Bioethanol from poplar: a commercially viable alternative to fossil fuel in the European Union

Pectin and Xylan Biosynthesis in Poplar: Implications and Opportunities for Biofuels Production

A potential method by which society's reliance on fossil fuels can be lessened is via the large-scale utilization of biofuels derived from the secondary cell walls of woody plants; however, there remain a number of technical challenges to the large-scale production of biofuels. Many of these challenges emerge from the underlying complexity of the secondary cell wall. The challenges associated with lignin have been well explored elsewhere, but the dicot cell wall components of hemicellulose and pectin also present a number of difficulties. Here, we provide an overview of the research wherein pectin and xylan biosynthesis has been altered, along with investigations on the function of irregular xylem 8 (IRX8) and glycosyltransferase 8D (GT8D), genes putatively involved in xylan and pectin synthesis. Additionally, we provide an analysis of the evidence in support of two hypotheses regarding GT8D and conclude that while there is evidence to lend credence to these hypotheses, there are still questions that require further research and examination.

Transgenic Poplar Designed for Biofuels

Members of the genus Populus (i.e., cottonwood, hybrid poplar) represent a promising source of lignocellulosic biomass for biofuels. However, one of the major factors negatively affecting poplar's efficient conversion to biofuel is the inherent recalcitrance to enzymatic saccharification due to cell wall components such as lignin. To this effect, there have been efforts to modify gene expression to reduce biomass recalcitrance by changing cell wall properties. Here, we review recent genetic modifications of poplar that led to change cell wall properties and the resulting effects on subsequent pretreatment efficacy and saccharification. Although genetic engineering's impacts on cell wall properties are not fully predictable, recent studies have shown promising improvement in the biological conversion of transgenic poplar to biofuels.

Hydrolysis of untreated lignocellulosic feedstock is independent of S-lignin composition in newly classified anaerobic fungal isolate, Piromyces sp. UH3-1

BACKGROUND

Plant biomass is an abundant but underused feedstock for bioenergy production due to its complex and variable composition, which resists breakdown into fermentable sugars. These feedstocks, however, are routinely degraded by many uncommercialized microbes such as anaerobic gut fungi. These gut fungi express a broad range of carbohydrate active enzymes and are native to the digestive tracts of ruminants and hindgut fermenters. In this study, we examine gut fungal performance on these substrates as a function of composition, and the ability of this isolate to degrade inhibitory high syringyl lignin-containing forestry residues.

RESULTS

We isolated a novel fungal specimen from a donkey in Independence, Indiana, United States. Phylogenetic analysis of the Internal Transcribed Spacer 1 sequence classified the isolate as a member of the genus Piromyces within the phylum Neocallimastigomycota (Piromyces sp. UH3-1, strain UH3-1). The isolate penetrates the substrate with an extensive rhizomycelial network and secretes many cellulose-binding enzymes, which are active on various components of lignocellulose. These activities enable the fungus to hydrolyze at least 58% of the glucan and 28% of the available xylan in untreated corn stover within 168 h and support growth on crude agricultural residues, food waste, and energy crops. Importantly, UH3-1 hydrolyzes high syringyl lignin-containing poplar that is inhibitory to many fungi with efficiencies equal to that of low syringyl lignin-containing poplar with no reduction in fungal growth. This behavior is correlated with slight remodeling of the fungal secretome whose composition adapts with substrate to express an enzyme cocktail optimized to degrade the available biomass.

CONCLUSIONS

Piromyces sp. UH3-1, a newly isolated anaerobic gut fungus, grows on diverse untreated substrates through production of a broad range of carbohydrate active enzymes that are robust to variations in substrate composition. Additionally, UH3-1 and potentially other anaerobic fungi are resistant to inhibitory lignin composition possibly due to changes in enzyme secretion with substrate. Thus, anaerobic fungi are an attractive platform for the production of enzymes that efficiently use mixed feedstocks of variable composition for second generation biofuels. More importantly, our work suggests that the study of anaerobic fungi may reveal naturally evolved strategies to circumvent common hydrolytic inhibitors that hinder biomass usage.

