Simultaneous improvement in production of microalgal biodiesel and high-value alpha-linolenic acid by a single regulator acetylcholine

Roadmap to sustainable carbon-neutral energy and environment: can we cross the barrier of biomass productivity?

The total number of inhabitants on the Earth is estimated to cross a record number of 9 × 10³ million by 2050 that present a unique challenge to provide energy and clean environment to every individual. The growth in population results in a change of land use, and greenhouse gas emission due to increased industrialization and transportation. Energy consumption affects the quality of the environment by adding carbon dioxide and other pollutants to the atmosphere. This leads to oceanic acidification and other environmental fluctuations due to global climate change. Concurrently, speedy utilization of known conventional fuel reservoirs causes a challenge to a sustainable supply of energy. Therefore, an alternate energy resource is required that can maintain the sustainability of energy and environment. Among different alternatives, energy production from high carbon dioxide capturing photosynthetic aquatic microbes is an emerging technology to clean environment and produce carbon-neutral energy from their hydrocarbon-rich biomass. However, economical challenges due to low biomass production still prevent the commercialization of bioenergy. In this work, we review the impact of fossil fuels burning, which is predominantly used to fulfill global energy demand, on the quality of the environment. We also assess the status of biofuel production and utilization and discuss its potential to clean the environment. The complications associated with biofuel manufacturing using photosynthetic microorganisms are discussed and directed evolution for targeted phenotypes and targeted delivery of nutrients are proposed as potential strategies to increase the biomass production.

Fusion transcription factors for strong, constitutive expression of cellulases and xylanases in Trichoderma reesei

BACKGROUND

The filamentous ascomycete T. reesei is industrially used to produce cellulases and xylanases. Cost-effective production of cellulases is a bottleneck for biofuel production. Previously, different strain and process optimizations were deployed to enhance enzyme production rates. One approach is the overexpression of the main activator Xyr1 and a second is the construction of synthetic transcription factors. Notably, these genetic manipulations were introduced into strains bearing the wild-type xyr1 gene and locus.

RESULTS

Here, we constructed a Xyr1-deficient strain expressing a non-functional truncated version of Xyr1. This strain was successfully used as platform strain for overexpression of Xyr1, which enhanced the cellulase and xylanase production rates under inducing conditions, with the exception of lactose-there the cellulase production was severely reduced. Further, we introduced fusion transcription factors consisting of the DNA-binding domain of Xyr1 and the transactivation domain of either Ypr1 or Ypr2 (regulators of the sorbicillinoid biosynthesis gene cluster). The fusion of Xyr1 and Ypr2 yielded a moderately transactivating transcription factor, whereas the fusion of Xyr1 and Ypr1 yielded a highly transactivating transcription factor that induced xylanases and cellulases nearly carbon source independently. Especially, high production levels of xylanases were achieved on glycerol.

CONCLUSION

During this study, we constructed a Xyr1-deficient strain that can be fully reconstituted, which makes it an ideal platform strain for Xyr1-related studies. The mere overexpression of Xyr1 turned out not to be a successful strategy for overall enhancement of the enzyme production rates. We gained new insights into the regulatory properties of transcription factors by constructing respective fusion proteins. The Xyr1-Ypr1-fusion transcription factor could induce xylanase production rates on glycerol to outstanding extents, and hence could be deployed in the future to utilize crude glycerol, the main co-product of the biodiesel production process.

Enhanced accumulation of alpha-linolenic acid rich lipids in indigenous freshwater microalga Desmodesmus sp.: The effect of low-temperature on nutrient replete, UV treated and nutrient stressed cultures

The indigenous microalga, Desmodesmus sp. produced alpha-linolenic acid (ALA) rich lipids in response to low temperature and UV treatment. Incubation at 5 °C showed a 1.5 fold increase in lipid content (34% w/w) with 44% ALA fraction of total fatty acids. The UV treatment (UV 60 min) exhibited a 1.4 fold increase in biomass productivity and 1.6 fold increase in lipid content (37% w/w) with ALA fraction as 31% of total fatty acids. The nitrogen stress enhanced the lipid content (39% w/w) with a reduced ALA fraction (18%) of total fatty acids. The UV treated cultures (UV 40 and 60 min) on incubation at 5 °C showed maximum lipid accumulation (59 to 62% w/w) with ALA fraction of total fatty acids as 39 to 42%. The incubation of nutrient-replete and UV treated cultures at low-temperature could therefore be used for the production of ALA-rich lipids in microalgae.

