The annually averaged global mean near-surface temperature in 2024 was 1.55 °C ± 0.13 °C above the 1850–1900 average. This is the warmest year in the 175-year observational record, beating the previous record set only the year before. While a single year above 1.5 °C of warming does not indicate that the long-term temperature goals of the Paris Agreement are out of reach, it is a wake-up call that we are increasing the risks to our lives, economies and the planet.

Over the course of 2024, our oceans continued to warm, sea levels continued to rise, and acidification increased. The frozen parts of Earth's surface, known as the cryosphere, are melting at an alarming rate: glaciers continue to retreat, and Antarctic sea ice reached the second-lowest extent ever recorded. Meanwhile, extreme weather continues to have devastating consequences around the world.

The human-caused increase in the concentration of CO₂ in the atmosphere is the largest driver of climate change. CO₂ accounts for around 66% of the radiative forcing by all long-lived greenhouse gases since 1750 and about 79% of the increase over the past decade. Current atmospheric concentrations of CO₂ are higher than at any time in at least 2 million years^[1].

Figure 1 Annual mean globally averaged atmospheric mole fraction of carbon dioxide from 1984 to 2023 in parts per million (ppm) Source: Data are from the World Data Centre for Greenhouse Gases (WDCGG)

The looming energy crisis and greenhouse effect are two of the greatest problems facing the sustainable development of humanity. Conversion of carbon dioxide (CO₂) into fuels and chemicals by organisms is a promising way to solve these problems.

The total CO₂ fixation capacity of autotrophic organisms on earth reaches up to about 380 billion tons per year, but the specific carbon fixation efficiency is relatively low. In addition to the low efficiency of the carbon fixation pathways, limited energy availability is also a bottleneck for carbon fixation. In nature, autotrophs can be classified into photoautotrophs and chemoautotrophs. Photoautotrophs use light as the energy source to fix carbon dioxide, while chemoautotrophs utilize reduced compounds such as molecular hydrogen, hydrogen sulfide and other inorganic compounds . While photosynthesis is widespread in nature, its energy efficiency is generally no higher than 3%. It can be seen that there is an urgent need for more efficient carbon sequestration processes, as natural biological carbon sequestration rates do not meet the needs of industry^[2].

Microbial Electrosynthesis refers to the process where microorganisms act as biocatalysts in an electrochemical system, utilizing external electrical energy to drive reduction reactions that convert carbon dioxide (CO₂) or other organic substrates into valuable chemicals. This technology integrates microbial metabolism with electrochemical reduction, enabling sustainable chemical production powered by renewable energy sources (e.g., solar or wind power). For instance, certain microbes (e.g., Sporomusa ovata) can directly accept electrons from a cathode to reduce CO₂ into acetate, bypassing traditional fermentation or photosynthesis pathways. The key advantage of microbial electrosynthesis lies in its environmental benefits, as it simultaneously sequesters greenhouse gases and generates renewable fuels or chemical feedstocks.

Figure 2 A high-level overview of the concepts associated with bioelectrochemical systems.

Microbial Electrolysis Cell (MEC) is a type of bioelectrochemical system that relies on external electrical energy to drive microbially catalyzed reactions, typically for converting organic matter in wastewater into hydrogen (H₂) or other reduced products. In an MEC, exoelectrogenic microbes on the anode oxidize organics and release electrons, while the cathode—under an applied voltage—reduces protons (H⁺) to H₂ or further converts CO₂ into compounds like methane. Unlike microbial fuel cells (MFCs), MECs require additional energy input to overcome thermodynamic barriers, but they achieve higher energy efficiency compared to conventional water electrolysis for H₂ production. For example, when treating wastewater, MECs can simultaneously purify water and generate clean energy (H₂), embodying a circular economy model of 'waste-to-energy'^[3].

Acidithiobacillus ferrooxidans is an extreme acidophilic, chemolithoautotrophic, Gram-negative bacterium that can obtain energy by oxidizing ferrous iron (Fe²⁺) or reduced sulfur compounds (such as sulfides, elemental sulfur, or thiosulfate) in environments with a pH range of 1.5-2.5, while using CO₂ as a carbon source for autotrophic growth^[4]. This bacterium is widely distributed in acidic mine waters, sulfur springs, and other environments, and is a core species in bioleaching, coal desulfurization, and heavy metal pollution remediation. The Fe³⁺ and sulfuric acid produced through its metabolism further promote mineral dissolution and metal recovery, making it an important microorganism in metallurgy^[5]. Additionally, its genome contains various iron- and sulfur-metabolism-related genes (such as the rus operon and sulfur oxidase system), which helps in understanding the adaptation mechanisms of microorganisms in extreme environments^[6]. More importantly, A. ferrooxidans can grow using electricity as its sole energy source, utilizing soluble iron as an electron shuttle^[7]. This means we can use A. ferrooxidans to build microbial electrochemical synthesis systems, facilitating the conversion of electrical energy into chemical energy.

The working principle of solar panels is based on the photoelectric effect. When sunlight (photons) strikes a semiconductor material, the photon energy is absorbed, exciting electrons from the valence band to the conduction band and creating electron-hole pairs. The built-in electric field in the P-N junction separates these charges—electrons move to the N-side and holes to the P-side—generating a voltage. When connected to an external circuit, the flow of electrons produces an electric current, converting solar energy into electricity.

Figure 3 Schematic diagram of the principle of solar panels

Our project

Figure 4 Three-electrode reactor system used in our project for biocathode development. Here, WE: working electrode (i.e. cathode); CE, counter electrode (auxiliary); RE, reference electrode. All the electrodes were immersed in the electrolyte.

Although traditional solar cells are efficient at converting light energy into electrical energy, energy storage remains challenging. On the other hand, the efficiency of biological photosynthesis is limited by its inherent design and is unlikely to improve in the short term^[8]. Therefore, combining solar panels with biological CO₂ fixation and fuel production, using electrical energy to drive microorganisms to convert CO₂ into liquid fuels, is a promising solution. In fact, the use of electrical energy to drive microorganisms to convert CO₂ into liquid fuels has already been reported in Ralstonia eutropha H16^[9].

In 2025, we attempt to use A. ferrooxidans to build a microbial electrochemical synthesis system coupled with solar panels, completing the conversion from solar energy to electrical energy and ultimately to chemical energy. During the process, CO₂ fixation and glycerol production will also be achieved.

The solar-to-electricity energy converter will be provided by the solar panels, while the electricity-to-chemical energy converter will be A. ferrooxidans. In recent years, solar panels have achieved relatively high theoretical energy conversion efficiency, making this process primarily limited by the energy conversion efficiency of the microorganisms. To improve the energy conversion efficiency of A. ferrooxidans, we plan to enhance electron transfer efficiency by increasing the intracellular c-di-GMP content. Additionally, we aim to boost carbon fixation efficiency by overexpressing a key enzyme in the CBB cycle, PRK. Lastly, we choose glycerol as the final organic compound to store chemical energy and have designed a glycerol synthesis module. We have introduced efficient glycerol-producing enzymes (Gpd1 and Gpp2) and glycerol channel protein (Fps1) from Saccharomyces cerevisiae to facilitate glycerol production and export.