Enhanced reducing sugar production and extraction for Chlorella vulgaris in mixotrophic cultivation using high hydrostatic pressure processing and ultrasound

ABSTRACTAlthough extraction of polysaccharides to convert reducing sugars (RS) from microalgae by acid or alkali pretreatments and enzymatic hydrolysis has been extensively studied, few reports exploring the use of high hydrostatic pressure processing (HHP) and ultrasonication (US) as emerging technologies for the extraction of sugars from microalgae biomass exist. Thus, the present study was conducted to determine the effects of mixotrophic growth and stress conditions (NaNO3 and CO2 concentration and light intensity) on RS and protein accumulation in the unicellular green alga Chlorella vulgaris in addition to optimization of the effectiveness of the sequential applications of HHP and US with dilute acid as well as simultaneous enzymatic saccharification on the production of RS from microalga cells. High light intensity, high CO2 concentration and limited nitrogen concentration promoted RS production. The maximum protein content (0.0683 mg g‒1) was achieved at 0.3 g l‒1 NaNO3 concentration, 7000 μmol photons m‒2 s‒1 and 6 l min‒1 CO2 concentration. The highest RS content of C. vulgaris after 48 h enzymatic saccharification (583.86 ± 13.23 mg g‒1) was obtained at 1% (w/w) acid concentration and 80% amplitude for 30 min with 79.4% RS yield. Combined US-assisted dilute acid pretreatment and enzymatic hydrolysis were also found to be more effective than HHP assisted dilute acid pretreatment and enzymatic saccharification. Therefore, microalgal biomass can be considered a suitable renewable feedstock used in fermentation.Highlights The cultivation period of Chlorella vulgaris was reduced from 25 days to 14 days using mixotrophic growing conditions.Mixotrophic conditions enhanced reducing sugar productivity.Novel extraction techniques enhanced the extraction of reducing sugar from microalgae.KEYWORDS: High hydrostatic pressure processinglight intensitymicroalgamixotrophic cultivationnitrogen starvationultrasonication AcknowledgementsThe authors would like to thank the Innovative Food Technologies Development Application and Research Center (YENIGIDAM) of Bolu Abant Izzet Baysal University (BAIBU) for HHP analyses, and I. Isci for his help with construction of the lab-scale photobioreactor.Disclosure statementNo potential conflict of interest was reported by the author(s).Author contributionsS. Uzuner: conducted a research and investigation process, specifically performed the experiments, or data/evidence collection, formal analysis (application of statistical or mathematical techniques to analyze or synthesize study data), wrote original draft (preparation and/or creation of the published work, specifically writing the initial draft); G.A. Evrendilek: specifically performed the experiments, or data/evidence collection, wrote review and edited specifically critical review, commentary or revision-including pre- or post-publication stages; S. Kurhan: performed the experiments, or data/evidence collection.Additional informationFundingThis work was supported by Republic of Turkey Ministry of Agriculture and Forestry General Directorate of Agricultural Research and Policies under a project [TAGEM/16/AR-GE/36].