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methanotrophs (methane-using bacteria), which convert methane and other components of natural gas into
liquid hydrocarbons that can be used to make fuels and chemicals (CBD 2014). Another approach for the
application of synthetic biology is the production of consolidated bioprocessing platforms for further use in
biofuel production. The process works via engineering microorganisms to generate properties that allowed
them to digest plant biomass and to convert it to hydrocarbons that have the characteristics of advanced
biofuels. The requirements for such an organism are multiple pathways for hydrocarbon production and the
capacity to sufficient enzymes to efficiently hydrolyse cellulose and hemicellulose. An example in this research
field is the biofuel production from ionic liquid-pretreated switchgrass (
Panicum viragtum L.) using engineered
Escherichia coli, without the addition of enzymes (Bokinsky et al. 2011).
Another, though long-term example are cyanobacteria that convert carbon dioxide, untreated water and
sunlight into liquid hydrocarbons that are the functional equivalent of diesel and ethanol (U.S. Patents
#7,981,647 and #7,968,321).
4.3.2
Review of existing applications in plant-like systems and higher plants
Many efforts aim at the manipulation of algae and higher plants physiology and metabolic pathways for the
production of desired products and compounds, of which biofuels (bioethanol, biodiesel and H
2
) and
pharmaceuticals currently attract most interest (Zurbriggen et al. 2012; Lee 2013). In this context, synthetic
biology approaches also allow for the production of compounds with novel chemical properties. Prokaryotic
(cyanobacteria) but also eukaryotic algae may be a target organism to produce advanced biofuels (e.g. butanol
through photosynthesis); they have the additional advantage that they may be used in a (photo)bioreactor (no
need for arable land), and may be programmed in a way not to require freshwater. Apart from biofuels, algae
may also be used to produce pharmaceuticals-related products like omega-3 fatty acids, e.g. DHA
(docosahexaenoic acid) and EPA (eicosapentaenoic acid), ARA (arachidonic acid, an omega-6 fatty acid),
chlorophylls, carotenoids, phycocyanins, allophycocyanin, phycoerythrin, etc. (Lee 2013).
Algae for the production of desired compounds
As molecular genetic manipulation of eukaryotic algae and/or prokaryotic blue-green algae (cyanobacteria) is
easier than that of higher plants this field is emerging much faster than the latter (Lee 2013). Potential
applications include biofuel-production, ultimately by direct conversion of sunlight to fuel. It may also be used
to replace biomass production on arable land and the use of freshwater by employing other sources like sea-,
ground- or even waste water.
Biofuels and byproducts can be synthesised from a large variety of algae (Menetrez 2012). Using algae and
microalgae for the production of biofuels is an attractive application due to their potential to accumulate high
amounts of lipids and due to high starch content providing a good source for bioethanol production (Safi et al.
2014). In addition, they can be cultured with an inexpensive nutrient regime, have faster growth rate
compared to terrestrial plants and high biomass production. As they provide an alternative to current biofuel
crops (e.g. soybean, corn, rapeseed and lignocellulosic feedstock) also less favoured environments (land that is
unsuitable for agriculture, brackish coastal water and seawater) may be used, adding to the potential to
provide remediation for waste (Safi et al. 2014; Menetrez 2012).
Microalgal growth may be autotrophic (photosynthesis under appropriate light conditions), heterotrophic
(without light, requiring an organic carbon source), and mixotrophic (combination of photosynthesis and an
external carbon source). Finally, co-cultivation and co-immobilisation with bacteria is possible. Microalgae are
harvested by centrifugation,
flocculation, flotation, or filtration (reviewed in Safi et al. (2014)).
