The history, state of the art and future prospects for oleaginous yeast research

Abstract

Lipid-based biofuels, such as biodiesel and hydroprocessed esters, are a central part of the global initiative to reduce the environmental impact of the transport sector. The vast majority of production is currently from first-generation feedstocks, such as rapeseed oil, and waste cooking oils. However, the increased exploitation of soybean oil and palm oil has led to vast deforestation, smog emissions and heavily impacted on biodiversity in tropical regions. One promising alternative, potentially capable of meeting future demand sustainably, are oleaginous yeasts. Despite being known about for 143 years, there has been an increasing effort in the last decade to develop a viable industrial system, with currently around 100 research papers published annually. In the academic literature, approximately 160 native yeasts have been reported to produce over 20% of their dry weight in a glyceride-rich oil. The most intensively studied oleaginous yeast have been Cutaneotrichosporon oleaginosus (20% of publications), Rhodotorula toruloides (19%) and Yarrowia lipolytica (19%). Oleaginous yeasts have been primarily grown on single saccharides (60%), hydrolysates (26%) or glycerol (19%), and mainly on the mL scale (66%). Process development and genetic modification (7%) have been applied to alter yeast performance and the lipids, towards the production of biofuels (77%), food/supplements (24%), oleochemicals (19%) or animal feed (3%). Despite over a century of research and the recent application of advanced genetic engineering techniques, the industrial production of an economically viable commodity oil substitute remains elusive. This is mainly due to the estimated high production cost, however, over the course of the twenty-first century where climate change will drastically change global food supply networks and direct governmental action will likely be levied at more destructive crops, yeast lipids offer a flexible platform for localised, sustainable lipid production. Based on data from the large majority of oleaginous yeast academic publications, this review is a guide through the history of oleaginous yeast research, an assessment of the best growth and lipid production achieved to date, the various strategies employed towards industrial production and importantly, a critical discussion about what needs to be built on this huge body of work to make producing a yeast-derived, more sustainable, glyceride oil a commercial reality.

Keywords: Biodiesel; Climate change; Industrial biotechnology; Microbial lipid; Oleaginous yeast; Single-cell oil; Sustainability; Triglyceride.

Fig. 1

Timeline of yeast lipid research. Displayed is the annual amount of oleaginous yeast research publications since 1975 in English language together with the annual global biodiesel production (biodiesel data obtained from [254]). Key developments associated to the field are displayed in the top half of the graph (year label in brown) and those directly concerning oleaginous yeasts in the bottom half (blue). The full methodology used to collect and analyse the presented data is given in Additional file 1

Fig. 2

Simplified schematic description of the yeast fatty acid metabolism with a focus on de novo lipid formation. ACL ATP citrate lyase, MD malate dehydrogenase, ME malic enzyme; the enzymes catalyse reactions specific for oleaginous yeasts. *Pyruvate enters the mitochondria and undergoes oxidative decarboxylation to mitochondrial acetyl-CoA or is converted to cytosolic acetyl-CoA via the pyruvate–acetaldehyde–acetate pathway requiring ATP; **citrate is transported from the mitochondria into the cytosol and cleaved to cytosolic acetyl-CoA

Fig. 3

Feedstock distribution of oleaginous yeasts. Displayed are the percentages of main carbon sources used in oleaginous yeast research. Please note that sometimes multiple carbon sources are used in a single publication. The category ‘single saccharide’ includes all single saccharides, sugar acids and alcohols; ‘hydrolysate’ also artificial hydrolysates; and ‘mixture’ the mixtures of all other carbon sources. The full methodology used to collect and analyse the presented data is given in Additional file 1

Fig. 4

The feedstock preference of the most prominent oleaginous yeasts. Displayed is the feedstock popularity according to the depicted definition. As an example, Yarrowia lipolytica has been cultured on an oil/fat four times more often than the average yeast. The feedstock popularity indicates certain feedstock preferences of the specified yeast. For instance, when a fatty acid is the preferred feedstock, Yarrowia lipolytica and Cutaneotrichosporon oleaginosus are likely suitable yeasts. The feedstock distribution of all oleaginous yeasts can be reviewed in Fig. 3. Please see the corresponding caption for details of the carbon sources comprising each category. The full methodology used to collect and analyse the presented data is given in Additional file 1

Fig. 5

The confirmed native oleaginous yeasts. Included are all native yeasts or yeast-like species with a lipid content over or equal to 20% (w/w) reported in at least three publications and clearly identified with their generic name and specific epithet, as their current name according to the corresponding culture collection and MycoBank (48 yeasts in total). The range of the highest reported lipid contents including average and standard deviation, as well as the number of publications, where a strain of the species has been reported oleaginous, are depicted. For some species, such as Saccharomyces cerevisiae, not typically classified oleaginous, only certain strains have been shown to accumulate over 20% (w/w) lipid. The labels indicate the maximum lipid content recorded or the number of publications, respectively. Further identified oleaginous yeasts (less than three publications, 113 yeasts) are given in Additional file 1: Tables S3 and S4. The full methodology used to collect and analyse the presented data is given in Additional file 1

Fig. 6

Influencing yeast lipid production through process development and genetic modification. a Percentage of oleaginous yeasts cultured in different operation modes used in research and b percentage of oleaginous yeast species used for genetic modification (including genetic engineering, evolution, and mutation). The full methodology used to collect and analyse the presented data is given in Additional file 1

Fig. 7

The proposed areas of future oleaginous yeast research towards producing a commodity oil substitute. Research in process or product development of oleaginous yeasts should ideally focus on the depicted areas