Is it time to consider plant power?
Global climate change is now accepted by a substantial majority of the world’s climatologists. Together with the dangers of the overpopulation, come the environmental and resource issues that suggest we may face an apocalyptic future. Is there any reasonable solution to this?” Dr. Alexander Ruban, Lecturer in Biological Chemistry, Queen Mary College at the University of London sets out the case for the optimisation of photosynthesis as a means of increasing the future viability of ‘bioenergy'
Part of the Government's strategy for tackling climate change will be an increased reliance on so-called bioenergy, the generation of electricity, heat or transport fuels from renewable (non-fossil) biological sources: typically by the combustion of fresh or processed biomass or, potentially, electricity generation through reactions in microbial fuels cells. Bioenergy has been used in countries such as Brazil for many years via ethanol produced from fermenting sugar cane, but development in the UK has been held back in previous years by the low cost of crude oil and resultant policy development. Commentators continue to argue about how much bioenergy could conceivably contribute to the UKs future energy needs, some argue it will only ever be a niche contributor while others argue it could be a major facet of energy supply (even with a nuclear revival possibly on the cards).
It does seem clear however that many factors will increase the attraction of bioenergy in future decades; notably the continued decline of fossil fuel reserves and increased cost of extracting those that remain, combined with the possibility of higher oil prices from continued instability in the Middle East. Another factor will be land use competition increasing with greater pressures for multi-use crops e.g. where the grain is harvested for food while the remaining biomass is collected for fermentation to produce ethanol for fuel. The final obvious benefit of such bioenergy is its low carbon credentials, something that is likely to be a preoccupation of governments for many years to come.
When considering bioenergy there are various strategies that could potentially be employed to increase the efficiency of energy conversion in bioenergy crops: and these are outlined in a recent government funded report by the Biotechnology and Biological Sciences Research Council (BBSRC). A large part of the report is given over to how photosynthesis, the process by which plants and certain microbes use light from the sun to convert carbon dioxide into sugars for energy, can be improved. To set this in context we must understand the source of the energy on our planet: the Sun.
To do this let us look at some global energy balance figures. The Earth is bombarded with Suns energy particles, photons, with a yearly input of 3 x10 Joules; this mainly remains unused by mankind and also by the organisms which have evolved to utilise this energy - plants and photosynthetic microbes. They use only a tiny 1/300th of the solar energy received by the earth. What about humans? We need only about 10 Joules of energy annually, which is 30000 times less than that offered by Sun and only 1/10000 of the manmade energy actually is actually taken from the big orange ball. Currently we use only slightly more than a one billionth fraction of the energy offered by Sun. We only currently use less than 1% of what plants produce as biomass every year and most of that is used as food and very little for bio fuels. For the rest of the energy we need, we utilise fossil fuels which even taking the most optimistic prognoses into account retain energy equal to that stored in all currently existing plants on the planet and will only last for the next 60 years or so.
From the figures above we can see that Photosynthesis is currently not an efficient process and therefore there is a great potential to increase its yield, first of all by optimising the light energy capture and, second, the efficiency of carbon dioxide fixation (which plants utilise to create carbohydrates). A small improvement in photosynthetic efficiency can lead to a gigantic gain in the energy stored by plants and other photosynthetic organisms. The benefit of increasing photosynthesis yields is a concomitant increase in the amount of carbon dioxide taken out of the atmosphere by plants. But how could such an increase in yield be achieved? Genetic manipulation of existing plant material could enable us to enhance the ability of plants to capture more of the light energy and enhance the tolerance of plants to extreme environmental conditions.
Despite this the Government funded report lacks focus on these two vital processes- increasing the yield of light energy capture and enhancing tolerance to extreme environmental conditions. This is particularly puzzling when it is these processes that cause the largest losses of energy that undermine photosynthetic efficiency.
Those losses are mainly due to the limitations in the ability of the plant pigments (such as chlorophyll) to capture light quanta of a sufficiently broad range of energies (plants do not absorb light in the green areas of the visible spectrum) and the losses of energy due to incomplete absorption of light (caused by scattering when light hits the leaf). Environmental stresses, such as low temperature and drought, cause losses of energy that affect the photosynthetic efficiency and so the biomass achieved. Therefore even if small improvements are achieved in the light energy absorption and stress management by genetic manipulation of photosynthetic organisms, a very large gain in the energy storage could be expected.
If we could achieve these increases in energy storage then there would be more biomass produced per unit land area and the viability of bio fuels as significant future energy source would be a step closer.
In summing up, improving the yield of photosynthesis is a potential panacea for both the development of economically viable bioenergy and in reducing global CO2 concentrations. However for such an approach to be properly explored it would require acknowledging the current expertise that exists in the UK by supporting those research laboratories maintaining the traditions vital to the kind of world-class photosynthesis research that will be required in the challenging times to come.
- Liquid biofuels and renewable hydrogen to 2050 DT (2004) see http://www.dti.gov.uk/energy/sepn/h2bioassesment.pdf http://www.dti.gov.uk/energy/sepn/h2bioassesment.pdf
- Review of Bioenergy Research BBSRC (2006) see "http://www.bbsrc.ac.uk/about/pub/reports/bioenergyreview.pdf"http://www.bbsrc.ac.uk/about/pub/reports/bioenergyreview.pdf
- Bolton, J.R.. and Hall, D.O. (1979). Photochemical conversion and storage of solar energy. Annu. Rev. Energy, 4, 353-401