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1: Introduction
acidity of fruit juices is not due to sulfuric acid, but to organic acids
such as
citric acid, acetic acid, and lactic acid, and the acid of stomach juices
is
hydrochloric acid.)
   The most dramatic development of sulfuric acid environments occurs in
geothermal areas. Volcanic gases, coming from deep within the earth, are
often rich in hydrogen sulfide, and as this gas reaches the surface of the
earth it meets with the oxygen of the air and oxidizes, first to elemental
sulfur, and subsequently to sulfuric acid. The classic habitat of this type
is
Solfatara (the Forum Vulcani of the Romans), a small steaming volcanic
crater along the Bay of Naples near Pozzuoli, familiar to tourists for
hundreds of years and popular today as a camping site. In Italian, solfatara
means "sulfur mine," so-called because sufficient crystalline elemental
sulfur was present so that at one time it was mined, and the name solfatara
has subsequently been given to the many other sulfur-rich acid geothermal
areas around the world. There are many solfataras in Yellowstone National
Park, most notably at Roaring Mountain and the Norris Geyser Basin.
Solfataras are most easily recognized by the crumbling and bleached nature
of the rocks. Sulfuric acid corrosively attacks the rock fabric and causes
the
rocks to disintegrate into crumbling bits. Elements such as iron, which give
rocks their normal color, are leached out, leaving behind a whitish residue
containing predominantly quartz, the only mineral stable in very acid
conditions. Often the rocks are so destroyed by acid that walking in
solfataras is unsafe; beneath a thin surface crust the earth is hollow and
hot,
and one can easily thrust a foot through into a steaming pool of hot sulfuric
acid.
   Yet in all these diverse kinds of acidic environments organisms not only
live, but flourish. Some, in fact, will live nowhere else. (See Chapters
5, 6, 9
and 12)
  Sulfuric acid environments may be as acidic as any environment on
earth. Values in solfatara soils are usually pH 2 or below, and acid mine
drainage usually has a pH between 2 and 3, but some volcanic crater lakes
are even more acidic, with values less than 1. I have even had water
samples from small steaming pools which have had pH values as low as 0.2.
In an area where sulfuric acid leaches into a hot soil, the evaporation of
water may lead to an unusual concentration of the sulfuric acid that is left
behind, and we once measured a pH in a Yellowstone solfatara soil of 0.05;
even at this low pH we found a living organism, the alga Cyanidium
caldarium.
Saline Environments
Aqueous habitats exist with total ion concentration varying from essentially
zero up to saturation. As ion concentration of the water goes up, some ions
precipitate before others. The most soluble are the cations Nat, K+, and
Mg2+ and the anions Cl-, SO24+ and HCO3 (or CO2- depending on pH). It
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