Fuelling the Future

Biofuels…the saviour of the energy crisis? Or a curse that diverts food resources from the needy?

Scientists are well aware of the arguments against using plant material to fuel the future. Current research is responding to this by investigating how the “non-foody” bits of plants – the dry matter called lignocellulose – could be converted into the next generation of biofuels. The problem is that this material was designed for strength and durability – think of a giant redwood’s canopy towering above the forest floor –not for rapid breakdown into a combustible fuel. This has led to two approaches:

1. Developing plants whose cell walls are more easily digestible WITHOUT compromising field performance

Or

1. Finding novel enzymes that allow a more effective pre-treatment.

These avenues of research are being coordinated by a new initiative: This: Meanwhile, Simon McQueen-Mason (University of York) described his research on a curious organism, the marine wood borer Limnoria (also known as “the gribble”). Most animals, including cows and rabbits, rely on bacteria in their guts to supply the enzymes necessary to break down tough plant material, yet Limnoria is unusual in that it has a completely sterile gut… hence it must manufacture itself the enzymes it uses to bore holes into wooden boats. These include the glycosyl hydrolase GH7 – more commonly found in filamentous fungi, Limnoria is the first known animal to possess it. The enzyme specific to the wood borer, however, has very high stability to salt – an adaption to the marine environment. An additional quirk is that the enzyme for Limnoria attacks the crystalline arrays of cell wall compounds, rather than the non-crystalline parts that would be easier to digest. This suggests that Limnoria targets wooden materials specifically to obtain glucose. It may be that the borer does not possess any enzymes for the non-crystalline hemicellulose material. Researchers have also found, however, that even before the ingested wooden material reaches the gribble gut with its arsenal of enzymes, it is subjected to a special “pre-treatment” to loosen the structure before digestion. This is achieved using a specific chemical secreted by a gland called the hepatopancreas. Besides reducing the need for bacterial enzymes, this organ also stores toxins released from the wood, preventing them from reaching the gut.

So how could this knowledge be used to keep us on the move in the future? Scientists are hoping to sequence the genes encoding the gribble enzymes in order to introduce them into bacteria, allowing mass-scale production. The pre-treatment process that the gribble uses meanwhile, could help develop new ways of preparing lignocellulose material for biofuel production that don’t require the prohibitively high temperatures currently used.

Peter Eastmond (Rothamsted Research), meanwhile, described the research efforts focused on diverting vegetable oil production away from oil palm, currently the highest yielding terrestrial oil crop but also the cause of large scale rainforest destruction to clear ground for plantations. Seeds, with their high content of lipids (especially triacylglycerides), are an incredibly dense source of vegetable oil, with this accounting for up to 70% of seed dry weight. As a result, oilpalm, soybean and rapeseed are favoured for biodiesel production. The proportion of the plant made up of seeds however, is relatively small, especially compared with the amount of woody/ lignocellulose material.  The woody material of Miscanthus grass, meanwhile, can be used to make bioethanol, although this material only contains approximately 0.1% of vegetable oil in dry weight. If Miscanthus could be engineered to convert 20% of its biomass into oil, it could produce a yield seven times that of rapeseed in terms of biofuel production. Besides growing rapidly and being very hardy, these grasses can be grown in Europe, thus potentially saving tropical regions from being converted into biofuel plantations.  This won’t be possible using traditional plant breeding (selecting the strains with the highest natural oil content and breeding these together) but will require completely re-engineering the cellular metabolism. Preliminary work on the model organism Arabidopsis (thale cress) has demonstrated that introducing certain combinations of genes CAN increase the oil content of vegetative tissues by up to 400 times… the challenge now is to repeat the process in a suitable crop plant for industrial applications. In the Arabidopsis system, oil accumulation was particularly enhanced when the gene SUGAR-DEPENDENT 1 was switched off – this seems to be responsible for oil breakdown. Hopefully, these exciting findings can be used to develop crops that can satisfy our need for fuels without compromising tropical biodiversity or global food security.

 

For more information about the work being done use gribble enzymes in biofuel production, see the BBSRC article here: http://www.bbsrc.ac.uk/news/industrial-biotechnology/2012/121128-f-meet-the-gribbles.aspx .

To read more about  the work on Miscanthus, see http://www.bbsrc.ac.uk/news/industrial-biotechnology/2013/130521-pr-turning-leaf-into-biodiesel-factory.aspx.