Objectives:
(1) To understand the ecology of deep sea hot springs, and to appreciate the interplay between biology and plate tectonics(2) Understand how life developed with time, and how life is classified into eukarya, bacteria, and archaea
(3) Explore how these issues relate to the origin of life on Earth (and possibly on alien worlds).
Robotic submarine visits to sites of active undersea vulcanism began in
1977. Immediately, a surprising diversity of life forms was identified:
crabs, clams, tube worms, and more. Why surprising? Because it was
thought the deep sea would have low diversity and low biomass, the
organisms there surviving solely on what little organic material might have
filtered down from the sunlit world above. Instead, concentrated areas of
high diversity and high biomass are supported by chemosynthesis, or a food
web that is based upon bacteria that derive energy from chemical reactions
and that have NO DEPENDENCE ON PHOTOSYNTHESIS OR SOLAR ENERGY.
We find active vulcanism at divergent plates (like the undersea mid-Atlantic ridge).
The juxtaposition of YOUNG HOT VOLCANIC ROCKS and COLD SALINE SEAWATER
gives rise to a convective system known as
HYDROTHERMAL CIRCULATION.
As seawater is warmed and interacts chemically with the volcanic rocks, it
becomes more acidic and picks up Fe, Cu, Zn, S, and many other constituents
from the rocks. When this water discharges at the seafloor, often at
temperatures of 350-400C, this dissolved load of chemicals is precipitated
out as Fe, Cu, and Zn sulfide minerals. If the discharge is at a lower
temperature, dissolved Ca and Ba may combine with seawater sulfate to form
CaSO4 and BaSO4.
These ideas explain
"black smokers" and "white smokers,"
respectively.
Unlike plants that rely on sunlight, bacteria living in and around the
dark vents extract their energy from hydrogen sulfide (HS) and other
molecules that billow out of the seafloor. Just like plants,
the bacteria use their energy to build sugars out of carbon dioxide and water.
Sugars then provide fuel and raw material for the rest of the microbe's activities.
This process is called
chemosynthesis.
Deep-sea bacteria form the base of
a varied food chain
that includes shrimp, tubeworms, clams, fish, crabs, and octopi. All of these animals
must be adapted to endure the extreme environment of the vents -
complete darkness; water temperatures ranging from 2C (in ambient seawater)
to about 400C (at the vent openings); pressures hundreds of times that
at sea level; and high concentrations of sulfides and other noxious chemicals.
Could similar hydrothermal vents provide an environment for life in
the oceans of Jupiter's moon Europa?
How did life evolve through time? The standard picture is based upon the fossil record. The geological record shows how and when different forms of life appeared. From fossils alone, we expect the earliest life to have been unicellular or prokaryotic.
Genetic studies from the 1970s onward have caused a revolution in ourthinking about microbes. When compared on the basis of their genetic similarities, life forms are now classified into three branches of life: eukarya, bacteria, and archaea. All of the most primitive examples of these three branches are thermophilic, leading many to conclude that the "progenitor" was also thermophilic.
Archaea look like bacteria - that is why they were classified as bacteria in the first place: the unicellular organisms have the same sort of rod, spiral, and marble-like shapes as bacteria. Archaea and bacteria also share certain genes, so they function similarly in some ways. But archaeans also share genes with eukaryotes, as well as having many genes that are completely unique.
Archaea are so named because they are believed to be the least evolved forms of life on Earth (archae meaning ancient). The ability of some archaea to live in environmental conditions similar to the early Earth gives an indication of the ancient heritage of the domain.
The archaea that live in extreme environments can cope with conditions that would quickly kill eukaryotic organisms. Thermophiles, for instance, live at high temperatures - the present record is 113C (235F). In contrast, no known eukaryote can survive over 60C (140F). Then there are also psychrophiles, which like cold temperatures - there's one in the Antarctic that grows best at 4C (39F). As a group, these hard-living archaea are called "extremophiles."
Many archaea are methanogens, producing methane as a by-product of chemosynthesis. These live under anoxic conditions in hot springs, subterranean environments, anoxic sediments, wetlands, rice paddies, and animal guts.
Caves: such as those in Romania where new species of spiders, crabs, scorpions, etc., thrive on a food chain that begins with chemosynthetic bacteria utilizing hydrogen sulfide in the water in the cave.
Deep drillholes in rock, with chemosynthetic methanogens. In the Columbia
River basalts, 1500 meters below the surface, these organisms thrive. The
reactions they employ are
FeO(rocks) + H2O = Fe2O3 + H2
(oxidation of iron in rocks produces hydrogen gas)
H2 + CO2 = CH4 + 2 H2O
(bacteria combine hydrogen and carbon dioxide)
The early Earth was hot, with a lot of extremely active volcanoes and an atmosphere composed mostly of nitrogen, methane, ammonia, carbon dioxide, and water. There was little if any oxygen in the atmosphere. Archaea and some bacteria evolved in these conditions, and are able to live in similar harsh conditions today. Many scientists now suspect that those two groups diverged from a common ancestor relatively soon after life began.
Astrobiologists are increasingly convinced that life on Earth itself might have started in the sulfurous cauldron around hydrothermal vents. Vent environments minimize oxygen and radiation, which can damage primitive molecules. Indeed, many of the primordial molecules needed to jump-start life could have formed in the subsurface from the interaction of rock and circulating hot water driven by hydrothermal systems.
If this idea proves true, then the vents offer a glimpse of life's genesis in Earth's distant past - and a glimpse of alien life yet to be discovered.