"Light-driven reactions can be integrated into a nonphotosynthetic microorganism and used to generate an electric current," says Claudia Schmidt-Dannert of the University of Minnesota in St. Paul. She described experiments in which genes encoding light-capturing rhodopsin molecules are inserted into the genome and the corresponding proteins and then integrated into the membranes of Shewanella, which before this reengineering produces electric currents.

However, in the engineered cells, light energy now drives that electric current, she said during the colloquium "Engineering a Better Bacterium," held during the 110th ASM General Meeting in May in San Diego, Calif.

In terms of long-term ambitions, Schmidt-Dannert envisions retailored microorganisms with a wide range of synthetic and energy-producing capabilities. "Can we augment the metabolic capabilities of Escherichia coli or yeast?" she asks. "Can we use a nonphotosynthetic host to drive metabolically expensive reactions or produce electricity?"

En route to those goals, Schmidt- Dannert and her collaborators are marking progress with smaller but impressive strides. Thus, she says, they started with the "simplest rhodopsin system," alluding to a microbial-membrane- embedded, light-absorbing protein. In its native setting, rhodopsin drives proton pumps to generate metabolic energy in the form of adenosine triphosphate (ATP) molecules. Found in several species of Halobacteria, rhodopsin also sometimes is involved as part of a phototaxis response system. In its light-harvesting capacity, bacteriorhodopsin is simpler than the several types of chlorophyll that are involved in photosynthesis. Those systems use light energy to fix carbon dioxide into organic molecules and to generate oxygen from water molecules.

Schmidt-Dannert and her collaborators chose Shewanella oneidensis, a gram-negative anaerobe, to serve as the recipient organism in which to insert the gene encoding rhodopsin. On their own, cells of S. oneidensis generate electricity and can use a range of electron receptors, including metal electrodes that researchers supply in experimental fuel cells (see Microbe, April 2010, p. 143; November 2009, p. 506).

Not only do the genetically engineered Shewanella cells each produce about 40,000 copies of the rhodopsin protein, the change benefits them by enhancing their survival during stationary phase-helping to keep them "energized," Schmidt-Dannert says. More spectacularly, when the rhodopsin-producing Shewanella cells are exposed to light, they produce from 30-100% more current than do the unmodified parent cells. "The current output is proportional to the light intensity," she says, adding, "The electrons being recovered are less than expected, [and] we're not sure where they go."

The current output is consistent with rhodopsin-captured light being a new source of electric current, according to Schmidt-Dannert. "To our surprise, every time we switch the light on, we get a rapid increase in current, with a sawtooth pattern that we can repeat many times," she continues, noting that current is no longer generated when the light source is switched off. The physical status of the rhodopsin- producing cells affects the current output. Thus, "we get higher current production at the beginning when the biofilm is thin," she notes. Once the biofilm thickens, however, less current is produced because of "light penetration problems."

"We were lucky to choose Shewanella," Schmidt-Dannert says. "Geobacter does not show this behavior." Although both types of microorganism generate electric current, rhodopsin-containing Shewanella "does not generate enough membrane potential to drive ATP synthesis," she says. In Geobacter, however, rhodopsin- captured light energy is channeled into making ATP instead of electric current.

Jeffrey L. Fox

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