Galápagos Rift Expedition 2011

Expedition Purpose

Why Are Scientists Exploring the Galápagos Rift?

A key purpose of NOAA’s Ocean Exploration Initiative is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on earth, new knowledge about the forces that cause natural hazards such as volcanoes and earthquakes, cures for human diseases, answers on how to achieve sustainable use of resources, and information to protect endangered species.

On Feb. 17, 1977, scientists exploring the seafloor near the Galápagos Islands made one of the most significant discoveries in modern science: large numbers of animals that had never been seen before were clustered around underwater hot springs flowing from cracks in the lava seafloor. Similar hot springs, known as hydrothermal vents, have since been discovered in many other locations where underwater volcanic processes are active.

These processes are often associated with movement of the tectonic plates, which are portions of the Earth’s outer crust (the lithosphere) about 5 km thick, as well as the upper 60 - 75 km of the underlying mantle. These plates move on a hot flowing mantle layer called the asthenosphere, which is several hundred kilometers thick. Heat within the asthenosphere creates convection currents (similar to the currents that can be seen if food coloring is added to a heated container of water). Movement of convection currents causes tectonic plates to move several centimeters per year relative to each other.

Where tectonic plates slide horizontally past each other, the boundary between the plates is known as a transform plate boundary. As the plates rub against each other, huge stresses are set up that can cause portions of the rock to break, resulting in earthquakes. Places where these breaks occur are called faults. A well-known example of a transform plate boundary is the San Andreas Fault in California. View animations of different types of plate boundaries . A convergent plate boundary is formed when tectonic plates collide more or less head-on. When two continental plates collide, they may cause rock to be thrust upward at the point of collision, resulting in mountain-building (the Himalayas were formed by the collision of the Indo-Australian Plate with the Eurasian Plate). When an oceanic plate and a continental plate collide, the oceanic plate moves beneath the continental plate in a process known as subduction. Deep trenches are often formed where tectonic plates are being subducted, and earthquakes are common. As the sinking plate moves deeper into the mantle, fluids are released from the rock causing the overlying mantle to partially melt. The new magma (molten rock) rises and may erupt violently to form volcanoes, often forming arcs of islands along the convergent boundary. These island arcs are always landward of the neighboring trenches. View the 3-dimensional structure of a subduction zone.

Where tectonic plates are moving apart, they form a divergent plate boundary. At divergent plate boundaries, magma rises from deep within the Earth and erupts to form new crust on the lithosphere. Most divergent plate boundaries are underwater (Iceland is an exception), and form submarine mountain ranges called oceanic spreading ridges. While the process is volcanic, volcanoes and earthquakes along oceanic spreading ridges are not as violent as they are at convergent plate boundaries. View the 3-dimensional structure of a mid-ocean ridge.

Volcanic activity can also occur in the middle of a tectonic plate, at areas known as hotspots, which are thought to be natural pipelines to reservoirs of magma in the upper portion of the Earth’s mantle. The volcanic features at Yellowstone National Park are the result of hotspots, as are the Hawaiian Islands. As the Pacific tectonic plate moves over the Hawaiian hotspot, magma periodically erupts to form volcanoes that become islands. The oldest island is Kure at the northwestern end of the archipelago. The youngest is the Big Island of Hawaii at the southeastern end. Loihi, east of the Big Island, is the newest volcano in the chain and may eventually form another island.

The Galápagos region is geologically complex. The Galápagos Islands were formed by a hotspot called the Galápagos Mantle Plume (GMP), which continues to produce active volcanoes (the Sierra Negra volcano erupted on October 22, 2005). These islands are formed on the Nazca Plate, which is moving east-southeast. On the western side of the Nazca Plate, this motion produces a divergent plate boundary with the Pacific Plate. This boundary is called the East Pacific Rise. On the northern side of the Nazca Plate, just north of the Galápagos archipelago, another divergent plate boundary exists with the Cocos Plate. This boundary is known as the Galápagos Spreading Center (GSC). A convergent boundary exists on the eastern side of the Nazca Plate, which is being subducted beneath the South American and Caribbean Plates. This subduction has caused some of the oldest seamounts formed by the GMP to disappear beneath the South American and Caribbean Plates, so it is not certain exactly how long the GMP has been active in its present position.

When the movement of tectonic plates causes deep cracks to form in the ocean floor, seawater can flow into these cracks. As the seawater moves deeper into the crust, it is heated by molten rock. As the temperature increases, sulfur and metals such as copper, zinc, and iron dissolve from the surrounding rock into the hot fluid. Eventually, the mineral-rich fluid rises again and erupts from openings in the seafloor. The temperature of the erupting fluid may be as high as 400°C, and contains hydrogen sulfide, which is toxic to many species. When the hot hydrothermal fluid meets cold (nearly freezing) seawater, minerals in the fluid precipitate. The precipitated mineral particles give the fluid a smoke-like appearance, so these vents are often called black smokers or white smokers, depending upon the types of minerals in the fluid. Precipitated minerals may also form chimneys that can be several meters high.

