Engineered microbes for climate action

The May 9 edition of Quantum featured an article based on the work of a scientist at Lawrence Berkeley University, USA, for capturing carbon dioxide using an electrochemical cell. Now, another scientist at the university has come up with a different method using bacteria.  

It is not difficult to capture carbon dioxide from air — you only have to flow the air over a bed of chemicals that pick up the gas. The real challenge lies in separating carbon dioxide from the chemicals, so as to re-use the chemicals.  

Peter Agbo’s approach is to convert the captured carbon dioxide into limestone, using seawater — the magnesium and calcium in seawater react with carbon dioxide to form carbonates. In this way, you convert a gas into a rock. 

This, in fact, occurs in Nature and can be replicated in a laboratory or factory, but is a very slow process. However, it is known that a catalyst — an enzyme called carbonic anhydrase — can quicken it. The key question is, how to produce sufficient quantities of carbonic anhydrase in an economically viable way. 

Agbo and his co-researchers Steven Singer and Ruchira Chatterjee reasoned that since enzymes are proteins that are synthesised by bacteria, why not genetically engineer the bacteria to produce carbonic anhydrase? 

The bacteria will keep multiplying as long as you feed them. The problem distils to finding enough nutrients to feed the bacteria.

“One way around this issue would be to use bacteria that can grow using energy and nutrients that are readily available in the natural environment. So, this pointed towards photosynthetic bacteria,” says Agbo. “They can use sunlight as their energy source and carbon dioxide as carbon source to feed on.” 

The idea, therefore, is to develop a genetically modified bacteria that can grow in light and produce a lot of carbonic anhydrase. Then if you put the bacteria in seawater, you would find a rapid formation of limestone.  

In Agbo’s plan, the bacteria would grow in a bioreactor. Seawater would flow into the bioreactor, where limestone forms. Then you centrifuge the seawater to eject the limestone and return the water to sea. A series of plants like this could make a meaningful dent in atmospheric carbon dioxide levels. Agbo reckons that a smallish reactor of 1 million litres could suck out one megatonne of captured carbon dioxide per year. 

Meanwhile, another Berkeley Lab researcher, Deepika Awasthi, wants to use a genetically engineered bacterium, Methylomicrobium alcaliphilum, to feed on methane, a greenhouse gas, and use carbon dioxide to produce the industrial chemical malonic acid. Currently, malonic acid is made by the petrochemical industry or through sugar fermentation. The chemical is used in the solvent industry and in automobile coating, commanding a “multibillion-dollar market”, according to Berkeley Lab. 

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