Sugar crop

Creating sustainable biocrops

The success of biofuels depends on creating a non-food feedstock that can be successfully converted into fuel, but as E&T discovers there is no way to rush nature.

The debate has raged long and hard about the effects that the growing demand for biofuel crops has had on the world food supply. A report from the World Bank in 2008 attributed the vast share of the increase in global food prices to the large increase in biofuel production'. Claims have regularly been met with counter claims in the media, but the fact remains that the increasing use of biofuels is putting pressure on scarce resources particularly land.

The basic feedstocks for the production of first-generation biofuels are products that would normally enter the animal or human food-chain such as seeds, or grains such as wheat. These crops yield starch that is fermented into bioethanol.

The drive is therefore on to develop second- and third-generation biofuels. Second-generation present the short-term solution, and are made up of biofuels derived from feedstock outside the food chain. These include waste biomass stalks of wheat and corn as well as special energy crops such as Miscanthus and fast-growing trees. The downside is that it is difficult to extract the cellulose from the feedstock. Third-generation biofuels are algae-based.

In the UK the Biotechnology and Biological Sciences Research Council (BBSRC) has established the Sustainable Energy Centre, a virtual organisation comprising of the Universities of Cambridge, Dundee, York, Nottingham and the Rothamsted Research Centre. These institutions collaborate to carry out a variety of research to speed up the adoption of second-generation biofuels along the biofuels supply chain, including cell-wall sugars and lignin, lignocellulosic conversion, bacterial biofuels and enzyme discovery.

But at the head of the supply chain is the research carried out at Rothamsted, where Dr Angela Karp and her team are looking at the initial crop and how to optimise sustainable biomass yield by genetic improvement of plants to increase the amount of sunlight captured, the amount of carbon a plant can assimilate over a growing season and the partitioning of the carbon in harvested biomass.

'Currently biofuels are made from crops that store sugars or starches and then it is a very simple process to convert that because crops quite nicely store the simple form of sugar,' Karp says. 'All you have to do is extract it and ferment it much like you would to make alcohol and in this case we use the alcohol for fuel instead of drinking.

'The problem with that is the same crops that store sugars and starches are food crops, and in order to produce that amount of sugar they require a lot of inputs, particularly fertilisers. These inputs are energy-intensive and as a consequence when you add up the entire lifecycle and look at the amount of energy you need to grow the system and produce the sugars into fuels, compared to what you actually get out of the system, the balance isn't particularly favourable.'

The research is focused on developing a crop that is not used for food and also which is a perennial. Getting a perennial crop is important because not having to cultivate it each year means that the soil can maintain its carbon sequestration. The crop would also replenish its own nutrient supply; at the end of a cycle it would drop its leaves, returning the nutrient goes back to the system.

'This means that the amount of fertilisers or external inputs in the system is a fraction of what is needed for food crops the bulk of which are annuals and have to be grown from seed every year,' Karp adds. 'When you add all that up and look at the lifecycle analysis the total energy inputs in and what you get out it's several fold better that you would get extracting fuel out of a food crop.'

The crops that are the centre of attention are fast-growing woody trees such as Willow and also grasses like Miscanthus, often called Elephant Grass. 'The thing that these crops have going for them is that they produce large amounts of material with very [few] inputs; they grow astonishingly fast they produce a huge amount of biomass from a small area which is exactly what we need,' Karp explains.

Lignocellulose extraction

The difficulty is that the sugars that need to be converted into fuel are not stored in a simple way. They are locked up in structure of the plant itself, in the cell walls as lignocellulose. There is a very good supply of it, but lignocellulose stores the sugars as cellulose fibres interwoven with other polymers that are there in order to supply strength to the plant. There are polymers of lignin knotted together with the cellulose in a very intricate and tightly locked pattern.

'To release the sugars out of that mesh, that network, is difficult because you have to allow access for the enzymes which are involved in the fermentation process they can't get into the network,' Karp explains. 'So what you need to do is somehow either improve the structure of the cell walls so that it is easier for the enzymes to get in which means trying to select plants whose composition is amenable to breakdown by the enzymes or there has to be work done on the microbial process where the microbes are used to break the cell wall down.'

Because there is a huge amount of diversity among the plants the team are working on naturally selecting plants that can fit the bill, but Dr Karp does not rule out the use of genetic modification.

'The legality of genetically modified produce depends on where you are in the world; here in Europe it is not allowed without very strict regulations,' she adds. 'However in the case of non-food crops, such as energy crops, there is so much natural diversity that people are using that for now. There may well be a time when some specific changes such as engineering the cell wall could profit from transformation, and we certainly use genetic modification as a tool in order to see whether we have the right gene and whether the changes that we are trying to introduce by breeding are the right changes.

'There is a huge amount of diversity as these are not highly domesticated crops, and we have a lot to play around with before we need to think about the GM route, but on the other hand it would be silly not to develop a GM route because in the long-term future who knows that could become more acceptable and there might be specific conditions around it.'

New breeds

Developing new breeds of plant is not a quick process, they have to be developed and then let to grow for a full season, so the timeline of the research is not short term. 'We have a massive amount of Willow growing so we can really look at the crop in terms of what a farmer could get out of his field, which is very important,' Karp says. 'That is underpinned by work in the laboratory all the way through to bioinformatics. It is a very integrated approach through from genes and biochemistry in the lab, right through to performance in the field.'

It is expected to be between 10 and 20 years until the research delivers advanced varieties of crop into the market, although they hope to have improved on the current crops before then with varieties that Dr Karp calls 'sub-optimal'.

It is likely that although the research at Rothamsted is at the head of the supply chain it will lag behind research carried out by the other teams. 'In terms of the chemistry that could move much faster because it is a question of working with micros and enzymes where you don't have the time-lag of having to grow things out and see how they perform,' Karp agrees.

If biofuels are going be a viable and sustainable replacement for fossil-based fuels the work of Dr Karp and her colleagues around the globe needs to deliver a feedstock that can tick all the boxes, and although research the work of nature cannot be rushed, the clock is ticking for biofuels to deliver.

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