In the 1990s, a team of education researchers attended graduation at Harvard University and MIT. Whenever possible, they pulled a graduate aside, handed him or her a seed and a heavy piece of a tree trunk, and said “Say I plant that seed and after many years get a tree large enough to have this thick a trunk; where does all the mass of the tree come from?” Interviewees typically replied along the lines of “water, nutrients from soil, and sunlight.”
What would your answer be?
The responses were intuitive. We learn at a young age that plants need water, soil and sunlight to live. However, the graduates neglected 45–50% of what makes up a plant, namely carbon. And plants capture carbon from carbon dioxide in air. Yeah, plants are made of air.
This carbon sequestered by plants from air is at the core of our very existence; and not only because plants clean the air by absorbing carbon dioxide and releasing oxygen. No. Energy as we currently know and use it is intimately linked to this carbon. Our own bodies are powered by chemical energy stored in the bonds of carbon-based molecules. Which is why we eat plants, or at least animals that at some point ate plants. Coal and oil, the feedstocks of the energy that powers our cities, cell phones, cars, and refrigerators are fossils of bacteria and plants that confiscated carbon from air over millennia. Every aspect of our daily life is in some way dependent on this carbon – from waking up in the morning (that takes energy) to allowing ourselves just one more episode of Stranger Things before going to bed. We are carbon junkies.
As we spend more time driving, charging our phones, and lighting up our homes, our addiction for carbon grows, and plants are having a hard time meeting our demand. Old fuels like oil are running out and new fuels like ethanol are limited. So, we are looking at ways of detoxing. Solar, wind, geothermal, and wave power are all means to circumvent that pesky carbon atom. Some scientists, however, are taking a different approach. They’re asking “why is it that plants are not keeping up, and can we do something about that?” To answer that question, they are looking back in evolution to the point where photosynthesis emerged on the planet and, as one of them appealingly puts it, are correcting an egregious mistake: chlorophyll.
Bacteria, algae and plants have been using photosynthesis to harvest sunlight and pack energy into carbon-based molecules for 3–3.5 billion years. Chlorophyll is their choice light photon trap and the reason why most of these organisms are green. The problem is, chlorophyll isn’t very good at catching sunlight. Theoretically, the best efficiency that plants can achieve converting sunlight into carbon-based mass is 11%. In practice, that efficiency lies at 0.1–2%.
Professor Peidong Yang and his former graduate student Kelsey Sakimoto addressed this inefficiency. They found a naturally occurring bacterium called Moorella thermoacetica, which does two things really well: first, it makes acetic acid from carbon dioxide; second, it protects itself from the accumulation of toxic levels of metals in its inside by bundling and packing the metal on its cell surface. Yang and Sakimoto fed cadmium to Moorella and the bacteria casually converted it into cadmium sulfide (CdS) and then festooned their surface with CdS nanocrystals. And guess what; CdS captures sunlight. In fact, cadmium is used to make thin film solar panels.
The small vats of cadmium-coated bacteria started churning out acetic acid using the solar energy captured by the CdS nanocrystals and did so at efficiency levels four times greater than photosynthesis. Furthermore, as living organisms, the bacteria in such a vat self-repair, self-replicate and self-produce their own photon-trap CdS nanocrystals. That means, such vats would be cheap to start, cheap to maintain, and would generate no waste.
But wait a minute. Acetic acid is vinegar. Why do we want bacteria that produce a bunch of vinegar? Well, acetic acid is a building block to manufacture a broad range of more useful compounds, such as fuels, polymers, pharmaceuticals and household chemicals. Chemical manufacturing facilities already synthesize upgraded compounds from acetic acid, but as it turns out, there are bacteria that can do the same. A cool vision of the future is a biological manufacturing chain, where different bacteria perform a series of chemical conversions that transform acetic acid created by Moorella from sunlight into, for example, methane, the main component of natural gas. Maybe we are at the start of a new era where adventurous “bioprospectors” scour every recess of the planet to find the yet undiscovered bacteria that can make jet fuel out of car exhaust. It’ll be like American Gold Prospectors combined with The Matrix, only starring Moorella, E. coli and Lactobacillus instead of Keanu Reeves.
Of course, there’s still some work to do before we can enjoy our new and improved, bug-powered carbon source. Cadmium, for one, will need to be replaced with a more benign and abundant light absorber. Cadmium is a toxic and rare metal, mostly harvested as a by-product of zinc production. Then, even Yang and Sakimoto admit that they got lucky with Moorella. The bacterium they chanced upon already produced a nice chemical building block and it naturally deposited CdS on its surface. How easy is it to find bacteria with such trait combinations? We have been looking at microbes since the 17th Century, but have only scratched the surface of the diversity and capabilities of the microscopic world. And there are questions about engineering bacteria to do the chemical conversions needed to meet our energy needs (oops! Bioprospectors are out of a job), and whether that can be done and managed safely.
I’m thinking more about the size and number of those sun-basking, energy-producing vats of Moorella. I enjoy looking out on an expanse of forest trees. Behemoth containers of yellow-colored, vinegar-smelling bacteria just don’t have the same charm. But, maybe I’m just an old-fashioned carbon junkie.