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The large phagotrophic dinoflagellate Noctiluca has become a prominent red tide organism in southeast Australian waters since the 2000s, raising concerns for beach tourism, grazing impacts as well as ichthyotoxicity for finfish aquaculture. Satisfactory culture growth rates (0.23–0.56 per day) were obtained by feeding with small Thalassiosira diatom and Tetraselmis flagellate diets, while optimal growth rates sustained for up to 8 months (0.69 per day) were achieved by feeding in a plankton wheel with the large chain-forming dinoflagellate Gymnodinium catenatum. Noctiluca was highly tolerant towards salinities from 20 to 35 and growth was stimulated by temperatures increasing from 10 to 23°C, which in combination with the key factor of prey abundance explains the incidence in southeast Australia of predominantly summer and spring but occasionally also winter blooms. Fatty acid biomarkers suggest that Tasmanian field populations indiscriminately feed on available diatom and dinoflagellate mixtures. Noctiluca exhibited very limited ichthyotoxicity, and only at the highest cell concentrations of 2 000 000/L (50% reduction in RTgill W1 cell viability). Only the densest red tide surface slicks contained acutely toxic levels of unionized ammonia of 242 to 510 μg/L while inshore slicks generated oxygen concentrations as low as 0–1.5 ppm. Lipid phycotoxins (eicosapentaenoic acid, docosahexaenoic acid) did not appear to contribute to Noctiluca ichthyotoxicity. The fatty acid 20:0 eicosanoic acid may serve as a potential Noctiluca biomarker in marine food webs and sediments.

Plant seeds have long been promoted as a production platform for novel fatty acids such as the ω3 long-chain (≥ C20) polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) commonly found in fish oil. In this article we describe the creation of a canola (Brassica napus) variety producing fish oil-like levels of DHA in the seed. This was achieved by the introduction of a microalgal/yeast transgenic pathway of seven consecutive enzymatic steps which converted the native substrate oleic acid to α-linolenic acid and, subsequently, to EPA, docosapentaenoic acid (DPA) and DHA. This paper describes construct design and evaluation, plant transformation, event selection, field testing in a wide range of environments, and oil profile stability of the transgenic seed. The stable, high-performing event NS-B50027-4 produced fish oil-like levels of DHA (9–11%) in open field trials of T3 to T7 generation plants in several locations in Australia and Canada. This study also describes the highest seed DHA levels reported thus far and is one of the first examples of a deregulated genetically modified crop with clear health benefits to the consumer.

The Labyrinthulomycetes or Labyrinthulea are a class of protists that produce a network of filaments that enable the cells to glide along and absorb nutrients. One of the main two Labyrinthulea groups is the thraustochytrids, which are becoming an increasingly recognised and commercially used alternate source of long-chain (LC, ≥C20) omega-3 containing oils. This study demonstrates, to our knowledge for the first time, the regiospecificity of the triacylglycerol (TAG) fraction derived from Australian thraustochytrid Aurantiochytrium sp. strain TC 20 obtained using 13C nuclear magnetic resonance spectroscopy (13C NMR) analysis. The DHA present in the TC 20 TAG fraction was determined to be concentrated in the sn-2 position, with TAG (16:0/22:6/16:0) identified as the main species present. The sn-2 preference is similar to that found in salmon and tuna oil, and differs to seal oil containing largely sn-1,3 LC-PUFA. A higher concentration of sn-2 DHA occurred in the thraustochytrid TC 20 oil compared to that of tuna oil.

