Читать книгу Biomolecular Engineering Solutions for Renewable Specialty Chemicals - Группа авторов - Страница 56

Biodiesel is a renewable liquid transportation fuel consisting of alkyl esters of fatty acids and produced from triacylglycerides (TAGs). It is synthesized by a process called transesterification of fats in the presence of catalyst (alcohol) to form fatty acid methyl esters (Subramani and Gangwal, 2008). Therefore, properties of biodiesel are dependent on the fatty acid from which it was made (Knothe, 2005). Majority of biodiesel currently is being formed by oilseed crops competing with food and cultivable land. Alternative feedstock for the oil includes oleaginous or grease microorganisms, such as microalgae, cyanobacteria, yeast, and bacteria (Hu et al., 2008). Out of these microalgae and cyanobacteria are of importance as they are autotrophic in nature. Some microalgae accumulate 20–50% TAG, which are same as found in oilcrops such as canola and sunflower (Gaurav et al., 2017). Like plants only they utilize sunlight as energy source and CO₂ as carbon source but can be cultivated on little barren land using wastewater (Pittman et al., 2011). Attempts are being made to improve lipid content of microalgae by metabolic engineering and genetic engineering. C. reinhardtii is the model organism for this purpose as its genome is well known.

TAG is derived from acylation or diacylglycerol through acyl‐CoA dependent and acyl CoA‐independent pathway. Rate‐limiting steps of these two pathways can be altered to increase or decrease the TAG production. Inactivation of ADP‐glucose pyrophosphorylase in Chlamydomonas resulted in 10‐fold increase in TAGs (Miller et al., 2010). In another study, Chlamydomonas mutant lacked subunit of ADP‐glucose pyrophosphorylase and accumulated 46.5% total lipids out of which 32.5% were neutral lipids (Li et al., 2010).

Knockout and overexpression of enzymes important for fatty acid synthesis like acetyl CoA carboxylase (ACCase) and type‐II fatty acid synthase (FAS) are well known to increase lipid content in the cell (Majidian et al., 2018). ACCase catalyzes the rate‐limiting step of fatty acid synthesis and therefore C. reinhardtii was overexpressed with ACCase and showed increase in fatty acid synthesis to 56.15% as compared to wild type (48.39%) (Chen et al., 2019).

In diatom like Thalassiosira pseudonana, deletion of multifunctional lipase/phospholipase/acyltransferase does not hamper growth and increases (up to threefold) lipid content (Trentacoste et al., 2013). Heterologous expression of two thioesterases in microalga Phaeodactylum tricornutum accumulates shorter chain fatty acids; lauric and myristic acid (Radakovits et al., 2011). These shorter fatty acids are not secreted and gets incorporated in TAG.