THIS place stinks. The rotten-egg smell of hydrogen sulphide hits before you
even enter the mouth of the cave. Acid drips from the walls and ceiling. Slime
coats the rock with coloured blotches like ghastly gelatinous wallpaper. And
then there are the 鈥渟nottites鈥�, white, wobbling versions of stalactites with the
consistency of phlegm.
Not your sort of place? But if you ever get to 鈥淪not Heaven鈥�, the part of the
cave with the nastiest smell and the most snottites, you鈥檒l discover why cavers
can鈥檛 keep away: the odour, the slime and the living strings of mucus make it
one of the strangest and most fascinating caves in the world.
鈥淲hen you first enter the cave, it appears rather ordinary except for the
smell,鈥� says Louise Hose, who led a team of 22 researchers into the cave in
January. 鈥淏ut soon you notice the white water and the extraordinary number of
fish. The closer you look鈥攁t the walls, the ceiling, the stream鈥攖he
more bizarre the cave becomes.鈥�
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In the darkness deep inside, things are happening that no one has encountered
before: sulphuric acid is carving out a labyrinth of tunnels by processes
geologists had imagined but never seen. And for a place that seems so
inhospitable, this cave is throbbing with life, from the massed ranks of
unidentified micro-organisms that create the slimes and snottites, to hordes
of midges, armies of spiders and tiny fish so numerous you can scoop them up
with your hand. All this life owes its existence to bacteria which make a living
from sulphur. These bacteria not only underpin the ecosystem, they also speed up
the processes of cave formation.
Cueva de Villa Luz, the cave of the lighted house, is a maze of passages
about 2 kilometres long, etched out of Cretaceous limestone in the state of
Tabasco in southern Mexico. Its name comes from its skylights, holes in the
rocky roof that let light into a few of its passages. Local people have
performed religious ceremonies in the cave entrance since prehistoric times, to
ask the gods underground for permission to collect fish to see them through the
hard times before the next harvest. In the 1940s and 1950s, a few
anthropologists and biologists ventured farther in, yet no one seemed to realise
how important the cave was until 1987.
Strange slimes
That year, Jim Pisarowicz, a cave guide at Wind Cave National Park in South
Dakota, explored the passages. To him, the ubiquitous encrustations of gypsum
and sulphur and the strange organic slimes, including those he called snottites,
added up to something special. He suspected that this was an example of 鈥渟ulphur
speleogenesis鈥� in action, with the added bonus of a thriving ecosystem based on
sulphur. Last year, Pisarowicz returned with Hose, America鈥檚 leading woman caver
and a geologist at Westminster College in Fulton, Missouri. She confirmed his
suspicions. In January, Hose, Pisarowicz and a team of 20 other cave experts
donned hard hats and gas masks and made the first detailed investigation of the
cave.
The gas masks are essential. The cave contains deadly hydrogen sulphide gas,
but just how much is unpredictable and varies from place to place. In a few
spots the air is healthy enough for six species of bat to roost in, including
vampires. But between these, bats and cavers must pass through places where the
concentration of hydrogen sulphide often reaches 60 parts per million. In Snot
Heaven the monitor registered a phenomenal 127 ppm. 鈥淯S health and safety
regulations say you should evacuate an area if it goes above 10 ppm,鈥� says Kathy
Lavoie, a caver and microbiologist from the State University of New York at
Plattsburgh.
The gas may be bad for cavers but it is a sign that the processes of cave
formation are in full swing. Most limestone caves are created from above ground
as rainwater mixes with carbon dioxide from the air and soil to produce carbonic
acid. This filters through cracks and fissures and slowly eats away the rock.
But a few caves are carved out by processes that start deep beneath the surface.
Geologists have long suspected that some caves are created when groundwater
laden with hydrogen sulphide flowed into water containing oxygen, reacting to
form sulphuric acid, which dissolves away the rock鈥攁 process called
sulphur speleogenesis.
The discovery of Lechuguilla Cave in New Mexico in 1986 strengthened this
view. Lechuguilla is probably the most spectacular cave in the US, at least 140
kilometres long and extending to a depth of 500 metres. The caverns are
decorated with beautiful deposits of sulphur and gypsum (calcium sulphate,
formed when sulphuric acid reacts with limestone). Huge 鈥渃rystal chandeliers鈥� of
gypsum are one of the hallmarks of the cave. Although this looked like a case of
a cavern carved out by sulphuric acid, the processes of creation have long since
ended. Today there is no hydrogen sulphide entering the cave, and only a few
isolated pools of water.
Microbes at work
Around the time Lechuguilla was discovered, Earth scientists were coming
round to the idea that microbes might also have a hand in carving out caves, by
speeding up the chemical reactions. The discovery of fossilised bacteria in
Lechuguilla led to even more speculation about the role of microbes. Now,
following explorations at Villa Luz, there is little doubt that they do help to
create caves. 鈥淭his is an example of what formed those caves millions of years
ago. But here we have the active processes,鈥� says Hose.