High-efficiency second generation ethanol from the hemicellulosic fraction of softwood chips mixed with construction and demolition residues

Using lignocellulosic residues for bioethanol production could provide an alternative solution to current approaches at competitive costs once challenges related to substrate recalcitrance, process complexity and limited knowledge are overcome. Thus, the impact of different process variables on the ethanol production by Saccharomyces cerevisiae using the hemicellulosic fraction extracted through the steam-treatment of softwood chips mixed with construction and demolition residues was assessed. A statistical design of experiments approach was developed and implemented in order to identify the influencing factors (various nutrient addition sources as well as yeast inoculum growth conditions and inoculation strategies) relevant for enhancing the ethanol production potential and substrate uptake. Ethanol yields of 74.12% and monomeric sugar uptakes of 82.12 g/L were predicted and experimentally confirmed in bench and bioreactor systems. This innovative approach revealed the factors impacting the ethanol yields and carbohydrate consumption allowing powerful behavioral predictions spanning different process inputs and outputs.

A novel hybrid first and second generation hemicellulosic bioethanol production process through steam treatment of dried sorghum biomass

Sweet sorghum was subjected to an impregnation step, which recovered most of the 1st generation sugars, prior to a steam-treatment extraction of the 2nd generation sugars, at three different severity factors (SF). A medium severity (3.56 SF) treatment proved to be an optimal compromise between the amount of sugars extracted and the fermentation inhibitors generated following the subsequent depolymerization approaches applied on the broth. Next, a series of detoxification approaches (ozonation, overliming and a combination of both) were investigated following a concentration and depolymerization step. Results show that higher steam-treatment severity required more intense detoxification steps. However, when combining the 1st and 2nd generation streams at a 2:1 ratio, the inhibitors did not affect the fermentation process and ethanol yields above 90% of the theoretical maximum were achieved.

A two-step optimization strategy for 2nd generation ethanol production using softwood hemicellulosic hydrolysate as fermentation substrate

Ethanol production using waste biomass represents a very attractive approach. However, there are considerable challenges preventing a wide distribution of these novel technologies. Thus, a fractional-factorial screening of process variables and Saccharomyces cerevisiae yeast inoculum conditions was performed using a synthetic fermentation media. Subsequently, a response-surface methodology was developed for maximizing ethanol yields using a hemicellulosic solution generated through the chemical hydrolysis of steam treatment broth obtained from residual softwood biomass. In addition, nutrient supplementation using starch-based ethanol production by-products was investigated. An ethanol yield of 74.27% of the theoretical maximum was observed for an initial concentration of 65.17g/L total monomeric sugars. The two-step experimental strategy used in this work represents the first successful attempt to developed and use a model to make predictions regarding the optimal ethanol production using both softwood feedstock residues as well as 1st generation ethanol production by-products.

Different Routes for Conifer- and Sinapaldehyde and Higher Saccharification upon Deficiency in the Dehydrogenase CAD1

In the search for renewable energy sources, genetic engineering is a promising strategy to improve plant cell wall composition for biofuel and bioproducts generation. Lignin is a major factor determining saccharification efficiency and, therefore, is a prime target to engineer. Here, lignin content and composition were modified in poplar (Populus tremula × Populus alba) by specifically down-regulating CINNAMYL ALCOHOL DEHYDROGENASE1 (CAD1) by a hairpin-RNA-mediated silencing approach, which resulted in only 5% residual CAD1 transcript abundance. These transgenic lines showed no biomass penalty despite a 10% reduction in Klason lignin content and severe shifts in lignin composition. Nuclear magnetic resonance spectroscopy and thioacidolysis revealed a strong increase (up to 20-fold) in sinapaldehyde incorporation into lignin, whereas coniferaldehyde was not increased markedly. Accordingly, ultra-high-performance liquid chromatography-mass spectrometry-based phenolic profiling revealed a more than 24,000-fold accumulation of a newly identified compound made from 8-8 coupling of two sinapaldehyde radicals. However, no additional cinnamaldehyde coupling products could be detected in the CAD1-deficient poplars. Instead, the transgenic lines accumulated a range of hydroxycinnamate-derived metabolites, of which the most prominent accumulation (over 8,500-fold) was observed for a compound that was identified by purification and nuclear magnetic resonance as syringyl lactic acid hexoside. Our data suggest that, upon down-regulation of CAD1, coniferaldehyde is converted into ferulic acid and derivatives, whereas sinapaldehyde is either oxidatively coupled into S'(8-8)S' and lignin or converted to sinapic acid and derivatives. The most prominent sink of the increased flux to hydroxycinnamates is syringyl lactic acid hexoside. Furthermore, low-extent saccharification assays, under different pretreatment conditions, showed strongly increased glucose (up to +81%) and xylose (up to +153%) release, suggesting that down-regulating CAD1 is a promising strategy for improving lignocellulosic biomass for the sugar platform industry.

