The developed bio-solar cell is more energy-efficient than marine ecosystems.
The world’s surface is about 70 per cent water. In our oceans, marine microbial ecosystems can be seen as a huge battery charged by solar energy, where microorganisms convert light into electricity.
In a new study published in Nature Communications, a team of scientists from the Institute of Microbiology of the Chinese Academy of Sciences replicated the fundamental ecological structure of marine microbial ecosystems and created a miniature bionic ocean-battery, which is a bio-solar cell that generates power from light.
In marine ecosystems, energy conversion begins with photosynthesis. In the uppermost layer of the water column that receives sunlight, primary producers, or photosynthetic microorganisms, take in solar energy and transform photons into electrons, which are then used to fix carbon dioxide into organic matter. The complex organic matter may be consumed by plankton dwelling in the water column, or mineralised to carbon dioxide via successive oxidation by facultative anaerobic or strictly anaerobic microbes in marine sediments.
Microbes in marine sediments can be further classified into two groups—primary degraders and ultimate consumers. Primary degraders break down complex organic matter into simpler organic compounds while ultimate consumers completely oxidise the simpler organic compounds and release electrons in the process that can be used for the biological reduction of elements like iron, manganese, nitrogen, and sulphur. Marine microbial ecosystems employ solar energy to power biogeochemical cycles through photosynthetic carbon fixation and mineralisation of organic materials.
When viewed from outer space, marine microbial ecosystems with the ability to convert light energy into electricity can be compared to a massive “ocean-battery” powered by solar energy. However, the efficiency of photoelectric conversion is low due to the large spatial and temporal distribution of microorganisms in marine environments and the slow and sluggish nature of electron transfer. The group of scientists suggested that it would be possible to create an ocean battery that is spatially and temporally compressed and has considerably higher energy efficiency.
In this study, the scientists took the basic structure of marine microbial ecosystems and created a synthetic microbial community for biophotoelectric conversion. This community is composed of four microbial species—cyanobacteria (primary producer), Escherichia coli (primary degrader), and Shewanella oneidensis and Geobacter sulfurreducens (ultimate consumers).
In this synthetic microbial community, the engineered cyanobacteria can produce sucrose from carbon dioxide and store light energy in sucrose. The engineered E.coli then breaks down sucrose into lactate, and S.oneidensis and G.sulfurreducens completely oxidise the lactate to carbon dioxide through successive oxidation and transfer electrons to the extracellular electrodes to generate electricity.
From this study, the scientists show that in terms of internal resistance, maximum power density, and stability, the four-species microbial community surpassed the three-species community lacking G.sulfurreducens and the two-species community lacking E.coli and G.sulfurreducens. This study suggests that maintaining the complete ecological structure of marine microbial ecosystems is crucial for achieving efficient biophotoelectric conversion.
The scientists also found that while the oxygen produced by cyanobacteria during photosynthesis allowed the aerobic respiration of E.coli and S.oneidensis, it prevented electricity generation by S.oneidensis and the strictly anaerobic G.sulfurreducens. This led to a negative effect on the overall performance.
To address this issue, the scientists blocked the aerobic respiration pathway of E.coli and S.oneidensis, and created a conductive hydrogel with an oxygen barrier. This conductive hydrogel was then used to enclose E.coli, S.oneidensis, and G.sulfurreducens to create an artificial sediment layer capable of electron transfer and oxygen isolation.
By combining the artificial sediment layer containing the primary degrader and ultimate consumers with a water column layer containing a primary producer, the scientists constructed an integrated bio-solar cell that directly converts light into electricity for over one month.
This bio-solar cell can be thought of as a miniature ocean battery as it closely resembles the fundamental biological and physical structures of the ocean battery while having a substantially smaller spatial-temporal scale and fewer species.
This study shows that the photoelectric conversion function of marine microbial ecosystems may be replicated by a species-minimised and spatially-temporally condensed synthetic microbial community. By overcoming the network-like and slow electron transfer model, this bionic ocean-battery has a greater energy efficiency than marine ecosystems.
The development of a miniature bionic ocean battery enhances the efficiency of biophotovoltaics and opens up a new path for the creation of dependable and reliable bio-solar cells. This study also illustrates the biotechnological potential of synthetic ecology.
Source: Zhu et al. (2022). A miniaturized bionic ocean-battery mimicking the structure of marine microbial ecosystems. Nature communications, 13(1), 1-14.