Australasian Science: Australia's authority on science since 1938

Artificial Photosynthesis: Feeding and Fuelling the Future

iStockphoto

Image: iStockphoto

By Thomas Faunce

A global scientific project using nanotechnology and synthetic biology to re-engineer photosynthesis may help solve our energy, food, water and greenhouse gas problems.

Nanotechnology has been one of the most hyped terms used in science over the past decade, but the toxicological risks of various forms of nanotechnology have also caused concern. Examples have included nanoparticle forms of zinc oxide and titanium dioxide to make sunscreens that were not only effective in repelling unwanted solar radiation but also more cosmetically appealing. However, my own research revealed that one sunscreen released onto the Australian market, and never officially recalled, degraded steel roofing when accidentally applied by workmen.

Another problem area was carbon nanotubes used to make strong and lightweight building materials. I discovered that although animal models indicated that these thin and bio­persistent nanomaterials were likely to cause asbestos-like lung injury when inhaled, they were commonly used to harden pavers that were later cut to shape by an electric saw.

Then there was the use of nanosilver employed to reduce odour in clothes and grime in washing machines, despite being likely to accumulate and destroy crucial microorganisms in waste treatment systems and waterways.

I began to think that it was time to find much more positive uses for the exciting new science of nanotechnology, and in 2009 I was awarded an Australian Research Council Future Fellowship to study how nanotechnology might help solve some of the critical public health and environmental problems of our time.

It seemed that what was most needed was some form of nanotechnology that provided cheap energy, assisted food and water security and removed carbon dioxide form the atmosphere. Sounds easy? Well, surprisingly I found there was just such an application known as artificial photosynthesis.

My interest in photosynthesis had been stimulated while in Namibia for a UNESCO meeting about its global health law database. In Namibia I came upon some stromatolites in the Zebra River region in the same area where one could find fields of stone-age hand tools. These stromatolites were the fossil remnants of cyanobacteria that were among the first photosynthetic organisms. They began creating oxygen on Earth 2.5 billion years ago. Photosynthesis is not only the source of our oxygen, but also our food and basic energy supplies (including oil from decayed cyanobacteria in shallow oceans), coal and natural gas (from decomposed old forests).

Photosynthesis can be viewed as the planet breathing, although in a reverse way to us, taking in carbon dioxide and releasing oxygen. But it can also be considered as the planet’s nervous system, generating a basic voltage that powers the world’s life. This is because photosynthesis takes light energy from the Sun and stores it in chemical bonds.

In the 1800s most people believed that only birds would ever fly. The first attempts at manned flight thus involved people standing on cliffs and flapping artificial wings. Likewise, most people today still believe that only plants or certain bacteria can “do” photosynthesis. They believe that it is only by genetically modifying those plants and bacteria that we can ever meaningfully improve photosynthesis.

Yet, imagine if nanotechnology allowed artificial photosynthesis to become a routine component of all our engineered structures on earth, providing a cheap, local source of hydrogen fuel, oxygen, carbon dioxide absorption and soil nutrients.

Major Scientific Challenges
The major scientific challenges in artificial photosynthesis are commonly divided into three areas (see box, next page):

1. light capture;

2. water splitting or catalysis; and

3. carbon dioxide reduction.

In each of these areas, nanotechnology and synthetic biology present opportunities for significant improvements, with nanotechnology potentially overcoming some of the limitations of natural photosynthesis, including:

• the need to co-locate the core components of light capture, water splitting and carbon dioxide reduction;

• the need to place electron flow right next to oxygen as it is toxic and therefore damaged photosynthetic proteins will need to be regenerated;

• significant water loss through transpiration during CO2 uptake; and

• radiation of large amounts of heat to assist the organism’s survival.

Once these limitations are overcome we are on the way to mass-producing artificial photosynthesis devices that can provide hydrogen that can then either be cooled and stored as a fuel or used to make electricity in a fuel cell. Such devices may also produce water and basic starches that can themselves either be used as fuel (such as methanol), food or fertiliser.

Barriers and Solutions
Large research teams in many nations are using nanotechnology to actively redesign natural photosynthetic components such as light capture proteins, artificial reaction centre proteins, organic polymers and inorganic catalysts. For example, the Obama Administration in the US recently awarded US$122 million to establish the Joint Center on Artificial Photosynthesis at Caltech. Other large artificial photosynthesis groups are already established in the US, Europe and Japan.

A major aim of artificial photosynthesis research is to achieve cheap, localised, “off-the electricity grid” use of sunlight to split water and produce hydrogen for fuel cells (or compression and hypercooling to form a liquid fuel that produces fresh water as a by-product of combustion), as well as starch-based fuels, fertiliser and food. Artificial photosynthesis is thus an important opportunity to power and feed a sustainable world – particularly given evidence that the human birth rate is likely to drop as domestic energy supplies increase and education is made more widely available.

