Thermophilic Life in Yellowstone National Park Challenges the Physical and Chemical Limits of Survival

Introduction

Yellowstone National Park was the world’s first national park, designated as such by President Ulysses S. Grant in 1872 because of its well renowned hydrothermal properties and geologic wonders (U.S. Department of the Interior, 2021; U.S. Department of the Interior, 2020b). The park contains the world’s most concentrated area of geysers, hot springs, mudpots, and steam vents, with over 10,000 in total. The country’s largest and most photographed spring, the Grand Prismatic Spring shown in Figure 1, is located in the Midway Geyser Basin of the park. The spring is over 10-stories deep, 370 feet in diameter, and contains a rainbow of colors produced by the thermophilic bacteria that call the spring home (Peglar, 2017). Yellowstone National Park is geographically divided into geyser basins, defined as locations with clusters of hydrothermal features. Hot springs are the most common hydrothermal features in Yellowstone, but basins can also include geysers, mudpots, and fumaroles.

The park’s thermophilic features are ideal locations to explore the fundamental questions regarding life under extreme conditions. Prior research on heat-resistant microbes has led to both medical and forensic scientific advancements, such as the polymerase chain reaction (PCR) – which has proved an extremely valuable tool for gaining information on poorly understood diseases. Life within the park has also provided insight into Earth’s past microbial communities, and it has even helped researchers develop proposals about life on other planets.

Hot Spring Formation and the Grand Prismatic Spring

Hot springs are formed when water from deep within the earth emerges through the cracks in its surface (Geiling, 2016). In this case, the internal water route does not contain any obstructions near the surface that are responsible for creating intense, sporadic eruptions, as seen in geysers. In contrast, the hot water that comes out of hot springs is able to continuously flow and create a nonstop cycle of hot water rising, cooling, and falling.

Tourists in Yellowstone National Park often take pictures of the beautiful hot springs, capturing the rainbow coloration provided by the spring’s large groupings of microbial life (U.S. Department of the Interior, 2019b). The different thermophilic microbes present within the spring all have their own desired temperatures, and they cannot tolerate much cooler or warmer conditions. Hot springs have a distinct color gradation where the temperature of one microbial group is reached and another microbial community begins as the water temperature changes. This doesn’t mean that all colors are produced by single microbial species – often the communities within the colored mats are mixtures of several different microbes – but depending on the temperature, some organisms will be more predominant than others (Geiling, 2016).

The outermost ring of bacteria in the Grand Prismatic Spring, Calothrix, is responsible for the dark brown mats that are seen in Figures 1 and 2. The bacteria thrives in temperatures between 86 and 113 degrees Fahrenheit (U.S. Department of the Interior, 2020a). Moving inward towards the region heated to a range of 95 and 135 degrees Fahrenheit, Phormidium is the predominant creator of the orange mat color. Calothrix and Phormidium are both bacterial genera within the phylum cyanobacteria, meaning that they derive their energy from the sun through photosynthesis. Further inward, the species Aquifex Hydrogenobaclum lives at a comfortable 131 to 162 degrees Fahrenheit, and it is responsible for the spring’s yellow and white colors. Although present at the Grand Prismatic Spring, it is found more abundantly in Yellowstone’s Norris Geyser Basins and Amphitheater Springs. Its metabolic pathways consist of using hydrogen, hydrogen sulfide, and carbon dioxide as energy sources, but it can also use arsenic in place of hydrogen sulfide if necessary.

The green and yellow mats are both created by the genus Synechococcus, which lives at temperatures between 126 to 165 degrees Fahrenheit and can actually produce a range of different colors. Synechococcus starts to be seen as yellow when the temperature exceeds 149 degrees (Geiling, 2016). That is the point in which the carotenoid pigment within its chloroplasts becomes more predominant than its chlorophyll pigments. Synechococcus, like Calothrix and Phormidium, are genera of the phylum cyanobacteria, and undergo photosynthesis. The primary pigment for photosynthesis (the conversion of energy from the sun into chemical energy channeled by molecules within the bacteria) is chlorophyll, which we see as green, but chlorophyll levels can become surpassed by carotenoid pigments in more extreme conditions, which can be seen as red, orange, or yellow. The Synechococcus genus inhabiting the yellow temperature band live under harsher conditions and produce more carotenoids than they would if they lived under optimal temperature conditions such as 64 and 69 degrees Fahrenheit (Mackey et al., 2013). Synechococcus living in less harsh sunlight would look more blue-green – and that is how it is commonly seen in most lakes and ponds elsewhere (Geiling, 2016). The carotenoids act like sunscreen to protect the bacteria from extreme sunlight as high-energy ultraviolet wavelengths are captured.

It’s important to keep in mind that each color of the Grand Prismatic Spring does not represent a distinct bacteria but is a composite color that represents a community of organisms, inhabiting a desired temperature range. One example of a bacteria that is found alongside a number of others in a very broad temperature ranges is the green non-sulfur metabolizing Chloroflexus, which lives throughout the 95 to 185 degree range. It is the only microbe present in the hot spring capable of anoxygenic photosynthesis – photosynthesis that produces no oxygen (U.S. Department of the Interior, 2020a).