Exploring Light Spectrum Effects on Large-Scale Neochloris oleoabundans cultivation

In recent years, microalgae cultivation has emerged as a rapidly expanding global industry due to the capability of these photosynthetic micro-organisms to produce high-value bioproducts and, more broadly, compounds potentially useful for biotechnological purposes. Among factors involved in industrial microalgae cultivation, light plays a fundamental role, and its effect largely depends on the species being cultivated. Light can influence not only cell density and size, but also metabolic processes, promoting or affecting different pathways. Among light features, wavelength is known to mainly impact the production and/or accumulation of primary and secondary metabolites; therefore, it is essential to investigate how the growth capabilities and biochemical composition of the biomass vary when exposed to different light spectra. Most studies on light induced biochemical and metabolic changes in microalgae are conducted on a laboratory scale and are essential for establishing baseline data. Obtaining comparable results from both lab- and larger-scale cultivation is challenging due to difficulties in controlling optimal growth parameters; consequently, it is crucial to validate the results achieved on a lab-scale through a scale-up system. This approach provides relevant biological insights and technical information for industrial-scale cultivation. The green microalga Neochloris oleoabundans (UTEX 1185) was chosen as target species due to its well-documented capability of producing great amounts of lipid-enriched biomass, its flexibility to cultivate, and its capacity to utilize organic substrates. These properties make this microalga suitable for biodiesel production, however interest has recently shifted towards various other high-value applications. Historically the focus has been on lipid production for biodiesel, although attention has now turned towards the production of bioactive metabolites, such as molecules with antioxidant and anti-inflammatory effects (e.g., pigments and lipids), providing new perspectives and opportunities. The present work aimed at testing growth capability and biomass characteristics of N. oleoabundans cultivated in photobioreactors and exposed to different light spectra. The baseline cultivations conditions were founded upon lab-scale optimisation studies conducted at the University of Ferrara (Italy). The conditions were adapted to test the process scalability at the Culture Collection of Algae and Protozoa (CCAP-ARIES Centre, Oban, UK). N. oleoabundans was cultivated in 70L photobioreactors, employing two light spectra with different ratios of red and blue light (60:40 as control and 90:20 as treatment). Growth (cell density and dry weight) was monitored over 14 days growth. The algal biomass quality was evaluated in terms of total protein and carbohydrate content, pigments (both total pigments and carotenoids), and antioxidant capacity. In terms of both growth and biochemical composition, the results indicated differences between the two trials, particularly towards the end of the cultivation. However, there were no significant differences in the final dry weight obtained, with yields of 0.14 and 0.15 g/L, but algal biomass composition appeared modified by light treatment. Results suggest that the treatment with a higher proportion of red light leads to an interesting composition (proteins, carbohydrates) and bioactivity (antioxidant properties) of the N. oleoabundans biomass which can be suggested for biotechnological applications, such as food, medicine, cosmetics, among others. The results obtained have identified a promising cultivation method suitable for large-scale application; they have also provided insights into the microalgal biomass produced under different light spectra, offering additional guidelines for species scalability. Finally, this study helps bridge the significant gap that still exists between laboratory-scale experimentation and larger-scale cultivation; a major bottleneck hindering widespread commercialization of microalgae.