Review articleFungal metabolites as precursors to renewable transportation fuels
Introduction
The demand for liquid transportation fuels is expected to increase as the world’s population continues to grow and greater global industrialization is realized [1], [2]. In a 2010 report, the National Research Council stated, “There seems to be little doubt that … internal combustion engines will be the dominant prime mover for light-duty vehicles for many years, probably decades. … it is important to maintain an active ICE (internal combustion engine) and liquid fuels R&D program at all levels….” [3]. The development and use of sustainable liquid transportation fuels produced from renewable resources has attracted attention for addressing concerns about the environment [1], [2], [4], [5], [6], [7], [8]. Various fungi are recognized as being excellent platforms for taking advantage of biosynthetic routes to producing hydrocarbon fuels and their precursors [9]. Fungal organisms synthesize energy-dense isoprenoid and polyketide hydrocarbons through known metabolic pathways with potential to be engineered for production of infrastructure-compatible biofuel precursors from lignocellulosic biomass. A considerable number of these compounds have properties that are similar to conventional petroleum-based fuels [4]. Although there are many secondary metabolites, they are naturally produced at low levels by the organisms that do not appear to need them under normal development and growth conditions [10]. As more and more of these compounds are discovered and candidates are selected for commercial applications, the overarching challenge is to increase titer, rate, and yield to economically sustainable levels so their benefits can be realized [11], [12]. This requires development of, or improvements to, 1) metabolic pathways and other aspects of the biocatalysts (fungi) and 2) bioprocesses to facilitate production of the desired compounds at the required levels [4], [12]. Advances in metabolic engineering are making the generation of applicable host microbes that produce biofuels with properties similar to petroleum-based fuels a reality with considerable potential [7].
For this study, first the literature was searched for isoprenoid and polyketide compounds produced or predicted to be produced by filamentous fungi or yeasts. Second, Pacific Northwest National Laboratory (PNNL) fuel chemists evaluated the chemical structures of approximately 600 terpenes and 250 polyketides found in the literature to identify compounds expected to have potential for liquid hydrocarbon fuels or fuel precursors. Third, based on the results of the initial evaluation, 33 compounds were selected for further assessment. Fourth, key fuel properties were used to assess the suitability of the 33 compounds for use in specific fuels such as gasoline, diesel, jet, or high-energy fuels. The primary purpose of this study was to evaluate non-lipid compounds produced by fungi that have a variety of chemical functional groups to provide examples of compounds having potential as fuel compounds, rather than an exhaustive list of every candidate compound.
Section snippets
Hydrocarbon metabolites derived from fungal biosynthesis
Fungi are known to produce a wide variety of secondary metabolites that can be further divided into several classes, namely, terpenes and terpenoids, polyketides, non-ribosomal peptides, and alkaloids [8], [13]. Secondary metabolites from fungi encompass much of the diversity found in organic chemistry with regard to structure and functional groups. They can be relatively simple linear, branched or ring structures, analogous to paraffins, isoparaffins and cycloparaffins, respectively, or they
Chemical upgrading of biologically derived compounds
Biologically derived compounds may require upgrading to produce fuel compounds suitable for use with liquid fuel blendstocks. Specifications and properties specific to the target blendstock fuel will dictate the type of upgrading required. For example, alcohols and olefins may be desired as gasoline blendstocks due to their potential to increase the research octane number (RON) of gasoline blends [25], [26], [27], [28], [29]. In contrast, for jet fuel blendstocks, the olefin content is limited
Fungal-based compounds selected for assessment
The 33 fungal-based compounds selected for assessment from the many hundreds reviewed from the literature are shown in Table 1.
The fungi reported in the literature as producing the 33 compounds of this study are shown in Table 2. Production titers for only six of the 33 compounds were reported in the literature (Table 2).
In the sections that follow, the potential of these 33 fungal metabolites as standalone fuels or precursors suitable for upgrading to gasoline, diesel, jet fuel, or high-energy
Conventional gasoline compounds
Petroleum-based gasoline comprises over 500 individual hydrocarbon compounds that have between 4 and 14 carbon atoms per compound. Gasoline compounds belong mainly to one of the following three groups: 1) paraffins, 2) olefins, and 3) aromatics [65]. A significant portion of gasoline is composed of saturated linear hydrocarbons (i.e., paraffins), which are the most chemically stable compounds [66]. Branched paraffins (isoparaffins) are more preferable than straight chain (n-paraffins) due to
Conventional diesel fuel compounds
Diesel fuel is a mixture of thousands of compounds most of which have carbon numbers between C10 and C22 [75], [76]. Diesel fuels consist of ∼75% saturated hydrocarbons (primarily n-, iso-, and cylo-paraffins) and ∼25% aromatic hydrocarbons (naphthalenes and alkylbenzenes) [4]. To meet diesel fuel specifications, a combination of different hydrocarbon groups is needed. For instance, a mixture of paraffinic and aromatic substructures can improve cold flow properties without negatively affecting
Conventional aviation turbine fuel compounds
Jet fuels are similar to diesel fuels, but have higher boiling point ranges and contain larger molecular-weight compounds compared to gasoline. They are suitable for use in compression ignition or turbine applications and are generally of higher quality. They consist predominantly of refined hydrocarbons derived from conventional sources including crude oil, natural gas liquid condensates, heavy oil, shale oil, and oil sands [32]. Kerosene-type jet fuels such as Jet A or JP-8 consist of
Conventional high-energy fuel compounds
High-energy density fuels like JP-10 are designed for special applications, such as air-launched cruise missiles, that require very clean burning fuel, superior volumetric energy content, and low-temperature performance [31], [88]. JP-10 is a high-density (0.94 g/cm3) synthetic missile fuel composed exclusively of C10 compounds [86], [89]. It has a single chemical structure exotetrahydrodi(cyclopentadiene) that is difficult to refine from crude oil [90]. It is prepared by hydrogenating
Advances for fungal biosynthesis of hydrocarbon fuel compounds
The Joint Genome Institute, through “The 1000 Fungal Genomes Project” and many other fungal genome projects involving a multitude of collaborators is making an ever increasing number of sequenced genomes available from across the entire fungal kingdom. Because the key enzymes for production of terpenes and polyketides, terpene synthases and polyketide synthases respectively, are easily recognized by sequence homology and thus annotated within genomes, these fungal genome projects will provide
Conclusions
Considerable research has been conducted to investigate the suitability of terpenes for use as liquid hydrocarbon fuels, and several have been found to have significant potential. However, the reported production titers of these terpenes for fuels are low except for the very significant 104 g/L titer reported for farnesene. That example indicates that this is a challenging but promising approach that requires considerable process development time and resources.
Polyketides have potential as fuel
Acknowledgements
The manuscript preparation was supported by the U.S. Department of Energy under Contract No. DE-AC05-76L01830 at the Pacific Northwest National Laboratory. The PNNL authors gratefully acknowledge the support of the Office of Energy Efficiency and Renewable Energy through the: 1) the Co-Optimization of Fuels and Engines (Co-Optima) Project, sponsored by the Bioenergy Technologies (BETO) and Vehicle Technologies Offices and 2) BETO sponsored analysis research at PNNL.
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