The mysterious love child of geology and biology: Hydrothermal Vents - Part 3

Hydrothermal Vents 101

Hydrothermal Vents are chimney like structure that form along with new sea-floor at divergent plate boundaries in the middle of oceans. They form because an oceanic plate is being pulled from two opposite sides where it meets continental plates and subducts below. The oceanic plate splits apart in the middle, under miles of water. This splitting process allows sea water to come in contact with hot magma from below Earth’s crust. This hot magma forms new sea floor in the form of basalt. Basalt is a rock that is low in silica, unlike granite which makes up the continental crust and is much more buoyant. The water that comes in contact with the magma becomes super-heated and therefore able to dissolve lots of minerals that water doesn’t otherwise contain. Heat adds energy to a system which causes chemical reactions to speed up, and so many reactions happen in the presence of hot water that wouldn’t be noticeable otherwise. These hydrothermal fluids can be up to 350^oC (662^oF), which means that they would be gas (water vapor) under pressures that we are used to on Earth’s surface. However, because pressures at the sea floor bottom can be up to 345 times what they are at sea level, these liquids remain just that. As soon as the heated fluid comes in contact with cold seawater (2^oC, 36^oF) the minerals it was holding on to immediately precipitate out of solution (in a process that is the opposite of dissolving) and form solid rock structures. These sulfides make up the chimney-like structures that are characteristic of these environments. Minerals such as pyrite (fool’s gold) and chalcopyrite (a crusting mineral often confused with fool’s gold) coat the inside of these chimneys with shimmering golden crystals. Hollow tubes remain in the center of these structures, and the chimneys grow taller as more hydrothermal fluid flows through them adding its minerals as it is suddenly chilled by the surrounding seawater. Eventually the cracks in the underlying rocks fill in with new rock, or small earthquakes occur forming new cracks. When this happens one chimney “dies” and others begin to form. A single chimney might last 20 years.

The mysterious producers referenced above that were found to inhabit this extraordinary window into the deeper Earth are microscopic bacteria and archaea. Archaea are a relatively recently defined ancient group of microscopic organisms as genetically different from bacteria as animals or plants. These microrganisms, referred to as microbes or “bugs” (affectionately by microbiologists) garner energy from the dissolved minerals in the hydrothermal fluids described above. Some of these microbes thrive in the pore space of the sulfide rocks and are constantly bathed by incredibly hot, mineral-rich water.

The rocks that make up the chimneys, as well as the basalt crustal rocks that they grow on top of, provide a network of cracks and pore spaces in which the heated, mineral-rich waters mix with cold overlying seawater. The result is a warm and hospitable area called a diffuse flow zone that supports most of the life (animal and microscopic) in these ecosystems. The microscopic producers that convert this geological energy into energy that other organisms can use are called chemosynthesizers, chemoautotrophs or sometimes chemoautolithotrophs! While the terminology can seem like jargon it is actually very specific and explanatory. Chemo- means chemical, auto- means self, litho- means rock, and troph- has to do with feeding. By the same naming conventions plants and certain plankton are considered photoautotrophs, while we are considered heterotrophs, because we require organisms other than our self (hetero means different) for food.

Scientists do not know how life appears seemingly out of nowhere at new vents. Some have proposed that the carcasses of dead whales (whale falls) or sunken logs (wood falls) provide an intermediate nutrient source. Others have found evidence that there is an underground reservoir of microbes (the Deep Subsurface Biosphere) that survive in mantle material and come up as new vents are formed. Scientists have identified seven different biogeographical provinces of vents that all share similar species assemblages. Two of these provinces are dominated by the charismatic white and red tubeworms, but the others are dominated by various combinations of giant mussels, enormous clams, amphipods, shrimp, crabs, polychaete worms, huge barnacles, snails and anemones.