Sustainable Solutions for Environmental Pollution. Группа авторов

Sustainable Solutions for Environmental Pollution - Группа авторов


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the 2 d SRT. Opposingly, the system B failed to enhance the lipid extractability. It initially (e.g., at a 2 d SRT) washed out the lipid fermenters, but it was unable to recover them when the system switched to a 6 d SRT. Ultimately, demonstrated by multiple studies, the further studies are warranted to bring this technology forward towards further development, scaling-up and potential commercialization.

      1.3.5 Acetoin

      In general, acetoin is a substance that cannot be generated as the sole end product during the anaerobic fermentation due to being more oxidized form than other typical substrates, such as glucose (Förster et al., 2017). Hence, the acetoin is generated under a specific condition during fermentation by Bacillus subtilis and serval Enterobacteria species, for instance: (1) occurs as an intermediate in the formation of 2,3-butanediol in butanediol fermentation; (2) produced in a two-step process from pyruvate (Cheynier et al., 2010; Förster et al., 2017; Härtig and Jahn, 2012; Krieg and Padgett, 2011). Briefly describing the two-step process, the acetolactate synthase (AlsS) first catalyzes the condensation of two molecules of pyruvate to acetolactate, where one molecule of CO2 is released during this reaction. Then, acetolactate is decarboxylated by the acetolactate decarboxylase to produce acetoin (Nicholson, 2008; Ramos et al., 2000).

      To date, several limitations in acetoin synthesis have been reported. For instance, only low acetoin yields can be achieved using wild-type strains because the majority of other products are transformed to 2,3-butanediol (Förster et al., 2017). Hence, significant efforts have been dedicated to engineering the strains (e.g., Bacillus subtilis, Serratia marcescens, Clostridium acetobutylicum, Candida glabrata) to maximize acetoin output (Bai et al., 2015; Li et al., 2015; Liu et al., 2015; Zhang et al., 2016). However, despite the high acetoin yields (e.g., ~70-96% of maximum theoretical), acetoin-producing strains can be pathogenic, which may limit their use in the markets without appropriate post-treatment (Bursac et al., 2017; Förster et al., 2017; Xiao & Lu, 2014). Moreover, the related studies were performed using oxic (either anoxic or aerobic) processes, which are highly unfavorable for the biosynthesis of value-added products due to the requirement of aeration and a higher ratio of anabolism over catabolism (Weusthuis et al., 2011). Therefore, a more suitable approach that is capable of producing a high amount of environmental-friendly acetoin is greatly needed.

      1.3.6 Biopolymer

      Different fermentation methods can be utilized for the production of bio-based plastics like PHA. First, a sugar-based fermentation, which was the conventional method, has been implemented for the production of bioplastics (Jabeen et al., 2015; Rossell et al., 2002), but this conventional approach was not favorable due to reasons such as accumulation of shortchain volatile fatty acids (discussed in the previous sections) and the redox imbalance (e.g., restraining the product selectivity) (Lai and Lan, 2020). A newer fermentation approach using methanotrophs has been developed recently (Liu et al., 2020a; Myung et al., 2017). However, the major challenge of using methanotrophic fermentation for the production of bioplastics was the high production costs (e.g., feedstock, aeration), low yields, and mass transfer limitations (e.g., oxygen transfer, low methane solubility in water) (Liu et al., 2020a). To address such challenges, Myung et al. (2016) grew methanotrophs in water-in-oil emulsions (e.g., using the fact that methane was more


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