Humans survive by breathing in oxygen and releasing carbon dioxide, while plants survive by taking in carbon dioxide and releasing oxygen. However, have you ever thought that anything could live off a completely different chemical reaction? Researchers have just discovered a type of bacteria in the Black Sea (near Eastern Europe) that survives in the absence of oxygen by taking in sulfur and manganese instead.
Microorganisms are microscopic lifeforms, such as bacteria. They can live in a wide variety of conditions, such as extremely hot or cold temperatures, very acidic pools, and environments without oxygen. Scientists have long theorized that a microorganism could survive by using sulfide and the metal manganese, though no one has been able to prove it. However, that has just changed based on a new discovery from the Black Sea.
The Black Sea has multiple layers in it with different properties. The top layer of water is in contact with the air, so it is filled with dissolved oxygen. The bottom layer is near a sulfur source and is thus filled with a dissolved sulfur compound called sulfide. In between, there is a layer that lacks both sulfide and oxygen. This layer contains small particles of a mineral called manganese oxide, which has the chemical formula MnO2. Although the conditions in this layer may not seem favorable to living organisms, life was found here.
Researchers took a sample of water from near the border of this layer and the bottom sulfidic layer, where sulfide was present but oxygen was not. They sealed it in an airtight bottle full of MnO2 and gave it daily doses of hydrogen sulfide (H2S), which is the form of sulfur found in this water. They also tried using another form of sulfur called thiosulfate (S2O32-) because it is nontoxic and easier to work with.
The result was the growth of a new species of bacteria: Sulfurimonas marinigri. This microorganism is an autotroph, which means it gets its carbon (food) from carbon dioxide in the air. This is somewhat like plants, which also get their food from the air, but unlike humans, who have to eat other plants and animals to survive.
The researchers also performed additional experiments. They found that continually adding hydrogen sulfide instead of thiosulfate resulted in further growth. The organism could not use iron for growth, even though iron is common in the Black Sea.
Using this information, the researchers put together the following reaction to explain how the organism survives.
manganese oxide + thiosulfate + hydrogen ion → dissolved manganese + sulfate + water
Or written out as a balanced equation (remember that from chemistry class?):
4MnO2 + S2O32- + 6H+ → 4Mn2+ + 2SO42- + 3H2O
The left side shows what the organism takes in to produce the energy needed to live and the right side shows what it produces as a result. This process is very similar to the way we breathe in oxygen and breathe out carbon dioxide.
While growing on this reaction, Sulfurimonas marinigri produced an unusual form of the mineral calcium carbonate that contained manganese. The chemical formula for this mineral is Mn(Ca)CO3, and it is found in the Black Sea. The authors also report that other species of Sulfurimonas can be found near hydrothermal vents and in marine sediments, where they may create similar manganese carbonate minerals.
So what is so important about this new microbe? These findings make Sulfurimonas marinigri the first organism we have discovered to live by using sulfide and manganese oxide, and further prove that life can exist using energy sources that were not previously known.