Real-time spectral techniques for the detection of buildup of valuable compounds and stress in microalgal cultures: Implications for biotechnology

Single-cell algae (microalgae) are among the most promising resources for the production of biofuels and bioactive compounds, as well as for CO₂biomitigation and bioremediation. Improvement of microalgal photobiotechnologies for the production of value-added products such as long-chain polyunsaturated fatty acids, storage triacylglycerols and carotenoids, requires fast and reliable, and preferably non-destructive techniques for on-line monitoring of the target product's content and the physiological condition of the algal culture. These techniques can provide essential information for timely and informed decisions on adjusting illumination conditions and medium composition, and on the optimal time for biomass harvesting. Often, such decisions must be taken within hours, and mistakes can lead to a significant reduction in productivity or a total loss of the culture. A promising approach for real-time non-destructive monitoring of laboratory and upscaled microalgal cultures is based on measuring the optical properties of algal suspensions, such as absorption, scattering and reflection of light by microalgal cells in certain spectral regions. To this aim, the following criteria should be met: i) reliable spectral measurements, ii) efficient algorithms for the processing of spectral data, and iii) a thorough understanding of the relationships between changes in physiological condition and/or biochemical composition of the algal culture and accompanying changes in its optical properties. This chapter presents a review of recent experimental work in this area, with an emphasis on investigations conducted by the authors and their colleagues in the fields of physiology, biochemistry and spectroscopy which have implications for the cultivation of biotechnologically important microalgal species.