Harvesting the Sun.

John writes ...

I started to write this blog post on February 4th, approximately half-way between the Winter solstice and the Spring equinox. In the Christian church, February 2nd is celebrated as Candlemas, remembering the presentation of the infant Jesus, the Light of the World, in the Temple. It is also the date of the ancient Celtic festival of Imbolc which hints at the ‘pregnancy’ of Earth in relation to the return of light in the lengthening days.

All of this reminds me of the relationship of our beautiful planet Earth with the Sun. Our distance from the Sun is one of the major factors in providing conditions that are ‘just right’ for the development and flourishing of life, so much so that several scientists, including Paul Davies and the late Stephen Hawking, have talked about planet Earth being located in the ‘Goldilocks zone’; indeed, one of Paul Davies books is entitled ‘The Goldilocks Enigma’ (Penguin Books, 2006).  As we sit in that zone, the amount of sunlight that hits Earth’s surface every hour is enough to meet the energy needs of human society for a year, a fact that I find totally amazing. Just pause and think about that for a minute or so.

And of course we are harnessing some of that energy, albeit a tiny fraction of the total available, to generate electricity via photovoltaic cells. However, all over the globe, there are living organisms that utilise many times more of that energy than humankind is able to do. Those organisms are plants, including algae, and blue-green bacteria, i.e. organisms capable of photosynthesis – the capture (‘fixation’) of carbon dioxide driven by solar light energy, with oxygen as a by-product. But there’s more to it than that. Life as we know it is nearly all dependent on photosynthesis: 99% of living organisms, including humans, are directly or indirectly dependent on photosynthesis. Without photosynthesis there would be no complex life on Earth. Nature as we currently know it, including humankind, would not exist. Let me put that another way: without photosynthesis, we would not be here – another awesome thought.

An array of photosynthetic organisms! Credit: John Bryant.

​So, as we continue to move away from using fossil fuels, is there any way by which we can use the power of photosynthesis to help us achieve that aim? Well, in one sense we already do. All of the carbon contained in all the biofuels in use or in development was initially fixed by photosynthesis (see Biofuels and Bioenergy, John Love and John Bryant, Wiley, 2017). But that’s not what I am thinking of. I am thinking of the possibility of using the light-harvesting mechanism of photosynthesis in a more direct way. This was first proposed as long ago as 1912 by the very forward-thinking Italian chemist Giacomo Ciamician. At that point, nothing was known about the actual mechanisms used by plants to transform light energy into chemical energy. Ciamician envisaged the invention of photo-chemical devices that mimic the photo-chemical events of photosynthesis in order to synthesise compounds that could be used as fuels. He believed that the switch from fossil fuels to radiant energy could decrease the wealth gap between the poorer nations of southern Europe and the richer nations of northern Europe and, on a wider scale, would contribute to human progress and happiness. 

Giacomo Ciamician (1857-1922). Credit: public domain.

At this point, we need to think briefly about how photosynthetic organisms actually capture and utilise light energy. The primary photo-receptive compound is chlorophyll which is ‘excited’ by light, leading instantly to the photolysis (light-driven splitting) of water, releasing oxygen and electrons. Cambridge biochemist Robin Hill discovered this process in 1937 and it has been named the Hill reaction in his honour. The oxygen produced in this reaction is dissipated to the atmosphere, while the electrons are passed along a short chain of electron-carrying molecules embedded in the chloroplast membrane, at the end of which this bio-electric energy is used to drive the synthesis of two energy-carrying chemicals. These are ATP (adenosine triphosphate) and the electron donor NADPH (reduced nicotinamide adenine di-phosphate). It is the energy in these molecules that enables the biochemical reactions involved in fixing carbon dioxide into sugars. Thus, the molecules that capture the energy from the ‘light reactions’ drive the ‘dark reactions’ (more details are available in Functional Biology of Plants, Martin Hodson and John Bryant; Wiley-Blackwell, 2012).

Chloroplasts, the sites within plant cells in which photosynthesis takes place. Credit: Rupert Sheldrake.

Robin Hill (1899-1991} (Hill was still active in Cambridge when I was a student. He was mainly doing research on details of the ‘light reactions’ of photosynthesis and was not doing much lecturing so, sadly, I never ’sat at his feet’). Credit: public domain.

In connection with our need to move away from using fossil fuels, is there any way in which we can either mimic these photosynthetic processes, as envisaged by Giacomo Ciamician, or even use them directly? Thinking first of artificial photosynthesis, the key problem is to find a way in which the excited state of a photo-receptor can be transformed into chemical energy. There has been some very good progress with this in the light-driven formation of hydrogen and of methane, both of which can be used as fuel. However, I need to say that neither of these has yet been scaled up to anything like the extent needed to make a real contribution to our need for non-fossil fuels. Indeed, anaerobic digestion of biological waste already produces many times more methane than can be currently produced by solar fuel cells. Nevertheless, these processes show promise, so ‘watch this space’.

However, as I briefly mentioned above, there is another approach. Is it possible to use more ‘natural’ systems? Researchers at Cambridge University, certainly think this is feasible. Several approaches are being taken, of which I will briefly mention two. The first approach, initially developed in the Department of Biochemistry several years ago, uses colonies of blue-green bacteria or of green algae. These are immobilised as a biofilm on a surface that acts as an anode in a ‘bio-voltaic cell’. Illumination of the biofilm results in the splitting of water as in normal photosynthesis but the resulting bio-electric energy, in the form of electrons released from the cells via an electron carrier, is then passed to an electrode thus completing the electrical circuit. The amount of electricity generated is not huge but is enough to power micro-processors and small electrical devices, such as digital clocks. Indeed, in more recent  improvements of the process, one of the photo-voltaic cells, the size of an AA battery, ran a micro-processor for a year, giving hope that wider applications may be possible
(see Algae-powered computing: scientists create reliable and renewable biological photovoltaic cell | University of Cambridge).

Prof Chris Howe and Dr Paolo Bombelli, Biochemistry Department, University of Cambridge. Credit: Paolo Bombelli.

The second approach is seen in a more recent development by a research team in the university’s Department of Chemistry (see This artificial leaf turns pollution into power | ScienceDaily).  This starts with non-natural photo-receptors which use the captured solar energy to split water (as in photosynthesis); the resulting bio-electric energy drives the fixation of carbon dioxide to form not a sugar (which happens in the natural ‘dark reactions’) but formic acid which can be used as a starting point for synthesis of several important biochemicals, including pharmaceuticals. All the components for this process, namely the photo-receptors, electron carriers and enzymes, are immobilised on an inert matrix to make a ‘semi-artificial leaf’ which is a ‘hybrid’ structure of non-natural and natural components.

Semi-artificial leaf, developed by Prof Erwin Reisner’s research team in Cambridge University’s Chemistry Department. Credit: Celine Yeung.

Overall then, we have some promising innovations in the direct use of photosynthesis, raising hopes that that these or similar processes may one day make a significant contribution to our energy needs. The research goes on!
 
John Bryant
 
Topsham, Devon.
February 2026