Second Generation Biofuels

As part of my course last year, I wrote an extended long essay concerning the use of bacteria in biorefineries. As I've had a very lazy weekend (and to celebrate crossing the hundred post mark) I've decided to reproduce some of it here. More may be forthcoming at some point, depending on the laziness of my weekends.

Second Generation Biofuels

Second generation biofuels consist of lignocellulose material, which is broken down into simple sugars via enzymatic reactions and then fermented to produce ethanol. As lignocelluloses can be found in inedible plant matter (e.g corn husks, rice stems, and wheat stalks) they have the advantage that, unlike first generation biofuels, their utilisation does not compete with food production.

The three main components of lignocelluloses material are celluloses, hemicelluloses and lignin. These cannot be fermented directly and must therefore be broken down:The pre-treatment of the biomass is necessary both to remove lignin (although effective ligninases have been found in white-rot fungi, their rate of product turnover in bacteria is still too slow to be commercially successful) and to partially break down the cellulose to allow easier digestion by microbial processes. As the pre-treatment consists of harsh chemical processes, it would be advantageous within a biorefinery to use microbes which are able to withstand high temperatures and low pHs. For example, the cloning of thermostable cellulases into Trichoderma reesei allows a higher hydrolysis temperature compared to commercial Trichoderma enzyme, reducing the energy needed to cool the system after pretreatment with steam. The ability to save energy in this way could have a large economic impact, making the biorefinery more commercially feasible.

Currently one of the most popular microorganisms for use in lignocellulose biofuel production is Clostridium thermocellum which has an optimum temperature of around 60°C and also contains a cellulosome; a multi-protein cellulose-degrading complex attached to the bacterial cell wall. Cellulosomes are found in several bacteria, both Gram negative and positive, although they can differ in their structure and organisation (particularly of the cohesins and dockerins).
As potentially the entire process of ethanol production from lignocelluloses could be carried out by the microbes within a fermentor, the use of second generation biofuels in biorefineries has generated a lot of interest. The three main economic obstacles are the high processing costs, the narrow margin between biomass and fuel prices, and the large capital investment needed to initiate a cellulosic biorefinery. This could however, be overcome by increasing the potential for the production of high-value goods alongside the biofuel, either by adding pathways for the production of oleochemicals or bioplastics to the fermenting bacteria, or by utilising the lignin. This would provide the biorefinery with a greater capital return.

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Gilbert, H. (2007). Cellulosomes: microbial nanomachines that display plasticity in quaternary structure Molecular Microbiology, 63 (6), 1568-1576 DOI: 10.1111/j.1365-2958.2007.05640.x

Blumer-Schuette, S., Kataeva, I., Westpheling, J., Adams, M., & Kelly, R. (2008). Extremely thermophilic microorganisms for biomass conversion: status and prospects Current Opinion in Biotechnology, 19 (3), 210-217 DOI: 10.1016/j.copbio.2008.04.007

Zhang, Y. (2005). Cellulose utilization by Clostridium thermocellum: Bioenergetics and hydrolysis product assimilation Proceedings of the National Academy of Sciences, 102 (20), 7321-7325 DOI: 10.1073/pnas.0408734102