If Lechuguilla is a mature cave, Villa Luz is still an infant. The initial
stage of creation, when rising acid water etched out the first spaces in the
rock, is over. The stream that flows through the cave, although milky with
sulphur, is also full of dissolved limestone that neutralises the acid and
prevents it from eroding the rock. But hydrogen sulphide still enters the cave,
dissolved in the warm water of more than a dozen springs that bubble up through
the cave floor.
Once the spring water enters the cave, hydrogen sulphide gas bubbles out,
only to re-dissolve in drops of water on the cave walls and roof. The drops
become acid, with an average pH of 1.4, powerful enough to dissolve the
limestone below. 鈥淚n places it鈥檚 dripping the equivalent of battery acid. If it
gets on you, you have to get in the water to wash it off,鈥� says Lavoie. The dark
sulphurous mud at the bottom of the stream is also highly acidic. 鈥淚t burns your
belly if you are crawling through it,鈥� she says.
As the acid reacts with limestone, gypsum forms and creates a barrier between
acid and rock and prevents more rock being eaten away. 鈥淭he walls are almost
completely covered by gypsum,鈥� says Hose. But that鈥檚 not the end of the process.
Acid dribbles down the gypsum coating and where it meets the stream it etches
little rills in the rock before losing its potency in the water. Blocks of
gypsum frequently fall from the walls, either under their own weight or
dislodged by floodwaters or animals seeking shelter. This exposes fresh rock to
acid attack. 鈥淭his is the major process today,鈥� says Art Palmer, the team鈥檚
hydrologist from the State University of New York at Oneonta.
There are other more mysterious processes at work鈥攏ot chemical but
biological. Microbes play an active part in changing the nature of the rock,
both by dissolving it and by helping to create a range of deposits on the cave
walls. Most important are bacteria that get their energy by oxidising sulphur
and acquire the carbon they need from the air, converting carbon dioxide into
organic material. In the lightless cave ecosystem, these 鈥渃hemosynthetic鈥�
bacteria form the basis of the food chain, taking the place of photosynthetic
organisms in the outside world.
These bacteria grab hydrogen sulphide from the air and combine it with
oxygen. As a waste product they excrete sulphuric acid, which eats away at the
limestone. 鈥淗ow much they contribute to the process is one of the big unsolved
questions. But they certainly speed up the reaction,鈥� says Palmer.
Such bacteria鈥攑articularly those that live in the snottites鈥攁re
causing a lot of excitement. Some snottites stretch down half a metre, while
others coalesce with their neighbours into curtains or form loops like melted
mozzarella. 鈥淚t鈥檚 rare in a cave to actually see bacteria; to see hanging veils
made up of bacteria. It makes a microbiologist鈥檚 heart beat faster,鈥� says Diana
Northup of the University of New Mexico in Albuquerque.
At the tip of each snottite is a drop of acid. 鈥淭he slower they drip, the
lower the pH. The slowest drippers can have a pH of 0.5 or
lower,鈥� says Hose. These can be a bit of a hazard to cavers in T-shirts and a
direct hit in the eye can be very painful. 鈥淚t burns the skin and eats clothes,鈥�
says Palmer.
After her first visit, Hose took samples of snottite to Norman Pace, a
microbiologist at the University of California, Berkeley. He found they
consisted of dozens of species of bacteria, which seemed to have formed a
vertical version of a microbial mat, a structure resembling woven fabric but
made of bacterial filaments
(see 鈥淪lime city鈥�, 31 August 1996, p 32). Mats are
fairly common, says Pace, but vertical veils are unique. 鈥淚鈥檝e never seen
anything like those,鈥� he says. One possible explanation is that the bacteria
start growing on dangling bits of spider鈥檚 web, which are found all over the
cave.
Under the microscope, a glob of snottite contains a dense mesh of fine
filaments embedded in a mess of sticky polysaccharides. The filaments are
bacteria and caught up with them in the slimy glue they produce are crystals of
sulphur and gypsum and a few larger organisms such as nematodes and mites.
According to Pace, while many of the bacteria in the snottite are sulphur
oxidisers, there are other sorts that probably live off the organic compounds
these manufacture. Higher organisms probably eat the bacteria. 鈥淚t鈥檚 a
consortium of organisms based ultimately on sulphur metabolism,鈥� says Pace.
The snottites may be the most exotic of the microbial gunks but there are
mucus-like colonies everywhere. 鈥淩ed slimes, white slimes, black slimes. There
are slimes all over the place,鈥� says Hose. Large areas are covered with a soft
鈥済ypsum paste鈥� with a pH ranging from 0.5 to 3. All seem to be a
mixture of mineral and microbe. 鈥淲e found critters in the red goo and critters
in the gypsum paste. For nasty acidic paste, it was absolutely teeming with
bacteria,鈥� says Penny Boston, a microbiologist with Complex Systems Research of
Boulder, Colorado.
