Life in Boiling Acid
In the bubbling mud pots and lurid hot springs 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 even
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 at and
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 Anarctic ice,
and even far below the terrestrial suraface in earth’s lithosphere. While these
environments are wildly disparate, the organisms that inhabit them share a common
theme: the 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.
Extremophiles are divided into groups according to their particular niche: Lassen
is home to both thermophilic (literally: heat loving) and acidophilic organisms, which
live extremely low pH levels. But extremophiles also include cold-tolerant psycrophiles,
whose comfotable temperature ranges hover near water’s freezing point 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 atmoshere, while halophiles exist high salinity environments where they must
contend with potentially desiccating salt concentrations.
To persist in such seemingly adverse physical and chemical settings,
extremophiles must rely on host of adaptations specific to their particular millieu. Indeed,
many extromophiles are so highly evolved to deal with the circumstances at the extremes
that they and would perish immediately at ranges we find quite comfortable.
It is easy to see why 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. But 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. The concept is fundamentally quite simple: the number of
genetic differences between two species is proportional to the length of time since those
two species took on separate lineages. When scientists look at the genetic blueprint of a
human and compare it with that of a chimp, we find a large portion of the sequence is
identical, or “conserved.” We share fewer genes with rabbits, fewer still with fish, and
even fewer with plants. This hierarchy of relatedness reflects evolutionary history, or
phylogeny, for the number of discrepencies begins to acculmulate only when two groups
diverge
As we refine the picture of evolution on earth, often depicted as “the Tree of Life”
the pattern that emerges reveals groups of thermophilic Bacteria and Archaea branching
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 common ancestor.
Perhaps this should not surprise us as the picture 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, in the time that life first flourished, 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 and ammonia dominating the atmospheric gases. Along
with the significant levels of sulfide and iron, this mixture would have met the energy
requirements of anaerobic metabolism no different then that of today’s chemolithotrophs
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 nor ancient relics. Each species that exists today, whether thermophilic
archaean or human being, represents a modern lineage that has grown and evolved right
up to today.
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 makes them
incredibly useful for a variety of industrial, biomedical applications. Thermophiles,
especially, have become the focus of industrial interest for their ability to carry out
biological processes and high temperatures. To accomplish these feats, they rely on
enzymes, or protein catalysts, that are stable even at high temperatures. While all living
organisms use enzymes to drastically speed up chemical reactions, most enzymes lose
function above about 40* C(?) when the bonds that hold them in the proper
conformations 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!
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 even forensic purposes. The enzyme that makes this widely used process
possible is called Taq polymerase, isolated from the Yellowstone thermoacidophile,
Thermus aquaticus. Polymerase enzymes are responsible for lengthening DNA chains
during replication but most break down at high temperatures. The PCR process require
cycles of high heat to separate newly formed DNA strands and T. aquaticus carries a
themally robust polymerase built for these very conditions. 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. Purely industrial
techniques also rely on extremophile enzymes. Applications ranging from food stabilizers
to detergent agents 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’s capabilities, refined our picture
of early evolutionary history and provided the basis for important new technologies.
Their importance stretches far beyond the sulfur springs that cradle them and scientists
and non-scientists alike now look to places like Lassen to develop an understanding of
life in boiling acid.