Diatoms are the most dominant phytoplankton species in oceans and they continue to receive a great deal of attention because of their significant contributions in ecosystems and the environment. Due to triacylglycerol (TAG) profiles that are abundant in medium-chain fatty acids, diatoms have emerged to be better feed stocks for biofuel production, in comparison to the commonly studied green microalgal species (chlorophytes). Importantly, diatoms are also known for their high levels of the essential ω3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and are considered to be a promising alternative source of these components. The two most commonly exploited diatoms include Thalassiosira pseudonana and Phaeodactylum tricornutum. Although obvious similarities between diatoms and chlorophytes exist, there are some substantial differences in their lipid metabolism. This review provides an overview on lipid metabolism in diatoms, with P. tricornutum as the most explored model. Special emphasis is placed on the synthesis and incorporation of very long chain ω3 fatty acids into lipids. Furthermore, current approaches including genetic engineering and biotechnological methods aimed at improving and maximizing lipid production in P. tricornutum are also discussed.

BackgroundPhotosynthetic microalgae are considered a viable and sustainable resource for biofuel feedstocks, because they can produce higher biomass per land area than plants and can be grown on non-arable land. Among many microalgae considered for biofuel production, Nannochloropsis oceanica (CCMP1779) is particularly promising, because following nutrient deprivation it produces very high amounts of triacylglycerols (TAG). The committed step in TAG synthesis is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT). Remarkably, a total of 13 putative DGAT-encoding genes have been previously identified in CCMP1779 but most have not yet been studied in detail.ResultsBased on their expression profile, six out of 12 type-2 DGAT-encoding genes (NoDGTT1-NoDGTT6) were chosen for their possible role in TAG biosynthesis and the respective cDNAs were expressed in a TAG synthesis-deficient mutant of yeast. Yeast expressing NoDGTT5 accumulated TAG to the highest level. Over-expression of NoDGTT5 in CCMP1779 grown in N-replete medium resulted in levels of TAG normally observed only after N deprivation. Reduced growth rates accompanied NoDGTT5 over-expression in CCMP1779. Constitutive expression of NoDGTT5 in Arabidopsis thaliana was accompanied by increased TAG content in seeds and leaves. A broad substrate specificity for NoDGTT5 was revealed, with preference for unsaturated acyl groups. Furthermore, NoDGTT5 was able to successfully rescue the Arabidopsis tag1-1 mutant by restoring the TAG content in seeds.ConclusionsTaken together, our results identified NoDGTT5 as the most promising gene for the engineering of TAG synthesis in multiple hosts among the 13 DGAT-encoding genes of N. oceanica CCMP1779. Consequently, this study demonstrates the potential of NoDGTT5 as a tool for enhancing the energy density in biomass by increasing TAG content in transgenic crops used for biofuel production.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0686-8) contains supplementary material, which is available to authorized users.

Photosynthetic microalgae have promise as biofuel feedstock. Under certain conditions, they produce substantial amounts of neutral lipids, mainly in the form of triacylglycerols (TAGs), which can be converted to fuels. Much of our current knowledge on the genetic and molecular basis of algal neutral lipid metabolism derives mainly from studies of plants, i.e. seed tissues, and to a lesser extent from direct studies of algal lipid metabolism. Thus, the knowledge of TAG synthesis and the cellular trafficking of TAG precursors in algal cells is to a large extent based on genome predictions, and most aspects of TAG metabolism have yet to be experimentally verified. The biofuel prospects of microalgae have raised the interest in mechanistic studies of algal TAG biosynthesis in recent years and resulted in an increasing number of publications on lipid metabolism in microalgae. In this review we summarize the current findings on genetic, molecular and physiological studies of TAG accumulation in microalgae. Special emphasis is on the functional analysis of key genes involved in TAG synthesis, molecular mechanisms of regulation of TAG biosynthesis, as well as on possible mechanisms of lipid droplet formation in microalgal cells. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Wax esters (WE) belong to the class of neutral lipids. They are formed by an esterification of a fatty alcohol and an activated fatty acid. Dependent on the chain length and desaturation degree of the fatty acid and the fatty alcohol moiety, WE can have diverse physicochemical properties. WE derived from monounsaturated long-chain acyl moieties are of industrial interest due to their very good lubrication properties. Whereas WE were obtained in the past from spermaceti organs of the sperm whale, industrial WE are nowadays mostly produced chemically from fossil fuels. In order to produce WE more sustainably, attempts to produce industrial WE in transgenic plants are steadily increasing. To achieve this, different combinations of WE producing enzymes are expressed in developing Arabidopsis thaliana or Camelina sativa seeds. Here we report the identification and characterization of a fifth wax synthase from the organism Marinobacter aquaeolei VT8, MaWSD5. It belongs to the class of bifunctional wax synthase/acyl-CoA:diacylglycerol O-acyltransferases (WSD). The protein was purified to homogeneity. In vivo and in vitro substrate analyses revealed that MaWSD5 is able to synthesize WE but no triacylglycerols. The protein produces WE from saturated and monounsaturated mid-and long-chain substrates. Arabidopsis thaliana seeds expressing a fatty acid reductase from Marinobacter aquaeolei VT8 and MaWSD5 produce WE. Main WE synthesized are 20:1/18:1 and 20:1/ 20:1. This makes MaWSD5 a suitable candidate for industrial WE production in planta. Keywords Marinobacter aquaeolei VT8 Á Neutral lipids Á Seed oil Á Storage lipids Á Wax ester Á Wax synthase/acyl CoA:diacylglycerol O-acyltransferase Lipids (2020) 55: 479-494. Abbreviations A. baylyi Acinetobacter baylyi A. calcoaceticus Acinetobacter calcoaceticus, nowadays: Acinetobacter baylyi A. thaliana Arabidopsis thaliana aa amino acids AbWSD1 Acinetobacter baylyi WSD1, Ac1, AtfA ACP acyl carrier protein ATP adenosine triphosphate C. sativa Camelina sativa Supporting information Additional supporting information may be found online in the Supporting Information section at the end of the article.

Wax esters (WE) are neutral lipids that are formed by the transesterification of an activated fatty acyl moiety to a fatty alcohol. Due to their diverse physicochemical properties, WE are used as industrial lubricants, in cosmetics, or for coating. There is substantial interest in producing WE in bacteria and plants by genetic engineering to improve their sustainability and to reduce production costs. However, we lack a detailed understanding of the catalytic mechanism and structural determinants that influence substrate specificities of WE-synthesizing enzymes, which is essential for tailored WE production. One class of well-studied WE-producing enzymes are the bifunctional bacterial wax synthases/acyl-CoA:diacylglycerol O-acyltransferases (WSD). Here, we report the 1.95 Å crystal structure of Acinetobacter baylyi WSD1 (AbWSD1) with a fatty acid molecule bound in the active site. The location of a cocrystallized myristic acid confirms a previously proposed acyl-CoA binding site. A comparison of this AbWSD1 structure to a published Marinobacter aquaeolei WSD1 (MaWSD1) structure of the apoenzyme revealed a major structural difference in the C-terminal part of AbWSD1. This leads us to propose a conformational change in AbWSD1 induced by substrate binding. This conformational change forms then a potential coenzyme A (CoA) binding site. Furthermore, we have identified an additional cavity in AbWSD1 and could show through mutational studies that two amino acids lining the cavity are crucial for the acyl-CoA:diacylglycerol O-acyltransferase (DGAT) activity of the enzyme. Our findings provide a foundation for designing WSD variants that lack DGAT activity.