New study shows way to fourth-generation biofuels: scientists uncover mechanism that regulates carbon dioxide fixation in plants

A team of Biotechnology and Biological Sciences Research Council (BBSRC) funded scientists at the University of Essex has discovered a new mechanism that slows the process of carbon dioxide fixation in plants. The research, to be published today in the Proceedings of the National Academy of Sciences, may ultimately lead to dramatic crop improvements and, they say, "fourth generation" biofuels that remove CO2 from the atmosphere. The scientists reveil a crucial mechanism of the Calvin cycle, which regulates the way plants deal with the ultimate variable: the amount of sunshine they receive.

From first to fourth generation fuels
Biofuel development is currently undergoing a transition from first to second generation fuels that can be made from any biomass source. But scientists are already going beyond the new generation and are thinking of terms of a third, and even fourth-generation (overview in this previous post).

The first generation of biofuels was based on utilizing easily extractible sugars, starches and oils. These carbohydrates and triglycerides come in the form of food: sugar from beets or sugarcane, grains like wheat and maize, or oil from rapeseed or oil palms. These fuels have received their fair deal of criticism, because the perception is that their production is a factor in increased food prices.

A far more sustainable way to make biofuels is to use biochemical and thermochemical conversion methods to turn lignocellulosic biomass into fuels. Some of these techniques, such as gasification and synthesis via the Fischer-Tropsch process are already competitive with oil at over $65. Others are receiving a lot of research attention. If these conversion technologies become cost-competitive and efficient, then the food versus fuel debate is set to end. Lignin and cellulose are the most abundant organic polymers on Earth.

Now a third-generation of biofuels goes a step further: it is based on dedicated crops the properties of which have been designed in such a way that they conform to a particular conversion process. An example would be maize which grows its own cellulase enzymes (previous post), or trees and crops with less lignin which means more cellulose can be converted (more here). These crops have already been developed and many more are under investigation.

The most radical and futuristic biofuels add a component to the production process: the production process is coupled to carbon capture and storage techniques, which allows for the production of carbon-negative bioenergy (contrary to the previous generations, which are merely 'carbon neutral'). In practise this means a transition to a decarbonised biofuel better known as bio-hydrogen the CO2 of which has been captured. Alternatively and perhaps more feasible is a transition to electric transport based on carbon-negative bio-electricity, generated from fourth generation biomass systems.

Crucial to make this transition more efficient is the development of crops that sequester more CO2 than normal plants. Such high-carbon plants withdraw the greenhouse gas from the atmosphere and use it to grow more lignocellulose. When during their conversion into biohydrogen (or bio-electricity) more CO2 is captured and stored, it means they become more carbon-negative. The first crops with a higher CO2 storing capacity have meanwhile been developed: an eucalyptus tree that stores more CO2 and grows less ligning but more cellulose (previous post), and a hybrid larch that sequesters up to 30% more CO2 (earlier post).

The strange world of carbon-negative bioenergy means that more one were to use these fuels in a car, the more one would be fighting climate change: driving more would be good for the planet and help end global warming (previous post). Contrary to other renewables like solar or wind power, which only yield "carbon neutral" energy, carbon-negative biofuels actively remove CO2 from the past from the atmosphere. They generate "negative emissions". Thus they are the most radical tool in the climate fight.

Cracking the Calvin cycle
In a finding that further paves the way for these fourth-generation biofuels and dramatic crop improvements, the scientists from Essex have discovered the mechanism which helps to regulate the way in which plants absorb CO2 from the atmosphere and turn it into sugars.

Plants are dependent on sunlight to capture carbon dioxide, which is turned into important sugars via a process called the Calvin cycle (schematic, click to enlarge). As a result, as the amount of sunlight varies during the day (e.g. through cloud cover or shading from other plants), they must also be able to vary the speed at which they capture carbon dioxide from the atmosphere. This ensures that when there is a lot of sunlight, it is taken full advantage of but that when sunlight drops, so does CO2 uptake. This ability to maximise energy use is important for plants and prevents the loss of important metabolic resources. Because they essentially stay in one place, plants must have many unique abilities to adapt to their environment as it changes around them.

Scientists have been trying to find out how this variable speed control actually works. For the first time they now show how the Calvin cycle can be regulated in response to a changing light environment via a molecular mechanism. There is a special relationship between two enzymes that are involved in the Calvin cycle: phosphoribulokinase (PRK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH):
energy :: sustainability :: biomass :: bioenergy :: biofuels :: lignocellulose :: carbon dioxide :: Calvin cycle :: plant biology :: efficiency :: carbon-negative :: climate change ::

When light levels decrease, the two enzymes tend to stick together and therefore cannot function, thus slowing the Calvin cycle. The darker it is, the more PRK-GAPDH partnerships are formed and the slower the Calvin cycle becomes. In the light, they break apart rapidly and the Calvin cycle is allowed to speed up.

This fundamental research has thus revealed a novel mechanism and provides a better understanding of the regulation of CO2 fixation in plants. This work will underpin strategies to increase the amount of carbon dioxide absorbed by plants thereby increasing yield for food and biofuel production, and may ultimately feed into the development of "fourth generation" biofuels.

Although this research focuses on the fundamental biological processes that plants use, ultimately, if we can understand these processes, we can use the knowledge to develop and improve food and biofuel crops. - Professor Christine Raines of the University of Essex, Research Leader

Dr Tom Howard, who contributed to the research, added that plants have evolved a fascinating way to cope with variations in their local environments. Unlike animals, they cannot move on to look for new food sources. This research helps to unlock one way that plants deal with the ultimate variable: the amount of sunshine they receive.

References:
Thomas P. Howard, Metodi Metodiev, Julie C. Lloyd, and Christine A. Raines, "Thioredoxin-mediated reversible dissociation of a stromal multiprotein complex in response to changes in light availability", PNAS, March 4 early edition [no link available at the time of publishing; check back later today].

Eurekalert: Scientists uncover a novel mechanism that regulates carbon dioxide fixation in plants - March 4, 2008.

Biopact: A quick look at 'fourth generation' biofuels - October 08, 2007

Biopact: Japanese scientists develop hybrid larch trees with 30% greater carbon sink capacity - October 03, 2007

Biopact: Scientists develop low-lignin eucalyptus trees that store more CO2, provide more cellulose for biofuels - September 17, 2007

Biopact: Third generation biofuels: scientists patent corn variety with embedded cellulase enzymes - May 05, 2007

Biopact: Scientists release new low-lignin sorghums: ideal for biofuel and feed - September 10, 2007

Biopact: The strange world of carbon-negative bioenergy: the more you drive your car, the more you tackle climate change - October 29, 2007