This overview of extremophilic life in boiling acid was created by Kurt Ingelman a recent graduate of Humboldt State University. Kurt focused much of his effort on the art of science writing and has been gracious enough to write a series of selected essays that paint a portait of what life would be like for organisms that exist in LVNP's acidic hot springs.
Life in Boiling Acid (by Kurt Ingelman)
In the bubbling mud pots, lurid hot springs, and steaming fumaroles of Lassen Volcanic National Park live a group of organisms that have quietly changed biologists’ ideas about what life is capable of. Thriving in chemical and thermal conditions that would tear apart the cells of more pedestrian life forms, these microbes have solved physiological problems once thought insurmountable, expanded our picture of the nature’s diversity and may provide insight into life on early earth.
Lassen Volcanic National Park provides habitat for many familiar plants and animals. Its conifer forests shelter black-tailed deer and golden-mantled ground squirrels; bald eagles and osprey prowl skies over Lassen’s numerous lakes and the cold, freshwater streams teem with trout. But until relatively recently, no one suspected that the volcanic lakes and hot springs that dot the landscape of the park could cradle entire microscopic ecosystems. While scientists have been aware of relatively heat-tolerant microorganisms for some time, the discovery that life could exist at temperatures above that of boiling water came as a surprise even to those most familiar with extreme environments. But since the discovery of living specimens in hot spring samples from Yellowstone National Park, made in the late 1960’s by Thomas D. Brock of the University of Wisconsin-Madison, our conception of life’s outer limits has steadily expanded.
As researchers have developed more sensitive techniques for locating living organisms, they have found life in almost every place that they have looked, from hydrothermal vents in the deepest trenches of the Pacific Ocean to frozen Antarctic ice, and even far below the terrestrial surface in earth’s lithosphere. While these environments are wildly disparate, the organisms that inhabit them share a common theme: they exist at the fringes, where punishing temperatures, pressures and chemical concentrations exclude all but the hardiest of inhabitants. Accordingly, biologists have termed these survivors extremophiles for their ability to make a living far outside the ranges of most creatures.
Life at the Extremes
Extremophiles can be divided into groups according to their particular niche: Lassen is home to both thermophilic (literally: heat loving) and acidophilic organisms (those that live at extremely low pH levels). But extremophiles also include cold-tolerant psycrophiles, whose comfortable temperature ranges lie below the freezing point of pure water, and alkaliphiles that inhabit extremely basic (high pH) water or soil. In addition, barophiles of the deep oceans or subterranean lithosphere operate at pressures hundreds of times greater than that of our atmosphere, while halophiles exist high salinity environments where they must contend with potentially desiccating salt concentrations.
One of the best studied residents of the Park’s thermal pools is Sulfolobus, a single-celled microorganism belonging to the domain Archaea (discussed in the next section). Far from being restricted to Lassen, species of Sulfolobus have been detected as far away as Italy, Kamchatka, Iceland and Japan. Like many other archaeans, Sulfolobus is a hyperthermophile, tolerating ambient temperatures of nearly 90° C, just shy of the boiling point of water. It also requires a strongly acidic environment and grows best at a pH of 2-3. This is a level of acidity between that of vinegar and battery acid! The cells of Sulfolobus are small, stretching only 1-2 microns across (a micron is one-thousandth of a millimeter), and are distinctly lobed, giving it the shape of a lumpy sphere.
Having evolved in the dynamic environment of thermal springs, Sulfolobus has maintained an impressively flexible lifestyle, taking the energy it needs from many different sources. Sulfolobus can metabolize carbon compounds in a process not very different from our metabolism. In the absence of organic molecules, cells may adhere to sulfur crystals, using these crystals as both substrate and energy source. Additionally, this master of adaptation may use the energy from the oxidation of iron in order to make a living. Unlike many other residents of the thermal pools, Sulfolobus requires an oxygenated environment to run its metabolic processes. As a result, certain oxygen-free regions of the hot springs remain off-limits to this organism.
Due to its unique hot spring habitat, Sulfolobus must deal with ecological pressures somewhat different than those of a forest or ocean. Predation by protozoa, a common interaction for microbes of other environments, is limited by the extreme temperature and pH ranges of the habitat. Most potential “grazers” simply cannot survive in a hot, sulfuric spring. Populations of Sulfolobus are limited by other factors, however. Viruses attack the cells of these microbes, spreading through a population and restricting its growth. Limited nutrient resources and substrate availability may also keep Sulfolobus colonies in check.
Its tolerance for oxygen, ubiquitous distribution and metabolic malleability make Sulfolobus an ideal study organism. Biologists now posses a complete record of its genome and are connecting specific genes its unique lifestyle. With a growing list of residents, researchers have begun to piece together the complex interplay of hot springs ecology. Further research will help to understand the role that Sulfolobus and other organisms play in this singular environment.