Bioethanol from poplar clone Imola: an environmentally viable alternative to fossil fuel?

BACKGROUND

Environmental issues, e.g. climate change, fossil resource depletion have triggered ambitious national/regional policies to develop biofuel and bioenergy roles within the overall energy portfolio to achieve decarbonising the global economy and increase energy security. With the 10 % binding target for the transport sector, the Renewable Energy Directive confirms the EU's commitment to renewable transport fuels especially advanced biofuels. Imola is an elite poplar clone crossed from Populus deltoides Bartr. and Populus nigra L. by Research Units for Intensive Wood Production, Agriculture Research Council in Italy. This study examines its suitability for plantation cultivation under short or very short rotation coppice regimes as a potential lignocellulosic feedstock for the production of ethanol as a transport biofuel. A life cycle assessment (LCA) approach was used to model the cradle-to-gate environmental profile of Imola-derived biofuel benchmarked against conventional fossil gasoline. Specific attention was given to analysing the agroecosystem fluxes of carbon and nitrogen occurring in the cultivation of the Imola biomass in the biofuel life cycle using a process-oriented biogeochemistry model (DeNitrification-DeComposition) specifically modified for application to 2G perennial bioenergy crops and carbon and nitrogen cycling.

RESULTS

Our results demonstrate that carbon and nitrogen cycling in perennial crop-soil ecosystems such as this example can be expected to have significant effects on the overall environmental profiles of 2G biofuels. In particular, soil carbon accumulation in perennial biomass plantations is likely to be a significant component in the overall greenhouse gas balance of future biofuel and other biorefinery products and warrants ongoing research and data collection for LCA models. We conclude that bioethanol produced from Imola represents a promising alternative transport fuel offering some savings ranging from 35 to 100 % over petrol in global warming potential, ozone depletion and photochemical oxidation impact categories.

CONCLUSIONS

Via comparative analyses for Imola-derived bioethanol across potential supply chains, we highlight priority issues for potential improvement in 2G biofuel profiling. Advanced clones of poplar such as Imola for 2G biofuel production in Italy as modelled here show potential to deliver an environmentally sustainable lignocellulosic biorefinery industry and accelerate advanced biofuel penetration in the transport sector.

Using Populus as a lignocellulosic feedstock for bioethanol

Populus species along with species from the sister genus Salix will provide valuable feedstock resources for advanced second-generation biofuels. Their inherent fast growth characteristics can particularly be exploited for short rotation management, a time and energy saving cultivation alternative for lignocellulosic feedstock supply. Salicaceae possess inherent cell wall characteristics with favorable cellulose to lignin ratios for utilization as bioethanol crop. We review economically important traits relevant for intensively managed biofuel crop plantations, genomic and phenotypic resources available for Populus, breeding strategies for forest trees dedicated to bioenergy provision, and bioprocesses and downstream applications related to opportunities using Salicaceae as a renewable resource. Challenges need to be resolved for every single step of the conversion process chain, i.e., starting from tree domestication for improved performance as a bioenergy crop, bioconversion process, policy development for land use changes associated with advanced biofuels, and harvest and supply logistics associated with industrial-scale biorefinery plants using Populus as feedstock. Significant hurdles towards cost and energy efficiency, environmental friendliness, and yield maximization with regards to biomass pretreatment, saccharification, and fermentation of celluloses and the sustainability of biorefineries as a whole still need to be overcome.