The most extraordinary hot-water habitat was discovered in 1977 when the geologist John Corliss of Oregon State University and John Edmond of the Massachusetts Institute of Technology boarded the submarine Alvin. The two scientists and a pilot climbed inside a two-metre diameter titanium sphere, built to withstand the massive pressures at the depths of the ocean floor. The vessel was dropped into the Pacific, two hundred and eighty kilometres north of the Galapagos Islands to search for hot springs associated with the mid-oceanic ridges, where the continents were being pushed apart by molten magma welling up from cracks in the earth’s crust. The craft (descending at a leisurely rate of 30 metres a minute) took about ninety minutes to reach the ocean floor, two and a half kilometres below the surface.
The crew stared through Plexiglas portholes to see a bleak terrain of black basaltic rock cut by faults and fissures. For thirty minutes they surveyed this monotonous sterile landscape seeing nothing unusual until a pair of large purple sea anemones drifted in front of their searchlights. The crew chased their prey over the crest of a ridge and were astonished to find themselves in the midst of a fabulous oasis of life. Sea anemones and snake-like pink fish with bulging eyes moved through shimmering warm waters, whilst crabs and miniature lobsters crawled amongst fields of giant clams and reefs of mussels. For the remaining five hours the crew took photographs and measurements and hastily collected as many of the animals as they could catch in Alvin’s specimen basket, before ascending to the surface.
Alvin made fifteen dives to the underwater oasis in 1977 and collected a mass of data, photographs and specimens. Since then several other expeditions have descended to discover more about the geology and biology of these unique habitats. As the team suspected, the hydrothermal vents form when seawater seeps into cracks a mile or two deep. The water is heated by hot magma to temperatures above 400°C (high pressure prevents the water from boiling), mixed with hydrogen sulfide and spewed out of the seafloor through lava-encrusted chimneys, known as black smokers. The animals inhabiting the vent live in the cooler waters that surround the hot springs. One of the most curious creatures is the giant tubeworm, which forms dense pink forests around the vents. The worms grow to several metres long but have no digestive system: no mouth or gut. Instead they depend on symbiotic bacteria that live within their tissue and utilize hydrogen sulfide as an energy source to make organic compounds such as sugars, which nourish the worms. Bacteria are present not only as symbiots but are prevalent in the surrounding cold waters and the hot walls of the black smokers. Massive temperature gradients are found within the walls of the smokers and, within the cooler zones, thermophilic bacteria flourish. The record is currently held by a bacterium named Methanopyrus, plucked out of a black smoker by Alvin, which can grow at temperatures as high as 112°C.
LIFE IN THE DARK
It is often stated that all life on Earth depends ultimately on the energy from sunlight. Plants need sunlight, animals eat plants and some animals eat other animals. But the oceanic trenches discovered by Alvin are thousands of metres below the ocean surface, far beneath the depths that light can penetrate. These ecosystems thrive in the dark by capturing chemical energy from the hot vents. The bacteria that form the basis of these deep ocean food chains are called lithotrophs, literally rock-eaters. Like plants, they extract carbon dioxide from seawater and string the atoms together to make sugars; but, unlike plants, they use minerals (principally hydrogen sulfide) spewed out of the volcanic vents as a source of energy. The bacteria eat hydrogen sulfide; everything else eats the bacteria.
Christian Lascu and Serban Sarbu discovered another lightless ecosystem in a limestone cave in southern Romania. The cave appears to have been isolated from the surface for five million years; yet Lascu and Sarbu found transparent crabs, blind spiders and water scorpions crawling through its dark, damp interior. Microbial mats that cover the surface of a ground-water lake and the limestone walls of the cave, nourish the whole ecosystem. The bacteria appear to be able to extract carbon from limestone (calcium carbonate), using energy derived from the oxidation of hydrogen sulfide dissolved in the ground water.
FIRE AND BRIMSTONE
The Christian Hell is an inhospitable place: ‘and he shall be tormented with fire and brimstone’ (Revelations, 14:11). The most vociferous hellfire preachers conjure up images of fiery mountains, scorching deserts and bubbling pools of brimstone to roast the souls of mortals deserving eternal damnation. Yet, harsh though such environments might appear, they would in fact provide quite comfortable habitats for many (perfectly virtuous) living creatures.
Brimstone is an archaic name for sulfur, which is found in meteorites, hot springs and sprayed out of active volcanoes. It is often visible as pale yellow streaks decorating volcanic slopes. The element itself is relatively harmless. It gets its infernal reputation from its ability to float on water and burn, releasing poisonous fumes of sulfur dioxide. Many of its other compounds are also noxious. The reduced (reduction is the opposite of oxidation and often involves the addition of hydrogen atoms to an element or compound) compound of sulfur, hydrogen sulfide, is a foul smelling and poisonous gas (the smelly gas generated by the stink-bombs so beloved of schoolchildren). Sulfuric acid is one of the most corrosive acids. Yet sulfur is essential for life. Proteins and fats are particularly rich in sulfur. Our bodies contain about two-hundred grams of sulfur. The source of all this sulfur is bacteria, able to eat or breathe both sulfur and its noxious compounds.
We have already met hydrogen sulfide-eating bacteria at the depths of the ocean; but this metabolic capability is widespread. Sulfur-eating bacteria, such as Thiobacillus, live in soil and in fresh and saltwater; and use sulfur and hydrogen sulfide as an energy source to fix carbon dioxide and generate biomass. Other sulfur bacteria, such as Desulfobacter, live in brackish water and animal intestines and use oxidized forms of sulfur (sulfate) as we use oxygen – to breathe (they exhale hydrogen sulfide). Other bacteria such as Chromatium use hydrogen sulfide as a hydrogen source for photosynthesis, depositing the leftover sulfur granules within their cells. The combination of differing survival skills of various sulfur bacteria allows it to be cycled through the entire ecosystem of the Earth. On a smaller scale, miniature sulfur cycles take place within some warm fresh-water lakes fed by sulfur-rich steams. In the sulphuretas of Libya and Japan, it is cycled between its oxidized and reduced forms and in the process, elemental sulfur accumulates in the lake and is harvested commercially. So, far from being an instrument of infernal torture, pools of brimstone can be healthy and productive environments for many microbes.
Many of the bacteria that metabolize sulfur also generate sulfuric acid as a by-product. Although science fiction films such as Alien (1979) and its many sequels featured monsters with acid for blood, it is microbes, rather than monsters, which are most tolerant to acid. Acidity is measured in units of pH: pH7 is neutral, below pH7 is acid and from pH7–pH14 is alkaline. Our cells function within a fairly narrow pH range, from about pH7.5 to 8.5: very slightly alkaline. Blood contains a bicarbonate-based buffering system that maintains its pH within this range. These stores can however be depleted during illness, such as severe diarrhoea, resulting in the drifting of body fluids outside their normal pH range, causing metabolic acidosis or alkalosis. The consequences can be disastrous, leading to tissue damage, shock and death.
Other animals are much more tolerant of acid. Acid rain from the burning of fossil fuels has caused the acidification of many lakes in Europe and the USA. Some fish can survive in lakes with water as low as pH4 but if it becomes more acidic, all