Old Faithful Virtual Visitor Center

In Yellowstone there are three main chemical types of hydrothermal environments that are determined by the rock layers through which the heated ground water passes as it rises to the surface. Different microorganisms thrive in each.

Environment	Description
Alkaline siliceous	Water is rich in silica from traveling through rhyolitic lava flows within the Yellowstone caldera. (pH of water: 8-11)
Acidic	Gases, such as hydrogen sulfide, come into contact with oxygen in the water. Microbes convert the hydrogen sulfide to sulfuric acid. (pH of water: 1-6)
Sulfide-rich carbonate	The terraces at Mammoth Hot Springs are formed of travertine, a rock made of calcium carbonate. Microbes may convert hydrogen sulfide to sulfuric acid, but the carbonate acts like an antacid. (pH of water: 7)

Microbes thrive in each of these environments, but each microbial species requires specific conditions for survival. Therefore, a microbe that is adapted to high levels of hydrogen sulfide may be found near a sulfide-rich source pool, but not downstream where the gas is gone, or in a spring where hydrogen sulfide is absent. Similarly, microbes adapted to low pH conditions are not found in areas that are alkaline.

Temperature
Colored rings around the edge of a hotspring represent different microbe species

Each microbe species within a hydrothermal feature has specific temperature requirements. Some live in the hottest water near the hot water source, while others inhabit the cooler runoff channels. The upper temperature ranges of different microbial mats are marked by distinct color changes, forming a living thermometer. In an alkaline siliceous hot spring, for example, pink, pale yellow, or cream colors appear in the hottest areas, green indicates the next warmest areas, then yellow, orange, and brown microbes appear in cooler portions of the runoff channels.

Light
Different microbes also have varying light requirements. As a microbial mat grows, the microorganisms underneath don't receive as much sunlight. Eventually they die and are replaced by species capable of surviving in less light. Similar to forest ecology, some microbial species thrive in the "canopy" closer to the sunlight, while others flourish in the "under story."