A series of novel C18–C22trans ω3 polyunsaturated fatty acids (PUFA) with a single trans double bond in the ω3 position was found in Northern and Southern Hemisphere strains of the marine haptophyte Imantonia rotunda. The novel ω3 PUFA were identified as 18:3(9c,12c,15t) (0.2–1.8 % of total fatty acids), 18:4(6c,9c,12c,15t) (1.9–4.1 %), 18:5 (3c,6c,9c,12c,15t) (0.7–8.8 %), 20:5(5c,8c,11c,14c,17t) (1.2–4.1 %) and 22:6(4c,7c,10c,13c,16c,19t) (0.3–4.3 %), and were accompanied by larger proportions of the all cis isomers: 18:3ω3(9,12,15) (2.7–3.5 %), 18:4ω3(6,9,12,15) (9.3–14.3 %), 18:5ω3(3,6,9,12,15) (7.8–18.5 %), 20:5ω3(5,8,11,14,17) (3.2–3.9 %), 22:5ω3(7,10,13,16,19) (0.1–0.3 %) and 22:6ω3(4,7,10,13,16,19) (2.3–5.2 %). GC analysis of FAME using a non-polar column did not reveal the trans isomers as they coeluted with the all cis PUFA. However, GC using a polar column resolved the trans PUFA from the all cis PUFA, with the trans isomers eluting before the all cis isomers. GC-MS of FAME fractionated by argentation solid-phase chromatography confirmed the molecular ions of all components. FAME were derivatized to form 4,4-dimethyloxazoline (DMOX) derivatives, and GC-MS revealed the same double bond positions for each trans and cis FAME. The results suggest that the ω3 trans double bond originated from the Δ15/ω3 desaturation of 18:2(9c,12c), suggesting that this desaturase has dual cis/trans activity in these species. These results indicate that 18:3(9c,12c,15 t) was the precursor trans isomer produced for the trans series and further desaturation by the common Δ6 desaturase to produce the trans tetraene and successive elongations and desaturations led to the subsequent series of trans ω3 PUFA isomers. To our knowledge, this is the first report of these trans ω3 isomers occurring in strains of I. rotunda. These trans ω3 PUFA may be used as biomarkers in marine food webs for this species and with their unique structure may be biologically active.

New and sustainable sources of long-chain (LC, ≥C20) omega-3 oils containing DHA (docosahexaenoic acid, 22:6ω3) are required to meet increasing demands. The lipid content of the oilseed of a novel transgenic, DHA-producing land plant, Camelina sativa, containing microalgal genes able to produce LC omega-3 oils, contained 36% lipid by weight with triacylglycerols (TAG) as the major lipid class in hexane extracts (96% of total lipid). Subsequent chloroform-methanol (CM) extraction recovered further lipid (~50% polar lipid, comprising glycolipids and phospholipids) and residual TAG. The main phospholipid species were phosphatidyl choline and phosphatidyl ethanolamine. The % DHA was: 6.8% (of total fatty acids) in the TAG-rich hexane extract and 4.2% in the polar lipid-rich CM extract. The relative level of ALA (α-linolenic acid, 18:3ω3) in DHA-camelina seed was higher than the control. Major sterols in both DHA- and control camelina seeds were: sitosterol, campesterol, cholesterol, brassicasterol and isofucosterol. C16–C22 fatty alcohols, including iso-branched and odd-chain alcohols were present, including high levels of iso-17:0, 17:0 and 19:0. Other alcohols present were: 16:0, iso-18:0, 18:0 and 18:1 and the proportions varied between the hexane and CM extracts. These iso-branched odd-chain fatty alcohols, to our knowledge, have not been previously reported. These components may be derived from wax esters, or free fatty alcohols.