But many significant governance barriers are likely to confront this new field. For example, excessive patenting of core aspects of artificial photosynthesis could stifle innovation rather than promote it. “Patent trolls” could acquire intellectual property chiefly for the purpose of earning royalties and thus delaying and frustrating the capacity of scientists to explore new openings and avenues.

One solution to counter this is a UNESCO Declaration charting the governance principles for this area, including the principle that the photosynthetic process should be declared the “common heritage of humanity” – just like the human genome, the world’s UNESCO-listed cultural and natural heritage, outer space, the Moon and the deep sea bed – so that it would not be subject entirely to private ownership through patents and other intellectual monopoly privileges.

There is also the issue of how to effectively foster collaboration between groups of researchers in many nations, with barriers likely to be created by sovereign national interests due to energy and food security issues (including lobbying by the fossil fuel industry).

Another governance challenge will be the impact of approaching parity between the cost of photovoltaic electricity and energy purchased from the electricity grid. A solution to this may require a more integrated renewable energy policy scheme that includes artificial photosynthesis.

One vision of a world powered by artificial photosynthesis involves large coastal plants using sunlight to split sea water as a source of energy for large cities. Such plants could use photons captured in desert areas to produce carbon-neutral hydrogen-based fuels. When burned, such hydrogen fuels would also provide fresh water for the city more efficiently than current desalination plants.

A different vision is where global artificial photosynthesis replaces globalisation as a model of economic growth, with economies restructuring to emphasise smaller locally powered units, minimising the energy used in gathering raw materials and low-cost labour to make and transport goods for use in other countries.

In its present natural form, photosynthesis each year converts 4 x 1018 kJ of solar energy into biomass, which is eight times global human energy use. There is thus a vast potential for globally marketed and installed artificial photosynthetic systems to not only absorb carbon dioxide and reduce greenhouse gas-related climate change problems, but to provide basic forms of starch-based fuel, food and fertiliser.

BOX
Light Capture

The first component of any natural photosynthetic system that nanotechnology needs to improve is the light-capturing surface. Photon absorption is currently restricted to light arriving with a wavelength of ~430–700 nm. However, nanostructured materials are being developed to absorb more photons from a much wider region of the solar spectrum.

For a start, the use of light-capturing titanium dioxide nanoparticles on any surface drastically increases its surface area. If you then coat those titanium dioxide nanoparticles with a dye (like the chlorophyll that makes plants green) and then immerse them in an electrolyte solution with a platinum cathode, the photons from sunlight can displace electrons in the surface and create a current.

A wide range of semiconductor nanoparticles and carbon nanotubes are already being tested to harvest and conduct the resultant electricity efficiently.

Another significant challenge in light capture is to transport electrons from the light-capturing surface to a water-splitting reaction centre over multiple cycles of photon absorption and charge separation. This may require the application of new knowledge about how photosynthetic molecules self-assemble in the face of damage from excess sunlight, heat and oxygen.

It has been known for many years that both energy and matter exist simultaneously as both particles and waves. Likewise it is thought that photosynthetic plants and bacteria don’t transfer packets of energy along discrete wire-like structures, but as waves travelling along all possible pathways simultaneously, collapsing into the fastest one as the destination is reached. If such quantum coherence methods can be engineered then they will be very important in ensuring rapid electron transfer from the artificial light-capturing surface to the reaction centre.

Splitting Water
Any successful nanostructured light-harvesting system will need to interact efficiently with artificial versions of the photosynthetic reaction centres PSI and PSII.

The oxygen-evolving complex of PSII is where one of the core processes of photosynthesis takes place. This is the use of photons from the light-capturing surface to split water into its molecular components, thereby storing that solar energy in chemical bonds, particularly in a reduced form of hydrogen.

Other techniques include screening large numbers of cyanobacteria to discover the enzymes most suitable for use in genetically enhanced or wholly artificial photosynthetic bacteria that can maximise hydrogen fuel production.

CO2 Reduction and Energy Storage
Natural photosynthesis uses CO2 and the solar energy trapped in chemical bonds to produce carbohydrates. This is achieved through the agency of a very complex enzyme called RuBisCO. Studies are now being done to obtain kinetic data on all the different forms of RuBisCO that nature employs. By finding the most efficient form we may then be able to synthetically engineer it or make a wholly artificial nanotechnology-based version that would allow buildings to make biofuels such as methanol, or to enrich the soil through their foundations.

A/Prof Thomas Faunce is an Australian Research Council Future Fellow at the Australian National University. He was the scientific and administrative coordinator of the first international conference dedicated to creating a Global Artificial Photosynthesis project at Lord Howe Island in August 2011.