Growth techniques for algae and microalgae include open pond systems that may be natural (lakes, lagoons,
ponds) or wastewater, artificial ponds, or containers and tanks (Safi et al. 2014; Menetrez 2012). Open ponds
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are the most common and cheapest way of large-scale biomass production and are in particular used for
strains with high oil content. However, open systems also require strict control systems to avoid pollution,
water evaporation, contaminants, invading bacteria and the risk of growth of other algae species. Stirring is
necessary, at least near the end of the exponential growth phase, due to potential low surface to volume ratio
and poor diffusion of CO
2
to the atmosphere. Alternatively, closed photo-bioreactors (flat-plate, tubular or
column) are used, which provide a managed environment that allows for higher cell concentration and the
production of pure pharmaceuticals, nutraceuticals and cosmetics. Major disadvantages of closed systems are
higher costs (construction, sterilisation) and small illumination area. Balancing the advantages and constraints
in both systems hybrid culture systems may be chosen.
Algae are among the most potentially significant sources of sustainable biofuels in the future of renewable
energy (Menetrez 2012). There are several areas of researches for the production of biofuels based on algae
and microalgae, classified as lower plants, and cyanobacteria, which have as bacteria the ability for oxygenic
photosynthesis. Lipids, such as triacylglyceride (TAGs), and carbohydrates, both derived by extraction from
algae, can be used as sources for the processing into biodiesel and ethanol (Menetrez 2012; Georgianna and
Mayfield 2012). Algae are commonly genetically engineered to allow modification for agricultural and
industrial biofuel production (Menetrez 2012). Therefore also synthetic biology tools get included in the algae
biomass and lipid production and further in optimisation strategies for these tools. Challenges are the finding
of the ideal algal strain for effective production of biomass and photosynthetic activity and the adaptation to
variations in temperature, light, salinity and pathogen load in agricultural systems. Strains of the algae classes
Chlorophyceae, Eustigmatophyceae (f.i.
Nannochloropsis sp.) and Haptophyceae are used to produce biofuels
and other industrial organic chemicals but many other types of algae and photosynthetic active cyanobacteria
get tested as further biofuel candidates. In agricultural production the cultivation of the algae is done in open
ponds and for industrial production bioreactors or photobioreactors are use (Georgianna and Mayfield 2012).
For efficient, large-scale and sustainable algal biofuel production further engineering strategies and
improvements are needed. In regard to applications of synthetic biology methods combined with algal
“farming” in open pond systems or bioreactors, also crop and environmental protection measures/strategies
will be required.
Biofuel production from biomass
Attempts to improve the energy-conversion efficiency and the production of biofuels from non-food
agricultural residues use the techniques of synthetic biology, and involve corn stalks, straws, grass clippings,
prairie grasses, wood chips,
and dedicated biomass crops, like sugar cane, corn, grain and switchgrass (Agrivida
2012b; Fesenko and Edwards 2014; PCSBI 2010). The goal is to produce fuels and other valuable chemical
products from simple, inexpensive and renewable starting materials in a sustainable manner (JBEI 2014). The
various synthetic biology alternatives to current biofuel production methods include producing cellulosic
ethanol (derived from cell walls rather than corn) and manufacturing other bioalcohols with synthetically
manipulated biomass. A potentially more promising bioalcohol made by synthetic biology and used for energy
production is butanol. Like ethanol, butanol is produced by the fermentation of sugars and starches or through
the breakdown of cellulose (PCSBI 2010). Companies and organisations, like Agrivida, Amyris, JBEI, British
Petroleum, DuPont, Gevo, Global Bioenergies, LS 9, Inc., try or already use synthetic biology to produce
biofuels (Lipp 2008; PCSBI 2010). However, the current market status varies and is difficult to assess (Table 4).
The efficiency in bioenergy production may be increased by modifying plant cell walls to enhance the
digestibility of lignocellulosic biomass (Lee 2013). Agrivida has created a proprietary INzyme
TM
molecular
engineering technology. The modified enzymes allow a significantly more efficient conversion of plant cell
walls into sugars through the production of embedded "dormant" enzymes, which are then activated under
specific post-harvest conditions (Agrivida 2012b; BIO 2013). The modified enzymes can break down a wider