To enable humans to capture more of the sun's energy than natural photosynthesis can, scientists have taught bacteria to cover themselves in tiny, highly efficient solar panels to produce useful compounds.

Researchers at the University of California, Berkeley focuses on harnessing inorganic semiconductors that can capture sunlight to organisms such as bacteria that can then use the energy to produce useful chemicals from carbon dioxide and water.

While photosynthesis provides energy for the vast majority of life on Earth, chlorophyll, the green pigment that plants use to harvest sunlight, is relatively inefficient.

Image courtesy of Kelsey K. Sakimoto

"Rather than rely on inefficient chlorophyll to harvest sunlight, I've taught bacteria how to grow and cover their bodies with tiny semiconductor nanocrystals," says Kelsey K. Sakimoto, Ph.D., who carried out the research in the lab of Peidong Yang, Ph.D. "These nanocrystals are much more efficient than chlorophyll and can be grown at a fraction of the cost of manufactured solar panels."

Many scientists have worked to create artificial photosynthetic systems to generate renewable energy and simple organic chemicals using sunlight. While progress has been made, to date the systems are not efficient enough for commercial production of fuels and feedstocks.

Though few CO2 reduction approaches can rival the energy efficiency and selectivity of biological CO2 fixation, the limited light absorption of natural photosynthesis pales in comparison to that of inorganic semiconductor based photovoltaics.

"The thrust of research in my lab is to essentially 'supercharge' nonphotosynthetic bacteria by providing them energy in the form of electrons from inorganic semiconductors, like cadmium sulfide, that are efficient light absorbers," Yang says. "We are now looking for more benign light absorbers than cadmium sulfide to provide bacteria with energy from light."

When Sakimoto fed cadmium and the amino acid cysteine, which contains a sulfur atom, to the bacteria, they synthesized cadmium sulfide (CdS) nanoparticles, which function as solar panels on their surfaces.

The hybrid organism, M. thermoacetica-CdS, produces acetic acid from CO2, water and light. "Once covered with these tiny solar panels, the bacteria can synthesize food, fuels and plastics, all using solar energy," Sakimoto says. "These bacteria outperform natural photosynthesis."

The bacteria operate at an efficiency of more than 80 percent, and the process is self-replicating and self-regenerating, making it a zero-waste technology.

"Synthetic biology and the ability to expand the product scope of CO2 reduction will be crucial to poising this technology as a replacement, or one of many replacements, for the petrochemical industry," Sakimoto says.

Commenting on whether the inorganic-biological hybrids have commercial potential, Sakimoto said:

"Many current systems in artificial photosynthesis require solid electrodes, which is a huge cost. Our algal biofuels are much more attractive, as the whole CO2-to-chemical apparatus is self-contained and only requires a big vat out in the sun."

However, he points out that the system still requires some tweaking to tune both the semiconductor and the bacteria. He also suggests that it is possible that the hybrid bacteria he created may have some naturally occurring analog. "A future direction, if this phenomenon exists in nature, would be to bioprospect for these organisms and put them to use," he says.

According to the researchers, the advances and variations of the inorganic-biological hybrid organism concept are driving the work towards out-competing traditional approaches to chemical production and augment the ever expanding portfolio of next generation green technologies.

Click here to watch a video about the research