By: Michelle Koh
What is a cyborg? One might imagine Terminator-esqe half-human, half-machine hybrids or other creatures with fantastic mechanical augmentations, but we must direct our attention down to the cellular level—to cyborgian beings that are far smaller. Despite these cyborgs’ underwhelming size, UC Berkeley researcher Kelsey Sakimoto and his fellow researchers of Professor Pei-dong Yang’s lab have engineered a new biohybrid bacteria that may make a formidable impact on the solar fuel industry.1 These originally non-photosynthetic organisms are able to grow their own tiny semiconductor “solar panels” to harness solar energy and store it in the chemical bonds of acetate, an essential natural and industrial building block (1).
WHAT IS SOLAR FUEL?
According to the Royal Society of Chemistry, more energy is delivered to Earth in one hour by the sun than all the energy that we consume through fossil fuels, nuclear energy, and other renewable sources of energy in a year (2). Plants and other photosynthetic organisms have mastered the process of capturing and storing this solar energy in chemical fuels or “solar fuels” (2). Since the 1950s, scientists have strived to mimic these processes to create more sustainable alternatives to traditional energy sources such as fossil fuels (2). Unlike the energy generated by other sources of renewable energy, like photovoltaic cells and wind turbines, the physical nature of solar fuels means that they can be much more easily stored and transported through existing distribution networks and methods.
Acetate and other carbon-based solar fuels can be used as feedstock, or raw material, for the production of many products such as fertilizers, pharmaceuticals, and plastics (2). Currently, the petrochemical industry produces much of the feedstock for these industries. However, solar fuel-derived feedstock, in addition to being more renewable, reduces the harmful greenhouse gas emissions.
PHOTOSYNTHETIC BIOHYBRID SYSTEMS
Humans have developed incredible scientific and technological capabilities so far as to not only replicate some of the most complicated biological and chemical systems in nature but also surpass them in efficiency. Nevertheless, some processes, like the conversion of CO2 and other small atmospheric molecules to more complex organic molecules, have been more difficult to mimic (3).
The reduction of, or the addition of electrons to, CO2 is surprisingly difficult. Electrons must be transferred from some catalyst, or an electron carrier, to CO2 , and new carbon-to-carbon bonds must form (3). Furthermore, as each process and chemical reaction in the biological world is highly specific in its reactants and products, scientists also have to replicate the high-accuracy selection of a single product (3). Attempts to reproduce these processes in the lab have often ended in tangles of chemical problems that seem to contradict one another, yet biological organisms have evolved so that the cell is able to incubate and facilitate a vast number of diverse reactions through a delicately-regulated chemical environment.
As a result researchers have developed photosynthetic biohybrid systems (PBSs) to take advantage of the existing biological systems that have been developed so elegantly through evolution (3). By combining these systems with high-efficiency inorganic light harvesters, they are able to enhance or induce photosynthetic capabilities in organisms (3). The key challenge of this field has been smoothly integrating the biotic and abiotic components of the PBSs. Some PBSs feed electrons collected by an inorganic light harvester to the biological part of the system, though engineering and producing nanowire arrays and intricate carbon cloths to do so can be costly (4). Other researchers have developed PBSs by isolating specific enzymes, such as hydrogenases, and combining them with semiconductor nanoparticles (4). However, whole-cell PBSs are favored due to their self-replication and self-repair capabilities (4).
Sakimoto et al., on the other hand, have been able to engineer microorganisms that not only facilitate CO2 reduction, but also synthesize their own inorganic light harvester materials (3). Sakimoto’s team discovered that the introduction of Cd2+ (cadmium) ions to initially non-photosynthetic Moorella thermoacetica bacteria can induce the bio-precipitation of cadmium-sulfide (CdS) nanoparticles on the cell surface (4). The growth of these semiconductor light harvesters is able to transform the M. thermoacetica bacteria into highly efficient photosynthetic systems, producing products that are 90% acetic acid and 10% biomass (4).
Since the bacteria are able to produce their own inorganic semiconductor light harvester particles, Sakimoto et al.’s new PBS is cost-effective (4). The complex micro-fabrication techniques, high-purity reagents, and high-temperature processes are essential in synthesizing the semiconductor components in PBSs, but they are incredibly energy and resource intensive (4). Aside from the initial set-up of the system, Sakimoto et al.’s system requires very low maintenance, as the bacteria are able to remake the CdS particles even after the particles degrade (4).
Sakimoto and colleagues aim to experiment with other semiconductor particles and bacterial species in order to optimize the efficiency of their PBS (4). Since cadmium sulfide is highly toxic, they hope to replace these nanoparticles with other less toxic semiconductor materials such as silicon (4). As other researchers strive to develop PBSs that not only reduce CO2 but also complete other crucial biological processes such as N2 fixation, Sakimoto et al.’s discovery may signify the advent of a new cyborgian evolution (3).
Michele Koh ’21 is a freshman in Holworthy Hall.
 Cottingham, K. Cyborg Bacteria Outperform Plants When Turning Sunlight into Useful Compounds. https://www. acs.org/content/acs/en/pressroom/ newsreleases/2017/august/cyborg-bacteria-outperform-plants-when-turning-sunlight-into-useful-compounds-video.html (accessed Oct. 1, 2017).
 Royal Society of Chemistry. Solar Fuels and Artificial Photosynthesis: Science and Innovation to change our Future Energy Options; Royal Society of Chemistry: Cambridge, U.K., 2012; 4-11.
 Sakimoto, K., Acc. Chem. Res. 2017, 50, 476-481.
 Sakimoto, K. Science. 2016, 6268, 74- 77.