Bacteria Power Harnessing Photosynthesis
Next time you’re admiring the colors of the marine life by a pond or the sea, just remind yourself that you’re staring at the raw materials of a potential multi-billion-dollar global industry for harvesting sunlight. In particular, what scientists are now learning about the bacteria and plant organisms that give rise to these pretty colors has the potential for revolutionizing the entire energy industry.
Think photosynthesis, and you probably remem¬ber that day in kindergarten when life’s most basic process was first explained to you. The sun’s energy allows plants to grow, and these plants then underpin the entire food chain. Easy, right? Wrong.
It turns out that the details of photosynthe¬sis — and hence how to efficiently harness it for next-generation energy conversion devices — are only just revealing themselves through imaging technology such as the atomic force microscope. In particular, recent work by our collaborator James Sturgis at l’Université de la Méditerranée in Marseille, France, showed that photosynthetic organisms such as purple bacteria contain highly adaptive machinery that can respond to major changes in the amount of sunlight.
Moreover, although such bacteria were among the first organisms on Earth more than 3 billion years ago, even preceding plants, the complexity of their nanoscale light-harvesting machinery puts even the most sophisticated photovoltaic designers to shame. And did I mention that they cost nothing to build?
A growing number of scientists believe that photosynthetic systems such as purple bacteria provide the magic bullet for unlocking cheap, dependable solar power generation. I say “systems” because any resulting commercial machinery need be neither completely natural nor man-made. Instead, it can potentially combine the best of both worlds simply by allowing purple bacteria to grow on conventional circuitry — thereby combining the robustness of 3 billion years of evolution with the common setting of everyday materials such as plastics, metals or semiconductors. Such systems would be cheap, robust and, most importantly, adaptable because any damage to the bacterial surface coating could self-repair through natural colony growth. Powering everything from mobile phones to entire buildings may be as simple as letting them get “dirty” via the buildup of a bacterial layer.
Among the wealth of naturally occurring photosynthetic organisms, purple bacteria are arguably the most amazing in terms of their internal light-harvesting machinery. Resembling a bowl of Cheerios cereal in milk, the ring-like nanoscale structures lie in a loose membrane arrangement ready to capture and pass on packets of energy from the sun. Their 3-billion-year experience of having to grow in extreme, harsh conditions, and having to adapt to unexpected changes in climate on both the daily and decadal scale, has left them with a detailed architecture that can change according to the amount of sunlight that they receive as they grow.
Our recent work shows that purple bacteria optimize this internal machinery in a very special way, according to the amount of incident sunlight.
This ability to thrive in multiple environments, such as intense or sparse sunlight, is far beyond the capabilities of any existing technology — in particular, the much-touted organic photovoltaics, which instead tend to disintegrate.
Most importantly, it looks as though it should be relatively easy to insert such photosynthetic organisms into conventional technology such as semiconductors, judging by recent experimental work by researchers in Israel. In technical jargon, the researchers successfully employed time-resolved photoluminescence to measure an extremely fast electron transfer. But hasn’t somebody thought of all this before? Yes and no.
Yes, the idea of harnessing photosynthesis is as old as the hills — possibly even older if we allow for the fact that plants learned to harness sunlight well before dinosaurs roamed the planet. But what is new, and what makes this particular discovery so promising in terms of commercialization, is the fact that this is the first time that we have truly understood the internal workings of these organisms.
And the possibilities are simply breathtaking. For example, allowing purple bacteria to colonize a given device circuit could create a system that can absorb sunlight across a wide range of frequencies, re-adjust itself by adaptation should the level of sunlight change and repair itself if damaged or scratched. Such self-repairing systems would be ideal for placement in hard-to-reach areas of buildings. Whatever happens in terms of external conditions, the bacterial growth will adapt.
Going beyond everyday applications, these hybrid devices could also provide a revolution in the booming high-tech nanotechnology arena. In addition to harvesting sunlight, the ability of these bacterial membranes to process electrical signals has hardly been explored.
Yet there are no laws of physics that prevent us from creating hybrid light-harvesting devices that are self-powered, adaptive, and can repair themselves or grow at will. After all, purple bacteria have been doing this quite happily by themselves for more than 3 billion years. All it would take is some light exposure and investment.
Neil Johnson is a physicist at the University of Miami.
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