High biomass crops have recently attracted significant attention as an alternative platform for the renewable production of high energy storage lipids such as triacylglycerol (TAG). While TAG typically accumulates in seeds as storage compounds fuelling subsequent germination, levels in vegetative tissues are generally low. Here, we report the accumulation of more than 15% TAG (17.7% total lipids) by dry weight in Nicotiana tabacum (tobacco) leaves by the co-expression of three genes involved in different aspects of TAG production without severely impacting plant development. These yields far exceed the levels found in wild-type leaf tissue as well as previously reported engineered TAG yields in vegetative tissues of Arabidopsis thaliana and N. tabacum. When translated to a high biomass crop, the current levels would translate to an oil yield per hectare that exceeds those of most cultivated oilseed crops. Confocal fluorescence microscopy and mass spectrometry imaging confirmed the accumulation of TAG within leaf mesophyll cells. In addition, we explored the applicability of several existing oil-processing methods using fresh leaf tissue. Our results demonstrate the technical feasibility of a vegetative plant oil production platform and provide for a step change in the bioenergy landscape, opening new prospects for sustainable food, high energy forage, biofuel and biomaterial applications. © 2013 CSIRO., We would like to thank Nathalie Niesner, Anne Mackenzie, Dawar Hussain, Anna Mechanicos, Cheryl Blundell, Luch Hac, Alex Miller, Lina Ma, Lijun Tian and Jeni Pritchard for technical assistance. We also thank Dr Damien Callahan at Metabolomics Australia, University of Melbourne and Dr. Matt Taylor at CSIRO Ecosystem Sciences for assistance with LC-MS, Dr. Rosemary White for help with microscopy and Dr. Allan Green for helpful discussions and advice. This work was funded by the CSIRO Food Futures National Research Flagship, CSIRO Plant Industry and the CSIRO Office of the Chief Executive Postdoctoral Fellowship Scheme. LC-MS analysis was supported by Bioplatforms Australia Services. Research in the UNT Center for Plant Research on oil accumulation in leaves is supported by a grant from the U.S. Department of Energy, BER Division, DE-FG02-09ER64812. Acquisition of the imaging instruments at UNT was made possible, in part, by grants from the Hoblitzelle Foundation and the US National Science Foundation (MRI#112605).

We describe a transgenic microalgal Δ9-elongase pathway transformed in both Brassica napus and Arabidopsis thaliana seed resulting in the production of arachidonic acid (ARA). This pathway is noteworthy for both the production of ARA in seed tissue and the low levels of intermediate C20 fatty acids that accumulate. We also demonstrate that the arachidonic acid is naturally enriched at the sn2 position in triacylglycerol. This is the first report of ARA production by the Δ9-elongase pathway in an oilseed.

The assembly of multistep recombinant pathways in stably transformed plants is a cornerstone of crops producing new products yet can be a laborious and time-consuming process. Any heterologous expression platform capable of providing a rapid estimation of the functional assembly of an entire pathway would guide the design of such transgenic traits. In this study, we use a Nicotiana benthamiana transient leaf expression system to simultaneously express five genes, from five independent T(DNA) binary vectors, to assemble a complete recombinant pathway in five days. In this study, we demonstrate the production of long-chain polyunsaturated fatty acids (LC-PUFA) requiring five transgene-encoded reactions to convert endogenous fatty acids to LC-PUFA. The addition of a triacylglycerol assembly enzyme, Arabidopsis thaliana diacylglyceride-O-acyltransferase, and fractionation of the total lipid profile demonstrated that leaf oils contained 37% newly synthesised LC-PUFA, including 7% arachidonic acid (AA), 6% eicosopentaenoic acid and 3% docosahexaenoic acid. The calculation of enzymatic conversion efficiencies at each step of LC-PUFA synthesis suggests that this transient assembly of a complicated multistep pathway is highly efficient. Unlike experiments using stably transformed plants our assembly of an intricate pathway maintained full gene-for-gene interchangeability and required a fraction of the time and glasshouse space. Furthermore, an exogenous LC-PUFA fatty acid substrate, AA, was fed and metabolised by a transiently expressed Delta17-desaturase enzyme, and provided results similar to those obtained in yeast feeding experiments. Although the assay was ideal for LC-PUFA pathways, this assay format may become a powerful tool for the characterisation and step-wise improvement of other recombinant pathways and multigenic traits.

The presence of long-chain polyunsaturated fatty acids (PUFA; ca. 9% of total fatty acids) in marine sediments near Dover, southern Tasmania, Australia prompted a search for their likely source. Analysis of a number of different species of benthic fauna isolated from these sediments revealed that the brittle star Amphiura elandiformis contained abundant PUFA including high contents of the uncommon long-chain fatty acid 24:6(n-3), but much smaller amounts of the more common animal PUFA 22:6(n-3). This is the first report of the lipid composition of this animal. Identifications of the unsaturated fatty acids were confirmed by formation of DMOX derivatives which gave characteristic and easily interpreted mass spectra. The 24:6(n-3) PUFA has been identified in some genera of brittle stars, but not others. It is rarely found in significant amounts in other marine animals. DMDS adducts were used to identify the positions of double bonds in the monounsaturated fatty acids. The major 20:1 isomer was identified as the rarely reported 20:1(n-13) fatty acid. The two fatty acids 20:1(n-13) and 24:6(n-3) may be useful biomarkers in food-web studies for identifying a brittle star diet and for recognising contributions of organic detritus from this benthic animal to marine sediments.

