An ecosystem without sun or sky
Deep within the Earth, in the oxygen-free groundwater surrounding the Koktau iron ore deposit, a unique form of life thrives. Communities of bacteria here derive all their energy from a process that seems alien compared to life on the surface. These microbes are chemolithoautotrophs, meaning they "eat" inorganic chemicals to live. Specifically, they oxidize dissolved ferrous iron (Fe²⁺), the same process that causes rust, but they do it without any oxygen. Instead of breathing oxygen, they use nitrate as their electron acceptor. This metabolism is ancient and demonstrates how life might have functioned on early Earth, before photosynthesis filled the atmosphere with oxygen.
These bacteria belong to the family Gallionellaceae. Genera like Gallionella and Sideroxydans are known for this metabolic strategy. Genomic analysis of these and similar iron-oxidizing bacteria reveals a specialized set of genes. For example, genes like cyc2 and mtoA code for proteins that facilitate the transfer of electrons from iron outside the cell, which prevents the bacteria from becoming entombed in the rust they create. Their genomes show they are fully adapted to this lifestyle, with complete pathways for fixing carbon dioxide from their environment to build their own organic matter, making them primary producers in a food web completely disconnected from sunlight.
A model for life on mars
The Koktau ecosystem is of great interest to astrobiologists. Mars is an iron-rich world that once had liquid water. If life ever arose there, it would have needed an energy source independent of light and oxygen, especially in subsurface environments shielded from harsh surface radiation. The iron-oxidizing bacteria of Koktau provide a powerful analogue for how Martian life could exist, or could have existed in the past. Researchers study these terrestrial microbes to develop biosignatures—chemical or mineral traces left by life—that could be searched for on other planets.
The discovery of life fueled solely by the oxidation of dissolved iron in an anoxic, subterranean environment expands the known limits of habitability. It demonstrates that complex ecosystems can be built on geological energy sources alone. The reddish-brown gelatinous slime they produce, a form of iron oxide, shows their activity. Similar mineral deposits on Mars could potentially point to locations where microbial life once flourished, using the same fundamental chemistry as the iron-breathing bacteria of Kazakhstan.