We often refer to microbes as "eating" chemicals. What does this really mean?
Up until a few decades ago, scientists thought all life on earth depended on energy from the sun. Plants form the foundation of familiar food chains: through the process of photosynthesis, they use energy from the sun and carbon dioxide from the air to build more-complex carbon molecules. In other words, they transform solar energy into chemical energy. It was commonly thought that everything else either ate plants or they ate plant-eaters.
In the 1970s, scientists started finding microorganisms in extreme environments that could digest chemicals from the Earth itself. Instead of using light energy from the sun, these organisms build carbon-based molecules using energy that's released when they break apart chemicals like hydrogen sulfide and methane.
The earliest life on Earth was probably fueled by processes that are similar to the ones used by extremophiles today.
Most extremophiles are simple, single-celled life forms, yet many are not. Extremophiles occur in all three domains of life: bacteria, archaea, and eukaryotes.
Microscopic, single-celled bacteria are Earth's simplest life forms. They are also some of Earth's most successful organisms. Different types of bacteria have adaptations that allow them to live in just about any environment. They fill our oceans and rivers, our soil, and even the insides of our bodies. In fact, the total mass of bacteria on our planet is much greater than the collective mass of all of Earth's animals.
Archaea look a lot like bacteria: they are tiny, single-celled, and relatively simple. But they are actually more closely related to eukaryotes, and they have some unique characteristics that put them in a category of their own. For example, archaea have a more stable membrane chemistry than bacteria and eukaryotes have, which may make them better able to survive in extreme environments. However, archaea aren't restricted to extreme environments; they live in most of the same places as bacteria.
Eukaryotes have larger, more-complex cells than bacteria or archaea. This category includes microscopic, single-celled organisms like protists and yeast, and also multi-cellular organisms like plants and animals. Eukaryotes that live in extreme environments often depend on bacteria and archaea for food, much like we depend on plants and plant-eaters for our energy needs.