Abstract
Marine bioresources were explored for their application to bioconversion strategies, primarily for the production of bioethanol. 58 yeasts isolated from marine environmental samples were screened for their tolerance to a range of chemicals. Of these yeasts, the marine yeast Wickerhamomyces anomalus M15 has been identified as a potential candidate for fermentation; in this study its tolerance of substrates such as glucose and xylose, products such as ethanol and common inhibitory compounds formed during pre-treatment processes were investigated and compared to a tolerant terrestrial reference strain Saccharomyces cerevisiae NCYC 2592. W. anomalus M15 was then used to ferment a range of different biofuel feedstocks, such as the macroalga Ulva, the lignocellulosic grass Miscanthus and sugarcane molasses.W. anomalus M15 was initially investigated for its osmotolerance, halotolerance, ethanol tolerance and tolerance of inhibitory compounds. Microplate reader growth assays were used to determine that W. anomalus M15 was capable of growth in glucose concentrations as high as 500 g/L and a theoretical tolerance of a maximum 570 g/L glucose. It was also capable of fermentation in high concentrations of glucose, achieving an ethanol concentration of 41.66 g/L. Further investigation of high gravity fermentations produced a maximum of 95.45 g/L ethanol from a fed-batch fermentation of 250 g/L glucose. When investigated using a microplate reader, its halotolerance was estimated to be around 100 g/L sodium chloride and 200 g/L ethanol, although when conducted in small scale fermentations its tolerance limit of ethanol was lower. Tolerance of 9 common inhibitor compounds was also assessed using a microplate reader for both W. anomalus M15 and S. cerevisiae NCYC 2592. W. anomalus M15 exhibited similar or better tolerance of most inhibitory compounds tested, and although it was slightly more sensitive to furans, it exhibited a higher tolerance compared to yeast strains assessed in other studies.
W. anomalus M15 was subsequently used for the fermentation of macroalgal biomass. Three different seaweeds native to the UK coastline: an Ulva, Laminaria and Porphyra sp. were assessed for their suitability for fermentation. Simulated hydrolysates prepared from pure sugars based on the composition of the three algae were initially used. Using a fed-batch fermentation technique and synthetic hydrolysates of U. linza, a maximum ethanol production of 84 g/L was achieved. The fermentation was carried out in both seawater and reverse osmosis water, and identical yields were produced. This is a promising result for seawater-based fermentations, a potential method of reducing the water footprint of bioethanol production. Ulva spp., foraged from UK beaches was pre-treated and used for fermentation. Actual Ulva hydrolysates were fermented to achieve 15.5 g/L ethanol, the concentration of hydrolysates and consequent increased salt content appeared to be the limiting factor on fermentation.
W. anomalus M15 was also used for the fermentation of Miscanthus biomass and sugarcane molasses. Miscanthus hydrolysates were pre-treated using a dilute acid method followed by enzymatic hydrolysis then were fermented to produce a maximum ethanol concentration of 18.04 g/L from 45.27 g/L sugars. Fermentation of a sugarcane molasses and seawater media was conducted in 2 L bioreactors using a 2-stage batch method, a maximum concentration of 60.28 g/L was achieved.
| Date of Award | 15 Dec 2023 |
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| Original language | English |
| Supervisor | Chenyu Du (Main Supervisor) |