The dilemma for lipid productivity in green microalgae: importance of substrate provision in improving oil yield without sacrificing growth

Abstract

Rising oil prices and concerns over climate change have resulted in more emphasis on research into renewable biofuels from microalgae. Unlike plants, green microalgae have higher biomass productivity, will not compete with food and agriculture, and do not require fertile land for cultivation. However, microalgae biofuels currently suffer from high capital and operating costs due to low yields and costly extraction methods. Microalgae grown under optimal conditions produce large amounts of biomass but with low neutral lipid content, while microalgae grown in nutrient starvation accumulate high levels of neutral lipids but are slow growing. Producing lipids while maintaining high growth rates is vital for biofuel production because high biomass productivity increases yield per harvest volume while high lipid content decreases the cost of extraction per unit product. Therefore, there is a need for metabolic engineering of microalgae to constitutively produce high amounts of lipids without sacrificing growth. Substrate availability is a rate-limiting step in balancing growth and fatty acid (FA) production because both biomass and FA synthesis pathways compete for the same substrates, namely acetyl-CoA and NADPH. In this review, we discuss the efforts made for improving biofuel production in plants and microorganisms, the challenges faced in achieving lipid productivity, and the important role of precursor supply for FA synthesis. The main focus is placed on the enzymes which catalyzed the reactions supplying acetyl-CoA and NADPH.

Transcriptomic responses associated with carbon and energy flows under high salinity stress suggest the overflow of acetyl-CoA from glycolysis and NADPH co-factor induces high lipid accumulation and halotolerance inChlorellasp. HS2

“…These results contradict the overflow hypothesis (OH), which postulates that the overflow of photo-synthase is the main driver of lipid and/or starch accumulation and is frequently discussed to explain increased carbon storage in photosynthetic microbes under stress conditions (Juergens, Disbrow, & Shachar-Hill, 2016; Neijssel & Tempest, 1975; Tan & Lee, 2016): indeed, substantial compromise in PSII activities at both transcriptional and phenotypic levels at early growth phase and the upregulation of enzymes associated with the accumulation of lipid throughout entire growth stages clearly suggest that the overflow of organic photo-synthase ab initio was not the main driver of high lipid accumulation under salinity stress. Rather, our results identified the pulling of the acetyl-CoA precursor generated from glycolysis towards lipid synthesis as a major driver of lipid accumulation; accordingly, the redirection of storage carbon likely resulted in the seeming conversion of carbohydrate to lipid as observed in the proximate analysis of harvested biomass.…”

“…In addition to acetyl-CoA precursors, the synthesis of glycerolipid necessitates NADPH as a cofactor (Tan & Lee, 2016). Being an electron donor, NADPH is synthesized along with ATP during the light reaction of photosynthesis, and has been acknowledged for its role as an oxidative stress mediator (Valderrama et al, 2006).…”

The Microalga Nannochloropsis during Transition from Quiescence to Autotrophy in Response to Nitrogen Availability

“…Numerous studies in microalgae have shown that N deprivation induces de novo FA biosynthesis and results in an accumulation of significant amounts of storage lipids in the form of TAG (Li et al, 2012a(Li et al, , 2014Yang et al, 2013;Tan and Lee, 2016). The core building block of FA biosynthesis, acetyl-CoA, is generated by one of two types of multienzyme pyruvate dehydrogenase complex, plastidial (pPDHC) or mitochondrial PDHC.…”

Toward facilitating microalgae cope with effluent from anaerobic digestion of kitchen waste: the art of agricultural phytohormones

“…Apparently for SDEC-11, clear thylakoids in chloroplast were observed after phytohormones treatment, which could be another reason for the higher lipid content in GIB-treated groups. Moreover, Tan and Lee [57] summarized that the generation of NADPH could mainly derive from the metabolism of starch. Compared with ultrastructural cells in ADE-KW control (Fig.…”

“…Over-reduction will occur in the photosynthetic electron transport chain due to richness of NADPH, which could lead to some reactions with highly reduced products, such as lipid synthesis [58]. In addition to carbon supply, de novo FA synthesis also requires a continuous provision of reducing power in the form of NADPH [57]. It may be the efficient stimulation by GIB of chlorophyll in algae that ultimately leads to an increase in lipid accumulation.…”