Desulforudis audaxviator is a bacterium capable of living in a South African gold mine, more than three kilometers below the surface . Without oxygen, without light, and at a temperature of 60ºC, this bacterium is able to survive thanks to the energy it obtains from the radioactive decay of uranium in nearby rocks, from which it obtains hydrogen and sulfate. You can also build your own organic molecules from water, and inorganic carbon and nitrogen from ammonia in surrounding rocks and liquids.
This is one of many examples of creatures that live underground and that are part of an extraordinary ecosystem of which we are now beginning to know some details.
70% of bacteria
Many types of bacteria have been found to live hundreds of meters deep, approximately one million to one billion bacteria per gram of rock . Life has even been found in places as inhospitable as underground lakes in Antarctica.
Charles Darwin himself went so far as to write how impressed he was to have found the first subterranean creatures in Diary of a naturalist’s journey around the world , published in 1845:
“We can well affirm that all parts of the world are habitable! Brine lakes or underground lakes hidden under volcanic mountains, sources of mineral waters, the wide and deep expanses of the ocean …”
The most common organisms underground are prokaryotes (microbes without a nucleus contained in a membrane), as well as bacteria and archaea (single-celled organisms with a different evolutionary history than bacteria).
According to Karen Lloyd, a researcher at the University of Tennessee in the United States and the scientists who participated in the Observatory project Carbon Deep (Deep Carbon Observatoryo), almost 70% of all microbes on Earth are under our feet. This means that underground organisms represent between 15 and 23 billion tonnes of carbon, hundreds of times more than is contained in all human beings. The deep biosphere is also estimated to have between 2 000 and 2 300 million cubic km , almost twice the volume of all oceans.
A study published by this researcher in the journal mSystems was the first to estimate the number of microbes under our feet, which have never been cultivated and whose functions are still unknown. To carry out the study, each DNA sequence deposited in public databases by researchers around the world, a total of 1.5 million, was collected and compared with 26,000 DNA sequences of microbes and bacteria that have already been cultured.
A quarter of the microbes on Earth could come from the taxonomic classification of about 30 taxa between kingdoms and classes of microbes that have never been cultivated . According to Lloyd:
“All vertebrates, all animals with backbones, are contained within a single phylum. This means that potentially we have 30 different types of microbes that are as different from any microbe already known as giraffes are from starfish.”
Referencia: Lloyd KG, Steen AD, Ladau J, Yin J, Crosby L. 2018. Phylogenetically novel uncultured microbial cells dominate Earth microbiomes. mSystems 3:e00055-18. https://doi.org/10.1128/mSystems.00055-18.
