Citation: Yamamoto, M., Takaki, Y., Kashima, H. et al. In situ electrosynthetic bacterial growth using electricity generated by a deep-sea hydrothermal vent. ISME J (2022). https://doi.org/10.1038/s41396-022-01316-6
The deep ocean holds a new surprise for every expedition, especially in the field of microbial ecology. In this deep, dark world without sunlight, microorganisms have some inventive strategies to get energy. While bacteria at the surface use photosynthesis or break down organic matter, deep sea bacteria get their energy from chemicals. Sulfur-spewing hydrothermal vents are an oasis where diverse microbial metabolisms can thrive. In a recent study, scientists discovered a new type of bacteria in hydrothermal vent ecosystems that can harness electric fields for energy! These little-understood microbes are called “electroactive microorganisms” and they get energy from reactions called “reduction-oxidation”, or redox reactions for short.
Redox reactions are all about the transfer of electrons from an electron “donor” to an electron “acceptor”. Batteries use redox to generate electricity, and like a battery, in the environment electrons flow from objects that acts as anodes to objects that acts as cathodes.
It turns out that the deep sea is full of redox chemistry, since it is abundant in metals and ores that are electrically conductive. Metals are spewed into the seawater by hydrothermal vents, and they settle on the seafloor around the vent, reacting with each other and other dissolved minerals to create ores. These ores automatically do redox reactions with the surrounding seawater, which generates a small electrical field. Researchers call this phenomenon the “deep sea hydrothermal field”, which has been found to conduct electricity over large distances. Essentially these large swaths of the seafloor act as batteries and researchers are just beginning to discover the microbes that evolved to capitalize on this deep-sea electrical field.
So far, microbiologists have described three different kinds of “electroactive microorganisms”:
- Electrogens: bacteria that make their own energy by releasing electrons to natural “electrodes” like metal ores
- Electrotrophs: bacteria that receive electrons from natural electrodes.
- Electro-synthesizers: bacteria that use carbon dioxide to grow (like plants) but use electric fields instead of sunlight to power that process.
Electroactive microbes have been described and studied before in a diverse range of ecosystems, from rice paddies to marine sediment, but this recent study by a team of Japanese researchers was the first to demonstrate the existence of electro-synthetic microorganisms in hydrothermal vents!
How the study worked
This study was part of a research expedition to a hydrothermal vent in the Okinawa Trough, which is a volcanically active rift in the East China Sea between the southern tip of Japan and northern edge of Taiwan.
The researchers wanted to know if the electric field generated around hydrothermal vents could support electroactive microbes, though the natural current was quite low and these kinds of bacteria are hard to study. For one, sampling the deep sea is difficult because hydrothermal vents are delicate environments and hard to access. Second, electroactive bacteria are often “masked” by chemoautotrophs (bacteria that metabolize chemical compounds like sulfur or methane) and these two types often grow together.
The researchers worked around this sampling issue by constructing an artificial hydrothermal vent in the deep sea a few miles away from the natural vent in order to study electroactive bacteria in a controlled environment where they could measure voltage.
To do this, they made a giant battery called a fuel cell that would be powered by hydrothermal fluid generated from an artificial vent. They created this vent by drilling into the volcanic hotspot just below the seafloor. The fuel cell contained a thermometer and voltmeter to measure temperature and electricity and had a sheet made of carbon felt which provided a habitat for microbes to grow on. The sheet had an insulated section which would not conduct electricity so they could compare which bacteria settled on the conductive versus non-conductive material. This fuel cell sat at the bottom of the sea, near the outflow of the artificial vent for 12 days before it was collected by the research team.
While the fuel cell sat on the bottom of the ocean, the researchers observed that the voltage gradually increased as the study period progressed. After they retrieved the cell and sequenced DNA to identify the bacteria on the carbon felt, they found that more bacteria settled on the cathode-end of the cell than the anode or the non-conductive area. Some of these bacteria were members of an undescribed species that had genes which matched those of other electroactive microbes that have been described in other studies.
The researchers hypothesized that the bacteria grew off of the carbon felt and the electrons that were flowing into the cathode of the fuel cell. This means the bacteria is likely electro-synthetic! They believe this microbe was responsible for the increase in electricity that they observed.
The researchers proposed a name for this new species: Candidatus Thiomicrorhabdus electrophagus.
Why is it important to study electrogenic bacteria?
Given all that is happening in the world today, some may wonder about the importance of studying electric bacteria. Newly discovered species capture our attention and imagination, broadening our understanding of life on earth. By studying novel microbes and metabolic strategies, we can better understand how life evolved on earth and how it may arise on other planets.
Over 80% of the ocean has been unexplored and unmapped, but the mining sector has turned its attention to the seafloor recently due to the presence of rare metals and ores that lie at the bottom of our seas. While still hard to reach, deepsea mining is a lucrative idea since it means mining operations may not have to destroy environments on land, which harms communities of people by polluting water resources and deforestation. However regions like hydrothermal vents are home to a lot of biodiversity and scientists discover new species of animals and bacteria with every expedition. The only way to understand how important deep sea ecosystems are is to study them. Policy makers can then use our growing knowledge of these environments to create mitigation strategies that will lessen the harmful effects of new industries like deep sea mining.
Cover photo: A smoking hydrothermal vent chimney in the Mariana’s Trench. Source: NOAA Ocean Exploration, Flickr
I am a masters student at the University of Rhode Island, studying the ecology of the harmful algal bloom genus, Pseudo-nitzschia. My work is interdisciplinary and revolves around coastal health, water quality, and public engagement with science. In my free time I enjoy doing crafts, writing, cooking, and exploring the outdoors on my feet or a pair of skis.