Isolation of a novel strain of Monoraphidium sp. and characterization of its potential for α-linolenic acid and biodiesel production

α-Linolenic acid (ALA) is an essential fatty acid which cannot be synthesized de novo in mammals and must be ingested regularly in the diet. In this study, a microalgal strain named HDMA-11 was isolated from Lake Ming, China, and was found to accumulate a high ALA content (39.2% of total lipids). Phylogenetic neighbor-joining analysis indicated that HDMA-11 belongs to the genus Monoraphidium (Selenastraceae, Sphaeropleales) and its 18S ribosomal DNA sequence seemed to be a new molecular record of a Monoraphidium species. The fatty acid profiles, biomass productivity and lipid content of HDMA-11 were also investigated in autotrophic conditions. The high levels of polyunsaturated fatty acids in HDMA-11, especially ALA, make it suitable as a source of nutritional supplementation for human health. Furthermore, HDMA-11 exhibited good properties for biodiesel production, characterized by high lipid content (28.5% of dry weight), moderate biomass productivity (31.5 mg L^-1 day^-1) and a promising lipid profile.

Traced on the Timeline: Discovery of Acetylcholine and the Components of the Human Cholinergic System in a Primitive Unicellular Eukaryote Acanthamoeba spp

Acetylcholine (ACh) is the neurotransmitter of cholinergic signal transduction that affects the target cells via muscarinic (mAChR) and nicotinic (nAChR) cholinergic receptors embedded in the cell membrane. Of the cholinergic receptors that bind to ACh, the mAChRs execute several cognitive and metabolic functions in the human central nervous system (CNS). Very little is known about the origins and autocrine/paracrine roles of the ACh in primitive life forms. With the recent report of the evidence of an ACh binding mAChR1 like receptor in Acanthamoeba spp., it was tempting to investigate the origin and functional roles of cholinergic G-Protein coupled receptors (GPCRs) in the biology of eukaryotes. We inferred the presence of ACh, its synthetic, degradation system, and a signal transduction pathway in an approximately ∼2.0 billion year old primitive eukaryotic cell Acanthamoeba castellanii. Bioinformatics analysis, ligand binding prediction, and docking methods were used to establish the origins of enzymes involved in the synthesis and degradation of ACh. Notably, we provide evidence of the presence of ACh in A. castellanii by colorimetric analysis, which to date is the only report of its presence in this primitive unicellular eukaryote. We show the evidence for the presence of homology of evolutionary conserved key enzymes of the cholinergic system like choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) in A. castellanii spp., which were found to be near identical to their human counterparts. Tracing the origin, functions of ACh, and primeval mAChRs in primitive eukaryotic cells has the potential of uncovering covert cholinergic pathways that can be extended to humans in order to understand the states of cholinergic deficiency in neurodegenerative diseases (ND).

Chlorella sorokiniana Extract Improves Short-Term Memory in Rats

Increasing evidence shows that eukaryotic microalgae and, in particular, the green microalga Chlorella, can be used as natural sources to obtain a whole variety of compounds, such as omega (ω)-3 and ω-6 polyunsatured fatty acids (PUFAs). Although either beneficial or toxic effects of Chlorella sorokiniana have been mainly attributed to its specific ω-3 and ω-6 PUFAs content, the underlying molecular pathways remain to be elucidated yet. Here, we investigate the effects of an acute oral administration of a lipid extract of Chlorella sorokiniana, containing mainly ω-3 and ω-6 PUFAs, on cognitive, emotional and social behaviour in rats, analysing possible underlying neurochemical alterations. Our results showed improved short-term memory in Chlorella sorokiniana-treated rats compared to controls, without any differences in exploratory performance, locomotor activity, anxiety profile and depressive-like behaviour. On the other hand, while the social behaviour of Chlorella sorokiniana-treated animals was significantly decreased, no effects on aggressivity were observed. Neurochemical investigations showed region-specific effects, consisting in an elevation of noradrenaline (NA) and serotonin (5-HT) content in hippocampus, but not in the prefrontal cortex and striatum. In conclusion, our results point towards a beneficial effect of Chlorella sorokiniana extract on short-term memory, but also highlight the need of caution in the use of this natural supplement due to its possible masked toxic effects.