Nine microalgal species from the classes Bacillariophyceae, Cryptophyceae, Prymnesiophyceae and Dinophyceae were isolated from Australian waters, cultured to stationary phase and analyzed for their lipid and fatty acid composition and yield. Five species (Pavlova pinguis, Heterocapsa niei, Proteomonas sulcata, Navicula jeffreyi and Thalassiosira pseudonana) produced high proportions of triacylglycerol (TAG: 22–57% total lipid). An unidentified Navicula-like diatom (CS-786), despite having a low TAG content, had the highest EPA yield (5.8 mg L−1), due to high biomass and a high relative proportion of EPA. Heterocapsa niei had the highest DHA yield (2.9 mg L−1), due to a high cellular lipid and DHA content (171 pg cell−1 and 13.7 pg cell−1, respectively) despite its relatively low biomass. The desirable PUFA composition and yield of both diatom CS-786 and H. niei make them potential candidates for optimization of biomass and PUFA production for use as live-feeds in aquaculture. In addition, H. niei may have potential as a source of DHA for other uses. Low proportions (< 1.2%) of 24:6(n−3) accompanied by trace proportions of 24:5(n−6) were detected in most strains, while 28:8(n−3) was found in dinoflagellates and also in the prymnesiophyte P. pinguis. All non-diatomaceous species contained 26:7(n−3) in minor quantities. This is the first time these unusual C24 and C26 PUFA have been reported in microalgae and the first report of C28 PUFA in a microalga other than dinoflagellates. Possible biosynthetic reasons why these might occur in stationary phase cultures are considered and the likely dietary transfer of these PUFA to higher aquatic life is discussed.

We have studied the effects of growth phase on the lipid composition in batch cultures of Gymnodinium sp. CS-380/3 over 43 days of culturing. The lipid content increased two fold, from late logarithmic (day 6) to linear growth phase (day 22) then decreased at stationary phase (day 43) while the lipid yield (mg l −1 ) increased 30-fold from day 6 to 30 mg l −1 at day 43. Changes in fatty acid content mirrored those observed for the total lipid, while the sterol content continued to increase with culture age through to stationary phase. The largest changes occurred in the lipid classes, especially the polar lipids and triacylglycerols (oil). The proportion of triacylglycerols increased from 8% (of total lipids) at day 6 to 30% at day 43, with a concomitant decrease in the polar lipid fraction. The proportions of 16:0 and DHA [22:6(n-3)] increased while those of 18:5(n-3) and EPA [20:5(n-3)] decreased with increasing culture age. The proportion of the major sterol, dinosterol, decreased from 41% (day 6) to 29% (day 43), while the major dinostanol epimer (23 R ,24 R ) increased from 33% (day 6) to 38% (day 22). Despite small changes in the proportion of the main sterols, the same sterols were present at all stages of growth, indicating their value as a chemotaxonomic tool for distinguishing between strains within the same genus. Growth phase could be a useful variable for optimising the oil and DHA content with potential for aquaculture feeds and a source of DHA-rich oils for nutraceuticals.

The compositions of 4-desmethyl sterols and 4-methyl sterols in four species of marine dinoflagellates of the genus Prorocentrum (viz., P. micans Ehrenberg, P. minimum (Pavillard) Schiller, P. balticum (Lev.) Lemm and P. mexicanum Tafall) were identified by capillary gas chromatography–mass spectrometry as part of a study to identify signature lipids for dinoflagellates in marine organic matter. Complex mixtures were found in each species with over 20 sterols identified in all. All species contained the same core group of sterols, but there were significant differences in the proportions of the various sterols. Two distinct groupings could be discerned in the sterol patterns. The 4-methyl sterol 4α,23,24-trimethyl-5α-cholest-22 E -en-3β-ol (dinosterol), which is common in many dinoflagellates, predominated in P. balticum and in P. minimum whereas in the closely related species P. micans and P. mexicanum the major sterol was cholesterol. A novel monounsaturated C 23 sterol having a much shortened side-chain was found in P. balticum and P. minimum and both P. balticum and P. minimum contained peridinosterol (4α,23,24-trimethyl-5α-cholest-17(20)-en-3β-ol). 24-Methylenecholesterol was only found in P. minimum , where it comprised over one-third of the sterols. The steroid ketone dinosterone occurred in P. balticum , but none of the other species contained steroid ketones. Although all the sterol distributions were broadly similar, the presence or absence of specific components might be a useful chemotaxonomic tool for distinguishing between closely related species.

The fatty acid and sterol compositions of five species of marine dinoflagellates (Scrippsiella sp. Symbiodinium microadriaticum Freud, Gymnodinium sp., Gymnodinium sanguineum Hirasaki, and Fragilidium sp.) are reported. All contained the major fatty acids that are considered common in dinoflagellates, but the proportions were quite variable, and some species contained low contents of some polyunsaturated fatty acids. Concentration ranges for the major fatty acids were: 16:0 (9.0%–24.8%), 18:4(n-3) (2.5%–11.5%), 18:5(n-3) (7.0%–43.1%), 20:5(n-3) (EPA) (1.8%–20.9%), and 22:6(n-3) (DHA) (9.9%– 26.3%). Small amounts of novel very-long-chain highly unsaturated C28 fatty acids occurred in all species. Each dinoflagellate contained a complex mixture of 4-methyl sterols and 4-desmethyl sterols. Four species contained cholesterol, although the amounts were highly variable (from 0.2% of total sterols in Scrippsiella sp. to 45.6% in Fragilidium sp.). All but G. sanguineum contained the 4-methyl sterol dinosterol, and all species contained sterols lacking a double bond in the ring system (i.e. stanols); in Scrippsiella sp. cholestanol composed 24.3% of the total sterols. Other common features of the 4-methylsterol profiles were the presence of 23,24-dimethyl alkylation and unsaturation at Δ22 in the side chain. In Scrippsiella sp., four steroidal ketones were identified: cholestanone, dinosterone, 4α,23,24-trimethyl-5α-cholest-8(14)-en-3-one, and dinostanone. The structures of these corresponded to the major sterols in this species, suggesting that the sterols and steroidal ketones are biosynthetically linked. Steroidal ketones were not detected in the other species. Although fatty acid profiles can be used to distinguish among algal classes, they were not useful for differentiating among dinoflagellate species. In contrast, whereas some taxonomic groupings of dinoflagellates display similar sterol patterns, others, such as the gymnodinoids studied here, clearly do not. The combination of fatty acid, sterol, and steroidal ketone profiles may be useful complementary chemotaxonomic tools for distinguishing morphologically similar species. The identification of steroidal ketones supports earlier suggestions that certain dinoflagellates might be a significant source of such components in marine environments.

Very-long-chain (C28) highly unsaturated fatty acids (VLC-HUFA), octacosaheptaenoic acid (28:7(n-6)(4,7,10,13,16,19,22)) and octacosaoctaenoic acid (28:8(n-3)(4,7,10,13,16,19,22,25)) were identified in seven marine dinoflagellate species: Prorocentrum mexicanum, P. micans, Scrippsiella sp., Symbiodinium microadriaticum, Gymnodinium sp., G. sanguineum and Fragilidium sp. The proportion of these fatty acids accounted for less than 2.3% of the total fatty acids in these species. The main VLC-HUFA in S. microadriaticum was 28:7(n-6); in the other species it was 28:8(n-3), with Scrippsiella sp., Gymnodinium sp. and Fragilidium sp. containing only 28:8(n-3) and Gymnodinium sanguineum containing a small proportion of 28:7(n-6) in addition to 28:8(n-3). These findings show that some marine dinoflagellates have the ability to produce VLC-HUFA, but precursors of these VLC-HUFA with chain lengths between 22 and 28 carbon atoms were below the detection level. Both 28:7(n-6) and 28:7(n-3), as well as other VLC-HUFAs, have been detected previously in Baltic herring; our results suggest that they may have originated from microalgae.

Microalgae are major sources of lipids in lacustrine and marine environments. This paper provides a review of some recent advances in our knowledge of the wide variety of lipid types that have been isolated from microalgae with an emphasis on those likely to be useful biomarkers for identifying sources of organic matter in sediments. Extensive data are now available on the fatty acids in all of the major classes of microalgae and some useful characteristic features have been observed in the abundance of particular polyunsaturated fatty acids. Despite several decades of study, it is now apparent that some of the biosynthetic steps leading to the formation of these unsaturated fatty acids are still not known with certainty as shown by the occurrence of C28 polyunsaturated fatty acids in some dinoflagellates and the likely involvement of chain-shortening reactions. Considerable data have also been obtained on the sterols in microalgae, but some classes of organisms are still not well documented (e.g. cryptomonads, eustigmatophytes, xanthophytes and raphidophytes). Diatoms show a great variety of sterol compositions and no sterol appears to be either unique or representative. However, 24-methylenecholesterol in sediments is probably derived in most cases from diatoms. High contents of C25 highly branched isoprenoid (HBI) alkenes have been identified in the diatom Haslea ostrearia and both C25 and C30 HBI alkenes have been found in diatom strains thought to be Rhizosolenia setigera. Genetic and environmental factors appear to be important controls on the relative abundances of the various homologues identified. Microalgae are also suspected to be a source of long-chain saturated fatty acids having an even carbon number predominance and of long-chain alkanes with no odd over even carbon number predominance, although the available data are not conclusive. An exciting development in recent years is the identification of highly aliphatic biopolymers (algaenans) in some species of marine and freshwater green algae and eustigmatophytes. This material persists in sediments and may be a source of the alkyl chains in ancient kerogens and crude oil constituents. Algaenans do not occur in all algal species and may be absent from some classes, such as diatoms. This implies that the organic matter preserved in sediments is strongly influenced by a subset of the microalgal contributors of organic matter. Although reasonable sources have been identified for many of the lipids in sediments, there are still many gaps in our knowledge and further studies are clearly required.

Background Omega-3 long-chain (≥C20) polyunsaturated fatty acids (ω3 LC-PUFA) such as eicosapentaenoic acid (EPA) and docosapentaenoic acid (DHA) are critical for human health and development. Numerous studies have indicated that deficiencies in these fatty acids can increase the risk or severity of cardiovascular, inflammatory and other diseases or disorders. EPA and DHA are predominantly sourced from marine fish although the primary producers are microalgae. Much work has been done to engineer a sustainable land-based source of EPA and DHA to reduce pressure on fish stocks in meeting future demand, with previous studies describing the production of fish oil-like levels of DHA in the model plant species, Arabidopsis thaliana. Principal Findings In this study we describe the production of fish oil-like levels (>12%) of DHA in the oilseed crop species Camelina sativa achieving a high ω3/ω6 ratio. The construct previously transformed in Arabidopsis as well as two modified construct versions designed to increase DHA production were used. DHA was found to be stable to at least the T5 generation and the EPA and DHA were found to be predominantly at the sn-1,3 positions of triacylglycerols. Transgenic and parental lines did not have different germination or seedling establishment rates. Conclusions DHA can be produced at fish oil-like levels in industrially-relevant oilseed crop species using multi-gene construct designs which are stable over multiple generations. This study has implications for the future of sustainable EPA and DHA production from land-based sources.

