Hydrothermal Vents: Frontiers in Ocean Discovery

Background biology n Prior to 1977, biologists thought that without the energy of sunlight to support a food chain, organisms in the deep sea ate only what debris fell from surface waters. Scarce food meant that organisms were few and far between. photograph by the Institute of Oceanographic Sciences, Deacon Laboratory, Wormley, England

Background geology n Geologists knew that cold water sunk into cracks in the ocean floor and hypothesized that this water was heated beneath the ocean crust. During heating, the hot water would dissolve minerals from surrounding rocks. The heated, mineral- laden water, hypothesized geologists, would rise from the seafloor at a vent.

Geologists test a hypothesis n In order to test their hypothesis, geologists decided send remote-controlled equipment to look for vents where oceanic plates pulled away from each other at the Galapagos Rift. n Since the temperature of the deep sea varies very little from 35.6 degrees Farenheit, geologists searched for small changes in temperature.

Geologists find a vent! n After temperature-sensitive equipment returned small temperature changes at one site along the Rift, cameras were sent to the same site and returned with pictures of heaps of clam shells. n Repeated submarine dives to the same site revealed temperatures as high as 46.4 degrees and a variety of unusual organisms.

The reaction of biologists n Biologists were astonished at the density of deep sea life in vent areas. What did all of these organisms eat? Were they related to the deep sea organisms biologists already knew about? Teeming vent shrimp

The unusual vent life forms were different from any others recorded by scientists to date. A new age in deep sea biology and ecology had begun! Spaghetti worms

What is a hydrothermal vent? n At a hydrothermal vent, sea water that has sunken into cracks in the ocean crust and been heated (sometimes to over 180 degrees Farenheit!) by the interior of the earth escapes through crust cracks back into the ocean. n Superheated water beneath the oceanic crust often dissolves minerals from nearby rocks. As hot vent fluids meet cold ocean water, minerals precipitate out of vent fluids.

n Precipitating minerals often give vent fluids different colored “smoky” appearances. n Precipitating minerals can fall out of vent fluids to form “chimneys”(like the one on the left) and other formations on the sea floor.

Without sunlight, how does a vent food web begin? n In areas of the earth that receive sunlight or are near areas that receive sunlight, photosynthesizing plants are the basis of the food chain. Using the energy of the sun, plants turn water and carbon dioxide into carbohydrates. Carbohydrates are energy for plants and for the organisms who eat plants. n Bacteria are the green plants of hydrothermal vents. Through a process known as “chemosynthesis”, bacteria use the energy in hydrogen sulfide dissolved in vent fluids to join water and carbon dioxide into carbohydrates. Vent communities food chains are based on these bacteria!

Who else is in the food web besides bacteria? n Most deep sea animals can not tolerate the chemicals or hot temperatures near vents. Vent animals are unique because they can withstand and even thrive upon conditions that kill most life. Some vent animals are related to more familiar organisms, but some, like the fluffy ball that vaguely resembles a dandelion, are not..

n Grazing vent animals include snails, crabs, and limpets. These animals graze on bacteria. Suspension feeders such as mussels, barnacles, and feather-duster worms remove food from the water. One kind of predator is the anemone. Anemones capture prey with their tentacles. White crabs and brittle stars are examples of scavengers that eat whatever live or dead animals and bacteria they can find Feather-duster worms Mussels

n Vents are often far apart. They often exist for just a few decades or years. New vents are populated very quickly. n Vent bacteria live in deep ocean water and in pores of deep ocean rocks all of the time in low numbers. When a vent pops up, bacteria populations flourish. n Hot water streaming out of vents often plumes for 200 meters above the sea floor because it is less dense than surrounding cold water. Plumes probably carry larva into nearby currents. However, this still may not account for the great distances between vents. Scientists continue to test other hypotheses that consider “stepping stones”, or intermediate sites, where vent animals may grow without a vent and release larvae. How are vents populated?

Tube worms and giant white clams n Tube worms and giant white clams are unique because they do not have a digestive tract. They have no way of “eating” food. Both organisms harbor symbiotic bacteria. The worms and clams transfer hydrogen sulfide and oxygen to a special area of their body filled with bacteria. Through chemosynthesis, the bacteria make carbohydrates to fuel themselves as well as their larger hosts.

Where are the vents and how many are there? n Nobody knows how many vents exist or where they all are. Hydrothermal vents are constantly being formed and destroyed, and many parts of the deep sea floor have yet to be seen by human eyes. Some of the known sites are marked on the map below. Most occur on or near boundaries between tectonic plates.