Elevated CO2 improves lipid accumulation by increasing carbon metabolism in Chlorella sorokiniana

Supplying microalgae with extra CO2 is a promising means for improving lipid production. The molecular mechanisms involved in lipid accumulation under conditions of elevated CO2, however, remain to be fully elucidated. To understand how elevated CO2 improves lipid production, we performed sequencing of Chlorella sorokiniana LS-2 cellular transcripts during growth and compared transcriptional dynamics of genes involved in carbon flow from CO2 to triacylglycerol. These analyses identified the majority genes of carbohydrate metabolism and lipid biosynthesis pathways in C. sorokiniana LS-2. Under high doses of CO2 , despite down-regulation of most de novo fatty acid biosynthesis genes, genes involved in carbohydrate metabolic pathways including carbon fixation, chloroplastic glycolysis, components of the pyruvate dehydrogenase complex (PDHC) and chloroplastic membrane transporters were upexpressed at the prolonged lipid accumulation phase. The data indicate that lipid production is largely independent of de novo fatty acid synthesis. Elevated CO2 might push cells to channel photosynthetic carbon precursors into fatty acid synthesis pathways, resulting in an increase of overall triacylglycerol generation. In support of this notion, genes involved in triacylglycerol biosynthesis were substantially up-regulated. Thus, elevated CO2 may influence regulatory dynamics and result in increased carbon flow to triacylglycerol, thereby providing a feasible approach to increase lipid production in microalgae.

Enhanced lipid accumulation of photoautotrophic microalgae by high-dose CO2 mimics a heterotrophic characterization

Microalgae possess higher photosynthetic efficiency and accumulate more neutral lipids when supplied with high-dose CO2. However, the nature of lipid accumulation under conditions of elevated CO2 has not been fully elucidated so far. We now revealed that the enhanced lipid accumulation of Chlorella in high-dose CO2 was as efficient as under heterotrophic conditions and this may be attributed to the driving of enlarged carbon source. Both photoautotrophic and heterotrophic cultures were established by using Chlorella sorokiniana CS-1. A series of changes in the carbon fixation, lipid accumulation, energy conversion, and carbon-lipid conversion under high-dose CO2 (1-10%) treatment were characterized subsequently. The daily carbon fixation rate of C. sorokiniana LS-2 in 10% CO2 aeration was significantly increased compared with air CO2. Correspondingly, double oil content (28%) was observed in 10% CO2 aeration, close to 32.3% produced under heterotrophic conditions. In addition, with 10% CO2 aeration, the overall energy yield (Ψ) in Chlorella reached 12.4 from 7.3% (with air aeration) because of the enhanced daily carbon fixation rates. This treatment also improved the energetic lipid yield (Ylipid/Es) with 4.7-fold, tending to the heterotrophic parameters. More significantly, 2.2 times of carbon-lipid conversion efficiency (ηClipid/Ctotal, 42.4%) was observed in 10% CO2 aeration, towards to 53.7% in heterotrophic cultures, suggesting that more fixed carbon might flow into lipid synthesis under both 10% CO2 aeration and heterotrophic conditions. Taken together, all our evidence showed that 10% CO2 may push photoautotrophic Chlorella to display heterotrophic-like efficiency at least in lipid production. It might bring us an efficient model of lipid production based on microalgal cells with high-dose CO2, which is essential to sustain biodiesel production at large scales.