So far, the identities of the microorganisms remain a mystery. More than 99
per cent of bacteria are impossible to grow in the laboratory, so the team must
rely on DNA analysis. They can then compare what they find with the DNA of known
bacteria. Even if they can鈥檛 identify the species they can probably discover
what they might be related to. Before analysing DNA, however, you have to
separate the bacteria from the goo, a tricky task, but one that Northup and
Boston think they have now cracked.
The chances are that the bacteria will be from species found in other
sulphurous ecosystems, such as hot springs and hydrothermal vents on the sea
floor, or at least their close relatives. The concentrations of hydrogen
sulphide emerging around hot vents is similar to that in Villa Luz. 鈥淐hemistry
not geography drives microbial diversity. If you have a type of chemistry then
you get the same sort of bacteria,鈥� says Pace. Some of the fossil bacteria found
at Lechuguilla are most closely related to species collected from ocean
vents.
Whatever they are, these bacteria support an unusually rich collection of
cave animals. 鈥淣ormally, productivity in caves is low. The organisms are sparse
and living life at a slow pace,鈥� says Boston. 鈥淏ut here it鈥檚 ripping right
along. It鈥檚 extremely rich.鈥� The place is swarming with midges, the walls and
rocks covered with their wriggling larvae. Gas masks come in handy, not just to
protect against acid fumes. 鈥淭hey also keep the midges out of your mouth,鈥� says
Hose. There are so many of these midges that the air in some passages is full of
their buzzing. 鈥淭he roar is so loud it鈥檚 like a chorus of cicadas on a summer鈥檚
night,鈥� says Hose.
Acid heads
One extraordinary aspect of these animals鈥� lives is that they don鈥檛 seem to
be harmed by the acid. Some unidentified creatures bore tunnels through the
slimes. 鈥淲hat on earth crawls in there where the pH is 2?鈥� asks Hose.
Even spiders build webs right next to dripping snottites.
And then there are the mollies, Poecilia sphenops, small fish up to
6 centimetres long that supplement the local diet. 鈥淭he density of fish is the
highest I鈥檝e seen in a cave anywhere,鈥� says Lavoie. The fish are odd in that
those living near the cave entrance show no sign of adaptation to cave life and
have the same green colour and plump shape as mollies in the local streams. 鈥淏ut
at the back of the cave they have reduced eyes and have lost their pigment,鈥�
says Lavoie. These fish appear red because their blood shows through the
colourless skin. They are also slimmer, with mouths positioned so that they can
feed at the bottom of the stream more easily. Between the entrance and the
innermost caverns are fish that look part way between the two.
The mollies feed on the bacterial masses in the water and on the midge larvae
which cluster at the stream edges. Some pools far inside the cave are filled
with reddish mats of bacteria that are so enticing fish struggle upstream to
reach them, jumping stone dams across the stream like salmon on a fish
ladder.
There are still many unanswered questions about Villa Luz. 鈥淥ne puzzle is
where the hydrogen sulphide is coming from,鈥� says Hose. The most likely source
is the oilfields of Villahermosa, about 60 kilometres to the north. The
reaction of hydro-carbons with overlying rocks creates -hydrogen sulphide at
depth, and the chemistry of the water in the Villahermosa basin is very similar
to that welling up into the cave. But there is an alternative: 50 kilometres to
the west is El Chich贸n, a volcano that last erupted in 1982. Its caldera
is filled with a sulphur lake.
There are also questions about how creatures in the cave tolerate their toxic
environment, and how the different types of mollies remain so distinct when
there seems to be nothing to stop them interbreeding. Such puzzles are likely to
keep the team busy for years. Fortunately, Cueva de Villa Luz is the ideal place
to study ecosystems based on sulphur. None of the other places where life
depends on sulphur bacteria is so easy to reach. Movile Cave in Romania, also
discovered in 1986, is a similar type of system鈥攂ut you have to dive
through flooded passages to reach the interesting parts. And the hydrothermal
vents along the mid-ocean ridges are even harder to reach. 鈥淚f you go to a
mid-ocean ridge, you need submarines and so on. But at Villa Luz you need only
tennis shoes, knee pads, a helmet and a light,鈥� says Hose.
Boston has another reason for probing the secrets of Villa Luz. She sees
caves like this as a possible model for what鈥檚 going on elsewhere in the
Universe. If life exists on Mars, for example, then it鈥檚 likely to consist of
bacteria and they are likely to dwell deep below the surface, perhaps surviving
on sulphur, like those at Villa Luz. To her, Villa Luz doesn鈥檛 even seem like an
Earthly ecosystem, which helps in dealing with the nastier aspects of working in
the cave. 鈥淵ou don鈥檛 think about it as wading around in acid, bat guano and
slime,鈥� says Boston. 鈥淚t鈥檚 a completely different world. It鈥檚 almost like being
on another planet.鈥�
