Abstract
Euglena gracilis produces paramylon, which is a feedstock for high-value functional foods and nutritional supplements. The enhancement of paramylon productivity is a critical challenge. Microalgae growth-promoting bacteria (MGPB) can improve microalgal productivity; however, the MGPB for E. gracilis remain unclear. This study isolated bacteria capable of enhancing E. gracilis growth and paramylon production under mixotrophic conditions. Enterobacter sp. CA3 and Emticicia sp. CN5 were isolated from E. gracilis grown with sewage-effluent bacteria under mixotrophic conditions at pH 4.5 or 7.5, respectively. In a 7-day E. gracilis mixotrophic culture with glucose, CA3 increased E. gracilis biomass and paramylon production 1.8-fold and 3.5-fold, respectively (at pH 4.5), or 1.9-fold and 3.5-fold, respectively (at pH 7.5). CN5 increased E. gracilis biomass and paramylon production 2.0-fold and 4.1-fold, respectively (at pH 7.5). However, the strains did not show such effects on E. gracilis under autotrophic conditions without glucose. The results suggest that CA3 and CN5 promoted both E. gracilis growth and paramylon production under mixotrophic conditions with glucose at pH 4.5 and 7.5 (CA3) or pH 7.5 (CN5). This study also provides an isolation method for E. gracilis MGPB that enables the construction of an effective E. gracilis–MGPB-association system for increasing the paramylon yield of E. gracilis.
Highlights
Microalgal biomass is a sustainable and renewable feedstock for value-added products, such as feeds and foods, food supplements, pharmaceuticals, personal care products, and biofuels [1]
E. gracilis clearly showed faster growth at both pH values when co-cultured with sewage-effluent bacteria than without (Figure S1)
This suggested that Microalgae growth-promoting bacteria (MGPB) for E. gracilis must have been present among the sewage-effluent bacteria and supported E. gracilis growth under mixotrophic conditions at pH 4.5 or 7.5
Summary
Microalgal biomass is a sustainable and renewable feedstock for value-added products, such as feeds and foods, food supplements, pharmaceuticals, personal care products, and biofuels [1]. Euglena gracilis, a freshwater unicellular flagellated microalga, is among the most promising industrially applicable microalga for producing commercial sources of high-value-added products [2]. Microorganisms 2021, 9, 1496 activities [15]. Paramylon is recognized as a feedstock for high-value functional foods and nutritional supplements. E. gracilis can grow and accumulate paramylon under both low and neutral pH conditions [9]. The cultivation of E. gracilis under acidic conditions gives it an important advantage over other microalgae, as it prevents microbial and bacterial contaminations and enables large-scale cultivation for the paramylon industry. Enhancing the growth and paramylon productivity of E. gracilis is the most important challenge to realizing highly efficient E. gracilis-based industrial paramylon production
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