Production of biofuels by microorganisms

Although currently the only biofuels produced on a large scale are ethanol and biodiesel, different classes of molecules have desirable properties for this purpose and some are capable of being produced microbially.

Others, although not normally synthesized by microorganisms, can become so with the use of biotechnological tools. This work reviews the use of fermentations in the production of biofuels and gathers information about those that are in the development stage and have application prospects. The objective here is not to exhaust the subject, but to enable the beginning of a more in-depth study in the lines of research that are considered most promising on the topic.

Ethanol production is based on ancient technology, considering the consumption of alcoholic beverages in times before Christianity. Incredibly, the ethanol production process currently uses the same microorganism (the yeast Saccharomyces cereviseae) and achieves practically the same concentration that has been obtained for centuries. Since currently the cost of the raw material (90% of ethanol production today is made from corn and sugar cane) typically represents between 60 and 70% of the final cost of ethanol, several studies have been carried out in around the world to develop large-scale production of ethanol from lignocellulosic raw materials. However, due to the great complexity of the composition of these materials, there are still no economically viable processes using such substrates. Therefore, to significantly reduce capital and operating costs, efforts have been made to identify and develop microorganisms suited to the characteristics of the raw material and capable of fermenting different sugars with high conversion factors.

Butanol is another alcohol that can be produced through fermentation. In this bioprocess, which was, in the 1960s, the most important in the world, acetone is produced together with butanol and ethanol (ABE 3:6:1) by fermentation with the microorganism Clostridium. acetobutylicum. n-Butanol has desirable characteristics as a gasoline substitute fuel, such as: energy density 50% higher than ethanol and being able to be transported through pipes, as it carries a lower water content. As its performance in engines is not yet well studied, it could be used as a blending fuel.

Biodiesel is currently obtained by a catalytic transesterification reaction of different oilseeds, as well as used frying oil and animal fat in the presence of methanol or ethanol. Despite the advantageous environmental aspects, chemically produced biodiesel has some limitations that could be avoided with a biochemical process.

Although microorganisms do not produce biodiesel through their typical metabolism, some, such as the bacterium Acinetobacter baylyi, produce significant amounts of storage lipids in the form of triacylglycerides and fatty esters. Thus, it was possible to produce a modified E. coli strain containing the genes involved in the synthesis of fatty esters and the ethanol-producing genes of the bacterium Zymomona mobilis. This recombinant strain, when in a medium containing fatty acids, produced biodiesel at 26% of the dry weight of the biomass, under ideal conditions. Although this conversion is far from necessary for the development of an industrial process, it was possible to prove the viability of this new approach.

Long-chain hydrocarbons are very common fuels today, mainly isooctane (the main detonator in gasoline) and hexadecane (one of the hydrocarbons in the diesel range), both derived almost entirely from petroleum. Although there are reports demonstrating the biosynthesis of alkanes by animals, plants and microorganisms, this occurs in low concentrations and its production mechanism is not known. The challenge of biotechnology is to increase conversion rates by using cheap carbon sources as appropriate sized feedstocks that are considered optimal fuels.

Hydrogen is promising as a biofuel due to its great energy potential and because its combustion product, water, does not create environmental problems. On the other hand, high volatility creates restrictions on storage, which makes transportation over long distances very complicated. Its current production technology through electrolysis is energy inefficient since an input with high energy conversion (electricity) is used to generate another with low energy (hydrogen, which has only 50% conversion efficiency). Alternatively, biological systems can be used to generate hydrogen, but these typically have a low conversion rate and several research efforts have been made to increase productivity using metabolic engineering techniques. Another promising way to utilize biochemically produced hydrogen is the use of microbial fuel cells, in which a compatible electrode (anode) captures electrons from the hydrogen produced by microbial metabolism, releasing H+ and then generating an electrical current. The success of this process will depend on optimizing microbial hydrogen production and developing electrodes to capture this hydrogen.

Biofuels produced from biomass can be produced thermochemically by processes such as pyrolysis and gasification or biochemically by microbial fermentation and are sustainable sources of energy with great potential for a favorable carbon balance. In the case of biofuels produced by microorganisms, as we intended to show in this article, there is still a lot to be done, and biotechnology will contribute decisively to this effort, both in the development of plants more suitable for bioprocesses and in the development of microorganisms. This constitutes a vast field for research and industry, which could create a new generation of biofuels that have an immense market and consequently high profit potential. It's a big challenge, but it will certainly be worth the effort.

Cristina Maria Monteiro Machado

Researcher at Embrapa Agroenergia, Brasília (DF).

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