To persist in such seemingly adverse physical and chemical settings, extremophiles must rely on host of adaptations specific to their particular milieu. Indeed, many extremophiles are so highly evolved to deal with the circumstances at the extremes that they would perish immediately at ranges we find quite comfortable. For example, Polaromonas vacuolata, a bacterium of the Antarctic sea ice, operates most efficiently at a chilly 4° C, and cannot reproduce if the ambient temperature rises above 12° C. In contrast, Sulfolobus prefers temperatures of up to 90° C and cannot maintain vital functions if the mercury drops below about 65° C. The idea, however, that these organisms live outside of the “normal” physical or chemical ranges of life is a rather anthropocentric one. For Sulfolobus, room temperature is extreme! Accordingly, biologists have sought out extremophiles to help answer physiological questions such as how a cell protects itself from high acid concentrations or freezing temperatures or what genes thermophiles require to carry out metabolic reactions at 100° C.
Extremophiles and Early Earth
These same microbes may also enhance our understanding of the earliest cellular life. Since the advent of genetic sequencing technology, investigations into the evolutionary history of life on earth have increasingly relied on genetic comparisons between related groups. As these comparisons help us to refine our picture of evolution on earth, often depicted as “the Tree of Life,” the pattern that emerges reveals groups of thermophilic Bacteria and Archaeabranching off near the “root” of the tree, the universal ancestor of all life. These thermophilic lineages thus represent the closest known living relatives to this earliest common ancestor! Certainly, the story of early earth as told by geology is not so different from the forbidding realm of sulfuric hot springs. For the first 1 billion years of our planet’s history, it is likely the earth’s surface was too hot for any life. As it cooled enough for life to first flourish, large regions of the planet may have resembled today’s volcanically active regions where we find players such as Sulfolobus. Furthermore, atmospheric conditions were far different when life first emerged, with methane, carbon dioxide and ammonia dominating the atmospheric gases. Along with the significant levels of sulfide and iron, this mixture would have met the energy requirements of many modern extremophiles.
Thus, for researchers interested in life’s origins, thermophiles may provide a glimpse into the ancient past and yield clues about the physiology and genetic makeup of our most ancient of predecessors. It is important to remember, however, that while these organisms may be closely related to early forms of cellular life, they are not themselves primitive species or ancient relics. Each species that exists today, whether thermophilic archaean or human being, represents a modern lineage that has continued to evolve right to the present day.
The March of Progress
Extremophiles exist so far outside of our everyday experience that one might expect their study to fall mainly under the category of scientific curiosity. But the very features that set them apart from more garden-variety organisms also make them incredibly useful for a variety of industrial and biomedical applications. Thermophiles, especially, have become the focus of industrial interest for their ability to carry out biological processes at high temperatures. To accomplish these feats, they rely on protein catalysts, called enzymes, which are stable even at very high temperatures. While all living organisms use enzymes to drastically speed up chemical reactions, most enzymes lose function above about 42° C when the bonds that hold them in the proper conformation begin to break. Thermophiles, however, have an enzymatic “tool-kit” shaped by the unique evolutionary pressures at high temperatures, and their proteins actually operate most efficiently when the heat is on!
Many commercial detergents make use of a class of enzymes called proteases, taken from bacteria such as Bacillus licheniformis. Besides being thermally stable, these enzymes have the additional attribute of functioning effectively in an alkaline environment such as that of laundry detergent solutions.
The most famous scientific application may be the revolutionary polymerase chain reaction (PCR), which allows users to greatly amplify DNA samples for industrial, scientific and forensic purposes. The PCR process requires cycles of high heat to separate newly formed DNA strands and most enzymes break down at these high temperatures. However, Thermus aquaticus, a thermoacidophile found in Yellowstone’s hot springs, carries thermally robust enzymes built for these very conditions. Since researchers first isolated these hardy enzymes, PCR has grown into a 7 Billion dollar per year industry!
Other thermophiles have proved useful in the medical field where a class of heat-synthesized substances, called cyclodextrins, are used to improve the uptake of medicines in the body. Further applications ranging from food stabilizers to industrial waste decomposition already make use of resilient extremophile proteins and numerous other processes may be just over the technological horizon.
Extremophiles, such as those living right here in the thermal pools of Lassen Volcanic NP, have expanded our understanding of life, refined our picture of early evolutionary history and have provided the basis for important new technologies. Their significance stretches far beyond the sulfur springs that cradle them and scientists and non-scientists alike now look to places like Lassen for the promise of life in boiling acid.
Our tour of life in Lassen Volcanic National Park continues with a look at: