Purple Photosynthetic Bacteria Could Convert Wastewater Treatment Plants into Zero-Carbon Fuel Generators

Some valuable thing has been flushed down the toilet today. Organic compounds in industrial wastewater and household sewage have been shown to be a rich potential source of bioplastics, energy, and even proteins meant for animal feed.

However, due to lack of an efficient extraction technique, treatment plants throw away these organic compounds as contaminants. Now, a research team has discovered a new solution that is both cost-effective and environmentally friendly.

The study, reported in Frontiers in Energy Research, is the first to demonstrate that when purple phototrophic bacteria are supplied with an electric current, they can recover almost 100% of carbon from any kind of organic waste and at the same time produce hydrogen gas for producing electricity. These bacteria are capable of storing energy from light.

One of the most important problems of current wastewater treatment plants is high carbon emissions. Our light-based biorefinery process could provide a means to harvest green energy from wastewater, with zero carbon footprint.

Dr Daniel Puyol, Study Co-Author, King Juan Carlos University, Spain.

Purple photosynthetic bacteria

Where photosynthesis is concerned, green always takes the limelight. However, chlorophyll leaves behind its red, orange, and yellow cousins when it retreats from autumn foliage. Actually, photosynthetic pigments come in a variety of colors, and also in a variety of organisms.

Cue purple phototrophic bacteria

These bacteria capture the sunlight energy with the help of a wide range of pigments, which convert them into shades of brown, red, or orange, and also purple. However, it is not the bacteria’s color but the versatility of their metabolism that makes them so attractive to researchers.

Purple phototrophic bacteria make an ideal tool for resource recovery from organic waste, thanks to their highly diverse metabolism,” Puyol explained.

Instead of H2O and CO2, the bacteria can utilize nitrogen gas and organic molecules to provide nitrogen, carbon, and electrons for the photosynthesis process. This implies that when compared to alternative phototrophic algae and bacteria, these purple bacteria grow faster and can produce proteins, hydrogen gas, or even a type of biodegradable polyester as metabolic byproducts.

Tuning metabolic output with electricity

The environmental conditions of the bacteria, like temperature, light intensity, and the types of nutrients and organics available, decide the type of metabolic product that will predominate.

Our group manipulates these conditions to tune the metabolism of purple bacteria to different applications, depending on the organic waste source and market requirements. But what is unique about our approach is the use of an external electric current to optimize the productive output of purple bacteria.

Professor Abraham Esteve-Núñez, Study Co-Author, University of Alcalá, Spain.

This approach called “bioelectrochemical system” works because purple bacteria have varied metabolic pathways that are linked by a common currency—electrons. For instance, electron supply is needed to capture light energy, while nitrogen is converted into ammonia to discharge surplus electrons, which need to be dissipated. If the electron flow inside the bacteria is optimized, then electricity provided through negative and positive electrodes, like in a battery, will not only delimit these processes but will also increase the rate of synthesis.

Maximum biofuel, minimum carbon footprint

In their new study, the researchers examined the optimum conditions for increasing the production of hydrogen by a combination of purple phototrophic bacteria species. The team also analyzed the impact of a negative current—that is, electrons that metal electrodes supplied in the growth medium—on the bacteria’s metabolic behavior.

The team’s first important finding was that the blend of nutrients that fed the maximum rate of hydrogen production also reduced the CO2 production.

This demonstrates that purple bacteria can be used to recover valuable biofuel from organics typically found in wastewater—malic acid and sodium glutamate—with a low carbon footprint.

Professor Abraham Esteve-Núñez, Study Co-Author, University of Alcalá, Spain.

What was even more incredible were the results obtained through electrodes, which showed for the first time that purple bacteria have the ability to use electrons from a “cathode” or negative electrode to capture CO2 through photosynthesis.

Recordings from our bioelectrochemical system showed a clear interaction between the purple bacteria and the electrodes: negative polarization of the electrode caused a detectable consumption of electrons, associated with a reduction in carbon dioxide production. This indicates that the purple bacteria were using electrons from the cathode to capture more carbon from organic compounds via photosynthesis, so less is released as CO2.”

Towards bioelectrochemical systems for hydrogen production

The researchers say that their study was the first to report the use of diverse cultures of purple bacteria in a bioelectrochemical system, and also the first to demonstrate the phototroph shifting metabolism because of the interaction with a cathode.

The excess CO2 generated by purple bacteria can be captured and used for reducing carbon emissions as well as for refining biogas from organic waste that can be used as fuel.

Conversely, Puyol concedes that the team’s actual objective lies further ahead.

One of the original aims of the study was to increase biohydrogen production by donating electrons from the cathode to purple bacteria metabolism. However, it seems that the PPB bacteria prefer to use these electrons for fixing CO2 instead of creating H2. We recently obtained funding to pursue this aim with further research, and will work on this for the following years. Stay tuned for more metabolic tuning.”

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