light gradient will develop determining microalgal growth. This light gradient depends on microalgal specific light absorption and biomass concentration. Based on the light gradient the local rates of photosynthesis are calculated and integrated over reactor volume. The model is connected to our current understanding of photosynthesis by adopting proven photosynthesis models developed by Blackman and Jassby & Platt, and employing efficiency parameters based on theoretical evaluations and practical experiments. The wavelength dependency of light absorption is included. Photosynthesis is then connected to microalgal growth adopting the model of Pirt and distinguishing between maintenance-related respiration and growth-related respiration. The model is used to analyze productivity of simple photobioreactor geometry (1-dimensional light path) and calculate the limits of light-use efficiency. At the end of the chapter the assumptions and simplifications made are discussed in the light of possible effects of photoacclimation, mixing along the light gradient, day/night cycles, and the complexity of accurately modeling the light field.