Characterization of the effect of UV-A light and agitation on the exopolysaccharide production of Chlorella vulgaris and Porphyridium cruentum

Abstract

Microalgae species are photosynthetic microorganisms that are a sustainable source of bioproducts due to their ability to reduce anthropogenic carbon dioxide in the atmosphere. The number of studies focused on their biologically active molecules, such as lipids, proteins, polysaccharides, and pigments, has been increasing in the last years due to their promising application as valuable products. In this context, the exopolysaccharides (EPS) from microalgal sources stand out as high-value molecules for their potential applications in the nutraceutical and pharmaceutical industries. However, studies aimed to find strategies and optimal conditions to promote the biosynthesis of EPS are still required to make these molecules economically feasible. The effect of ultraviolet light A (UV-A) is studied on biomass and EPS productivity of the red microalgae Porphyridium cruentum and the green microalgae Chlorella vulgaris testing three levels: L0, L1 and L2. Likewise, the effect of the agitation factor on cell growth and EPS productivity are analyzed for the two microalgae species testing two levels: A0 and A1. After recovering the EPS using centrifugation and diafiltration, their potential antioxidant activity was tested using the 2,2-Diphenyl-1-prykylhydrazyl (DPPH) radical scavenging assay. The UV-A light (315-400 nm), along with photosynthetic active radiation (PAR) increased EPS productivity in both C. vulgaris and P. cruentum to 1.21-fold and 2.43-fold, respectively, compared to light control conditions (PAR at 35.6 µmol m⁻² s⁻ ¹). The highest P. cruentum EPS productivity was 8.67 mg/g DW biomass at the highest tested UV-A level and lowest agitation(L2A0), and the highest biomass concentration was 8.57 g/L at the highest agitation (A1) due to a possible improvement in nutrient distribution. For C. vulgaris, the highest productivity was 1380 mg/g DW biomass at intermediate UV-A light level (L1). The highest biomass concentration of C. vulgaris was 0.34 g/L at the highest UV-A level (L2), treatment that conversely displayed the lowest EPS productivity (73.08 mg/g DW biomass). The study of antioxidant activity revealed that EPS show DPPH radical scavenging activity. The mean highest radical scavenging effect (%) of P. cruentum and C. vulgaris EPS was 56.48 ± 4.46 and 46.31 ± 3.2 at 4 mg/mL and 2 mg/mL, respectively. This work contributes to the characterization of abiotic factors that could potentially influence the production of high-value EPS molecules that display bioactivity of interest for health applications.