O efeito da luz em cultivos fotossintéticos microalgais

Abstract

Microalgae are the main natural source of a wide range of valuable compounds such as lipids, proteins, carbohydrates, pigments, and others. Despite many applications, only a few microalgae species are cultivated commercially because of the poorly developed cultivation process. Nowadays, some culture strategies have been used to increase the production of biomass and valuable compounds. In this case, the use of different light strategies in terms of quality, intensity, and photoperiods are recognized as key parameters in improving the yields of microalgae cultures, as well as in optimizing intracellular products. Given this scenario, the present work is based on an exploratory study about the effect that different lighting characteristics can affect microalgal photosynthetic cultures in order to improve the bioprocesses and compounds of interest. The work carried out in this doctoral thesis demonstrated that although the area of development of microalgae production systems has evolved a lot in recent years, until now, there is no system free of limitations, and these are closely related to the input of light energy in the cells. Thus, it was observed that the amount of light energy received and stored by cells in different photoperiods is directly related to biomass productivity and cell growth rate. The photoperiod conditions used in the study were separated into three groups: long-term photo-cycles (24:0, 22:2, 20:4, 18:6, 12:12, and 10:14 (h:h)), frequency photo-cycles (2, 4, 8, 12, 24, and 48 times per day (t/d)), and short photo-cycles (0.91:0.09, 0.83:0.17, 0.75:0.25, and 0.50:0.50 (s:s)) of light:dark, respectively. The results showed that the microalgae Scenedesmus obliquus CPCC05 was able to store enough energy to maintain its metabolism for up to 2 continuous hours of darkness. Among all the photo-cycles tested, the 22:2 h when divided into 2 cycles per day showed the highest cell productivity (21.43 mg/L.h), with a growth of up to 3700 mg/L. The results also showed that the modulation of light cycles improves the amount of energy received by the photosynthetic apparatus, positively increasing the production of lipids, fatty acids, protein amino acids, and sterols in Scenedesmus obliquus. The 2 t/d photo-cycle was the best condition for the production of total sterols, while the values of polyunsaturated fatty acids, lipids, and protein amino acids obtained higher yields in the short photo-cycle of 0.75:0.25. In general, although the use of sunlight as a light source in microalgae cultures is free and abundant, the ease of manipulating the ideal conditions of artificial lighting in photobioreactors can reduce the demand for light energy and, consequently, develop and optimize the systems. However, it is necessary to optimize these processes to mitigate the costs related to artificial energy and subject them to sustainability assessments so that they are acceptable at a commercial level.