With the rise of renewable energy, the world faces a new problem to solve. During the years of the fossil fuel monopoly, energy storage was not a problem, since nature itself was responsible for providing these deposits. In contrast, the strategy to be followed with most renewable energies is different: the aim is to capture mechanical and thermochemical energy through electrical conversion. We have the materials to manufacture electrical energy from natural elements. Now we just need to find a way to store this energy without significant environmental or economic loss.
Microorganisms may have some things to say on this matter. There are some unicellular organisms called extremophiles. Despite their apparent simplicity, they have managed to colonize niches close – from an anthropic point of view – to the limit of life-sustaining chemistry. The most famous examples are acidophiles, which are usually also thermophiles, although there are others such as barophiles (cells whose membranes withstand extreme pressures at the bottom of the ocean) and electrophiles (they use electricity to grow). It is well known that there are electrogenic bacteria and archaea, capable of modifying their chemical environment to make electricity. It is worth remembering the nature of electricity itself. The nerve impulses of any higher animal are no different from a wire.
The action potential travels along the myelinated axons in a manner analogous to that of a toaster wire. That electronic current advances toward the point of least resistance, or highest electrical potential. Therefore, we are dealing with a ubiquitous form of energy wherever there are chemical reactions in fermionic matter. In the same way, microorganisms take advantage of these changes in the electrical gradient generated by the selective permeability of their membranes to certain ions to nourish themselves or interact. One application of Geobacter sp., for example, is the bioremediation of heavy metals in contaminated soils.
Apart from these functionalities, it should be noted that the greatest example of renewable energy recycling is found in photosynthetic biology. They are capable of recover “4,000 EJ yr-1 (corresponding to an annually averaged rate of ≈ 130 terawatts (TW))”1 from the Sun. This is estimated to be about 6 times more energy than the annual consumption by human society, which is about 20TW. Therefore, it is not unreasonable to think that the same organisms that store solar energy can also receive input from other renewable energies such as wind or even non-renewable energies such as nuclear. However, the performance of photosynthesis is not perfect. One of the causes is that photosynthesis has evolved so that carboxylation and assimilation of sunlight occur at the same time in the same cell (or reaction container). Attempts to overcome this mismatch between CO2 fixation and water photolysis are called re-wired carbon fixation or microbial electrosynthesis.
Finally, long-range electronic transport would be required. An interesting model is known as SmEET (solid matrix extracellular electron transport). It consists of three pillars: electron transport from the electrode to the cell surface, electron transport from the membrane to the cellular electronic transport chain, and the formation of reducing agents that will be incorporated into CO2 fixation. The problem is that no known organism should be able to perform rewired carbon fixation and SmEET at the same time, thus establishing a new area of work for systems biology and genetic engineering. In a nutshell, we are talking about a living solid matrix system, called electroactive biofilm, connected to electrodes. An intelligent reaction would be to question the capacity to host electrical energy in this system, or it can also be expressed as the maximum conductivity of the biofilm. Different calculations and estimates have been made, ranging from 5 x 106 S cm-1 to 5 x 106 S cm-1 at 30°C.
One aspect often forgotten during the review of this topic is that carbon-containing macromolecules can be custom designed for energy accumulation. A good candidate would be natural co2 binders that accumulate co2 in the form of plastic (polyhydroxybutyrate, PHB). The best bacterium studied so far is Ralstonia eutropha, capable of producing 15g of PHB per liter per hour. To access this energy, it can use its own oxidative metabolism and its release to external electron transport.
This is an uncertain technology since the challenges to be overcome are still very great and there is no guarantee that they can be solved. This means that there are no start-ups betting on this winning card yet. After all, one thing must be very clear: the capacity of biological systems to capture energy of any kind and transform it into other more stable types of energy, such as chemical energy, is unique.
On the other hand, and although it may seem like something out of a science fiction movie, it has been experimentally proven that Geobacter sulfurreducens is capable of supporting adhesion to electrodes while performing its normal electrogenic metabolism, so they can be classified as biological micro-batteries. Apart from this, a large-scale energy storage system requires incredible discoveries in the field of systems biology, an efficient and low-cost long-range electronic transport model that is simple in design, safe and effective, and above all gene editing of the heterologous microorganism that will express the carbon-fixing apparatus. This whole idea is likely to be greatly enhanced when nanotechnology joins the ranks of scientists trying to solve the emerging problem of renewable energy management.