Photosynthesis: Frontiers

A plant’s efficiency at turning CO2, water and light into biomass is extremely low – typically around 4 to 5 per cent at best. But where do these limits come from? Can they be overcome to produce crops with higher yields? Plants rely on chlorophyll molecules to collect light, yet these pigments don’t absorb over the entire spectrum – light at wavelengths above 750 nanometres is not used. This means plants waste about half of the energy in the solar spectrum, so researchers are attempting to tackle this by combining plant reaction centres with light harvesting antenna from purple bacteria which absorb light wavelengths from 800 to 1000 nanometres. Plants also make far more chlorophyll than they need. This is a survival mechanism: with extra chlorophyll in their leaves, little light will reach competitors growing below. But this also means that in strong sunshine, plants absorb more light than they can use. Under these conditions up to 80 per cent of the light collected is wasted, with excess energy dissipated as heat. Researchers hope that reducing a plant’s light harvesting capacity will increase its photosynthetic efficiency, so several teams are making mutant green algae with reduced pigment content. Another major inefficiency occurs during carbon fixation, thanks to the enzyme rubisco. C4 plants, such as sugar cane and sorghum have partially solved this thanks to a CO2 concentration mechanism (see “The light and dark reactions”, page ii) that raises the efficiency of photosynthesis to about 6 per cent. Cyanobacteria have a similar strategy: they contain carboxysomes, protein assemblies containing rubisco where CO2 can be concentrated while O2 is excluded. To improve crop yield, researchers are trying to convert C3 plants into C4 ones, and see if C3 plants can be modified to make their own carboxysomes. One team based at the University of Cambridge is attempting to change the leaf anatomy of a C3 plant so that it produces carboxysomes in its chloroplasts. To do this, the plant must not only synthesise all the components needed but they must also be delivered and assembled inside the chloroplast. Even if these efforts succeed, it is hard to envisage photosynthetic efficiency rising above 10 per cent.

Waste not: sugar cane, the world’s largest crop, is turned into biofuel, with left-over bagasse burned to make heat

vi | NewScientist | 2 February 2013

Daniel Nocera (below) is developing a solar cell (right) to make H2 fuel

New sources of fuel

bottom: VOLKER STEGER/spl Isaac Hernandez/IsaacHernandez.com top: Dominick Reuter

Jason Larkin/Panos

Improving crop yields

Global carbon emissions are rising steadily and if our planet is to avoid catastrophic warming, we must work rapidly to replace fossil fuels. Can photosynthesis help? Plant power has already been harnessed for biofuel. US distilleries produce more than 50 billion litres of bioethanol annually, mainly from fermented corn. Most is blended with conventional petrol and used to power vehicles. Yet questions remain over the sustainability of this biofuel. The conversion of solar energy into bioethanol is very inefficient, meaning huge areas of land are needed if production is to be scaled up. Another way that photosynthesis can offer us fuels is if we can mimic the way in which plants and algae use light to split water, to generate Derek Lovley (above) H2 as well as O2 (see “From ocean hopes to modify a to atmosphere”, page vii). bacteria so it produces Scientists already do this in the hydrocarbon fuel from lab – photovoltaic cells connected sunlight and CO2 to a pair of platinum electrodes immersed in water will generate bubbles of H2 fuel. However, this technique would be prohibitively expensive on a large scale because of the high cost of platinum. The challenge is to mass-produce electrodes at lower cost. One contender is a system devised by Daniel Nocera and colleagues at the Massachusetts Institute of Technology. Their oxygen evolving electrode uses a structure inspired by the plant’s oxygen evolving centre, but with cobalt instead of manganese. This splits water to release oxygen, creating hydrogen ions that combine with electrons at the other electrode – an alloy of nickel, molybdenum and zinc – to form hydrogen gas. Electricity is provided by a special silicon-based solar cell. Still further off is the “electric leaf”. This is a concept for a hybrid fuel generation system, using photovoltaic panels that supply electricity to living cells. These cells will be engineered to create not H2, but energy-rich hydrocarbons. A bacteria called Geobacter might provide the basis for the biological half of this double act. Geobacter isn’t photosynthetic. Instead, it extracts electrons from minerals and uses them to power its metabolism. Derek Lovley at the University of Massachusetts Amherst, near Boston,

has shown that Geobacter can grow using electrons provided by a photovoltaic cell and that the bacteria can extend wire-like hairs called pili to make electrical connections. This raises an intriguing question: could we modify Geobacter so it turns electrons into hydrocarbon fuel? Jay Keasling, of the University of California, Berkeley, has shown that the metabolic pathway required to synthesise hydrocarbons called terpenes can be engineered into E. coli. In principle, the same thing could be done with Geobacter, creating a hybrid system that converts sunlight into a petrol substitute.

An “electric leaf” constructed from a genetically engineered bacterium could convert sunlight and carbon dioxide into vehicle fuel

LIGHT PHOTOVOLTAIC PANELS

CO2

+

– GEOBACTER BACTERIA

HYDROCARBON FUEL

FRONTIERS OF PHOTOSYNTHESIS The complexity of photosynthesis is a huge challenge for those trying to unpick its details. And with the twin threats of climate change and food shortages, scientists are looking to photosynthesis for help. Can plants and algae be the key to new carbon-neutral fuels, for example? Might we even be able to supercharge the photosynthetic process and increase the yields of vital food crops?

From ocean to atmosphere

A false colour image shows how CO2 emission (red) gives way to CO2 absorption (green) as photosynthesis switches on at dawn

Jamison Daniel, ORNL/NCCS

”Geobacter bacteria could provide a hybrid system that converts sunlight into a petrol substitute”

Photosynthesis, and therefore all life on Earth, has been able to proliferate because of oxygen evolution. Plants, algae and cyanobacteria release oxygen by splitting water using a protein structure called the oxygen evolving centre. At its heart are four manganese ions, held in specific orientations by a protein scaffold. Although scientists can mimic the way the centre splits water using electricity and a platinum catalyst, this requires roughly twice the energy used by photosynthetic organisms. The oxygen evolving centre reduces its energy needs by dividing up the chemistry into a series of small steps. In particular, the manganese ions have four oxidation states and give up their electrons one at a time, steadily increasing their oxidation power until molecular oxygen is formed. Yet despite decades of study, scientists have so far failed to unravel every fine detail of the way the centre functions. We know the key features of its structure, thanks to X-ray measurements. However, these only provide a static snapshot: how the metal ions and the protein’s amino acids cooperate as the reactions proceed is still unknown. To find out more, researchers are using techniques such as time-resolved X-ray crystallography, in which ultrashort X-ray pulses are beamed through a sample of oxygen evolving centres. These measurements can reveal structural changes on a picosecond timescale, with a resolution of a few nanometres. Such experiments could allow us to follow the detailed changes that occur in the centre’s structure as a single molecule of oxygen is released, solving a scientific riddle and perhaps pointing the way to new kinds of photovoltaic technology. 2 February 2013 | NewScientist | vii