Bioethanol production from defatted biomass of Nannochloropsis oculata microalgae grown under mixotrophic conditions.

Nashwa A H Fetyan,Mahmoud W Sadik,Fatma M Ibrahim,Yasser A Attia,Abo El-Khair B El-Sayed

doi:10.1007/s11356-021-15758-6

Abstract

In order to improve the economic feasibility and environmental sustainability of microalgal bioethanol production, a nontoxic, copious agricultural waste, sugarcane bagasse aqueous extract (SBAE) was used for cultivating Nannochloropsis oculata microalga (NNO-1 UTEX Culture LB 2164) as potential sources of substitutes for traditional nutrition to reduce the costs in cultivation through acid digestion and enzymatic treatment before being fermented by Saccharomyces cerevisiae (NRRLY-2034). The primary target of this research was to find out the ethanol from hydrolysate of the defatted biomass of N. oculata grown mixotrophically on SBAE and CO2 as carbon sources. For acid hydrolysis (AH), the highest carbohydrate yield 252.84 mg/g DW has been obtained with 5.0% (v/v) H2SO4 at 121 °C for 15 min for defatted biomass cultivated mixotrophically on sugarcane bagasse aqueous extract (SBAE) regarding 207.41 mg/g DW for defatted biomass cultivated autotrophically (control treatment). Whereas, the highest levels of reducing sugars has been obtained with 4.0% (v/v) H2SO4 157.47±1.60 mg/g DW for defatted biomass cultivated mixotrophically compared with 135.30 mg/g DW for the defatted control treatment. The combination of acid hydrolysis 2.0% (v/v) H2SO4 followed by enzymatic treatment (AEH) increased the carbohydrate yields to 268.53 mg/g DW for defatted biomass cultivated mixotrophically on SBAE regarding 177.73 mg/g DW for the defatted control treatment. However, the highest levels of reducing sugars have been obtained with 3.0% (v/v) H2SO4 followed by enzyme treatment that gave 232.39±1.77 for defatted biomass cultivated mixotrophically on SBAE and 150.75 mg/g DW for the defatted control treatment. The sugar composition of the polysaccharides showed that glucose was the principal polysaccharide sugar (60.7-62.49%) of N. oculata defatted biomass. Fermentation of the hydrolysates by Saccharomyces cerevisiae for the acid pretreated defatted biomass samples gave ethanol yield of 0.86 g/L (0.062 g/g sugar consumed) for control and 1.17 g/L (0.069 g/g sugar consumed) for SBAE mixotrophic. Whereas, the maximum ethanol yield of 6.17±0.47 g/L (0.26±0.11 g/g sugar consumed) has been obtained with samples from defatted biomass grown mixotrophically (SBAE mixotrophic) pretreated with acid coupled enzyme hydrolysis.

Highlights

Bioenergy has gained more attention in recent decades due to the depletion of fossil fuel forms and increasing requirements for eco-friendly energy to alleviate global warming
For acid hydrolysis (AH), the highest carbohydrate yield 252.84 mg/g DW was obtained with 5.0% (v/v) H2SO4 at 121°C for 15 min for defatted biomass cultivated mixotrophically on sugarcane bagasse aqueous extract (SBAE) with respect to 207.41 mg/g DW for defatted biomass cultivated autotrophically, Whereas, the highest levels of reducing sugars was obtained With 4.0%(v/v) H2SO4 157.47 ± 1.60 mg/g DW for defatted biomass cultivated mixotrophically in compared with 135.30 mg/g DW for the defatted control treatment
The highest levels of reducing sugars were obtained with 3.0% (v/v) H2SO4 followed by enzyme treatment gave 232.39 ± 1.77 for defatted biomass cultivated mixotrophically on SBAE and 150.75 mg/g DW for the defatted control treatment

Summary

Introduction

Bioenergy has gained more attention in recent decades due to the depletion of fossil fuel forms and increasing requirements for eco-friendly energy to alleviate global warming. Microalgae in autotrophic cultivation depend on light energy to produce carbohydrates, while in heterotrophic cultivation organic carbon sources are directly used instead of light (Ceron-Garcia et al 2005). Microalgae may use both inorganic carbons fixed by photosynthesis and organic carbons such as glucose, acetate, and sucrose (Wen et al 2019; Cheah et al 2018; Dragone et al 2010). In order to overcome economic problems combined with microalga production (cultivation and processing), the biomass remaining after extracting oil and other valuable compounds should be used in the production of ethanol after proper pretreatment to liberate fermentable sugars (Sivasankar et al 2017; Nobre et al 2013; Chaudhary et al 2014). Before being used as a raw material for ethanol processing, microalgae biomass can be mechanically, chemically, or enzymatically pretreated (Maurya et al 2016; ElSayed et al 2017 and 2015)

Objectives

Methods

Results

Discussion

Conclusion