BACKGROUND: Omega-3 long-chain (≥C(20)) polyunsaturated fatty acids (ω3 LC-PUFA) have critical roles in human health and development with studies indicating that deficiencies in these fatty acids can increase the risk or severity of cardiovascular and inflammatory diseases in particular. These fatty acids are predominantly sourced from fish and algal oils, but it is widely recognised that there is an urgent need for an alternative and sustainable source of EPA and DHA. Since the earliest demonstrations of ω3 LC-PUFA engineering there has been good progress in engineering the C(20) EPA with seed fatty acid levels similar to that observed in bulk fish oil (∼18%), although undesirable ω6 PUFA levels have also remained high. METHODOLOGY/PRINCIPAL FINDINGS: The transgenic seed production of the particularly important C(22) DHA has been problematic with many attempts resulting in the accumulation of EPA/DPA, but only a few percent of DHA. This study describes the production of up to 15% of the C(22) fatty acid DHA in Arabidopsis thaliana seed oil with a high ω3/ω6 ratio. This was achieved using a transgenic pathway to increase the C(18) ALA which was then converted to DHA by a microalgal Δ6-desaturase pathway. CONCLUSIONS/SIGNIFICANCE: The amount of DHA described in this study exceeds the 12% level at which DHA is generally found in bulk fish oil. This is a breakthrough in the development of sustainable alternative sources of DHA as this technology should be applicable in oilseed crops. One hectare of a Brassica napus crop containing 12% DHA in seed oil would produce as much DHA as approximately 10,000 fish.

Long-chain (> or = C(20)) polyunsaturated fatty acids, such as docosahexaenoic acid and eicosapentaenoic acid, are nutritionally important and provide protection against cardiovascular disease, stroke, and cancer. Structural variants of these compounds may have the potential to be used as pharmaceuticals. Marine microalgae are the key producers of long-chain polyunsaturated fatty acids in the global food web. Assuming vast biological and biochemical diversity, we devised a screen to identify microalgae that produce novel fatty acids. The herbicide metolachlor, an inhibitor of long-chain fatty acid biosynthesis, was used in microcosms containing field-collected microalgae to identify naturally resistant strains. We show that one diatom, Melosira cf. moniliformis, is naturally resistant to concentrations of metolachlor, which were cytostatic or lethal to all the other microalgae. Gas chromatography and gas chromatography-mass spectrometry revealed three fatty acids that have not previously been described-18:4 (Delta5,8,11,14), 18:4 (Delta5,9,12,15), and 18:5 (Delta5,8,11,14,17). We propose that this type of screen may be generally applicable to the search of novel compounds produced by marine microorganisms.

BACKGROUND: Monoacylglycerol acyltransferases (MGATs) are predominantly associated with lipid absorption and resynthesis in the animal intestine where they catalyse the first step in the monoacylglycerol (MAG) pathway by acylating MAG to form diacylglycerol (DAG). Typical plant triacylglycerol (TAG) biosynthesis routes such as the Kennedy pathway do not include an MGAT step. Rather, DAG and TAG are synthesised de novo from glycerol-3-phosphate (G-3-P) by a series of three subsequent acylation reactions although a complex interplay with membrane lipids exists. METHODOLOGY/PRINCIPAL FINDINGS: We demonstrate that heterologous expression of a mouse MGAT acyltransferase in Nicotiana benthamiana significantly increases TAG accumulation in vegetative tissues despite the low levels of endogenous MAG substrate available. In addition, DAG produced by this acyltransferase can serve as a substrate for both native and coexpressed diacylglycerol acyltransferases (DGAT). Finally, we show that the Arabidopsis thaliana GPAT4 acyltransferase can produce MAG in Saccharomyces cerevisiae using oleoyl-CoA as the acyl-donor. CONCLUSIONS/SIGNIFICANCE: This study demonstrates the concept of a new method of increasing oil content in vegetative tissues by using MAG as a substrate for TAG biosynthesis. Based on in vitro yeast assays and expression results in N. benthamiana, we propose that co-expression of a MAG synthesising enzyme such as A. thaliana GPAT4 and a MGAT or bifunctional M/DGAT can result in DAG and TAG synthesis from G-3-P via a route that is independent and complementary to the endogenous Kennedy pathway and other TAG synthesis routes.