Hydrothermal vent communities and other deepwater chemosynthetic ecosystems are fundamentally different from other biological systems on Earth, and there are plenty of unanswered questions about the individual species and interactions between species found in these communities. Many of these species are new to science, and include primitive living organisms (Archaea) that some scientists believe may have been the first life forms on Earth. Although much remains to be learned, new drugs and other useful products have already been discovered in hydrothermal vent organisms. At present, almost all drugs produced from natural sources come from terrestrial plants, but marine animals produce more drug-like substances than any group of organisms that live on land. Some chemicals from microorganisms found around hydrothermal vents (the exopolysaccharide HE 800 from the bacterium Vibrio diabolicus) are promising for the treatment of bone injuries and diseases, while similar chemicals may be useful for treating cardiovascular disease. Other examples of useful products include Thermus thermophylus, a microorganism that is adapted to live under extremely high temperature conditions near hydrothermal vents. One of these adaptations is a protein (Tth DNA polymerase) that can be used to make billions of copies of DNA for scientific studies and crime scene investigations. Another microorganism found in vent communities (genus Thermococcus) produces a type of protein (an enzyme called pullulanase) that can be used to make sweeteners for food additives.

One of the key questions about hydrothermal systems is how their biological and geological processes are affected by different sources of magma. The geological complexity of the Galápagos makes this region an ideal natural laboratory to study this question because hydrothermal systems may receive magma from the GMP hotspot as well as from rifts associated with the spreading center.

In 2002 and 2005, NOAA’s Office of Ocean Exploration and Research sponsored expeditions to the Galápagos Rift. A major objective of the 2002 expedition was to revisit a hydrothermal vent site named Rose Garden to investigate changes that might have occurred in the community of living organisms around the vent since it was discovered in 1977. The Rose Garden site is on the GSC near longitude 86°W. Scientists found that significant changes had, in fact, taken place: Rose Garden had completely disappeared! In its place was a fresh sheet of lava that apparently buried the vent and all of the surrounding organisms. About 300 meters away, a new vent field (which the scientists named Rosebud) was discovered with typical hydrothermal vent species beginning to colonize cracks in recently formed lava. These discoveries underscored a growing awareness that the deep ocean environment can change much more quickly than was previously believed.

The 2005 expedition focused on a portion of the GSC between longitude 89.5°W and 94.5°W that had never been explored for hydrothermal vents. This area includes a segment of the spreading center that is located above the GMP, and receives a greater supply of magma that other parts of the GSC. The purpose of the expedition was to discover the geological and biological effects of an increased magma supply, and scientists hoped that they would find black smokers because at that time high temperature (several hundred degrees C) vents had not been found in the Galápagos region; only vents with temperatures of less than 50°C. Using chemical and physical clues, explorers first found evidence of high temperature hydrothermal activity and eventually made the first discovery of black smokers on the Galápagos Rift! Three groups of black smokers were discovered: the Navidad, Iguanas, and Penguinas Vent Fields, which included tall chimneys (the highest was 10 m tall), dense communities of giant tubeworms and dinner-plate-size clams and mussels.

These discoveries set the stage for the Galápagos Rift Expedition 2011, which will use the state-of-the-art exploration capabilities of NOAA Ship Okeanos Explorer to obtain detailed information about the biology and geology of Galápagos hydrothermal ecosystems, and determine whether different ecosystems are found at different vent fields within the Galápagos region.

Expedition Questions

The Galápagos Rift Expedition 2011 has five major objectives:

  • To capture high-resolution images of vent fields and associated living organisms in the Galápagos region using the Little Hercules remotely operated vehicle (ROV), and to biologically characterize and map selected sites and habitats;
  • To conduct exploratory surveys and mapping of known hydrothermal vent sites, unexplored regions of the Galápagos Rift, and seamounts to the north of the Rift that may be “stepping stones” that allow organisms to disperse away from the Rift;
  • To explore whether the transform fault at longitude 91°W is a barrier that divides the eastern and western portions of the GSC into separate biogeographic regions;
  • To compare species composition between vent sites on the Galápagos Rift and East Pacific Rise to assess whether these sites are separated by an evolutionary barrier; and
  • To conduct detailed bathymetric surveys of the Paramount Seamount north of the Galápagos Rift to explore for endemic communities and species, hydrothermal vent activity, and deepwater coral ecosystems.

Exploration Technology

To achieve these objectives, the full suite of Okeanos Explorer exploration capabilities will be utilized: telepresence technology, multibeam mapping, a Conductivity, Temperature and Depth Sensor (CTD), and a Remotely Operated Vehicle (ROV). The Galápagos Rift Expedition 2011 is divided into two legs. Leg I will include precise multibeam mapping and CTD casts in the vicinity of Paramount Seamount and along the GSC from longitude 101°W to 86°W. Following a port call in Puntarenas, Costa Rica, Leg II will focus on ROV exploration of sites identified during Leg I and from previous expeditions, as well as additional CTD sampling and multibeam mapping. For more information about the overall Okeanos Explorer exploration strategy, see the lesson, “To Explore Strange New Worlds” (PDF, 1.7 MB), and the Galápagos Rift Expedition 2011.


[Note: The following discussion is adapted, in part, from the July 9, 2010 mission log by Webb Pinner and Kelley Elliott.]

Telepresence is a group of technologies that allow people to observe and interact with events at a remote location. Aboard the Okeanos Explorer, the foundation for telepresence is advanced broadband satellite communication. Telepresence allows live images to be transmitted from the seafloor to scientists ashore, classrooms, and newsrooms, and opens new educational opportunities that are a major part of Okeanos Explorer’s mission for the advancement of knowledge. In addition, telepresence makes it possible for scientists in shore-based Exploration Command Centers (ECCs) to be directly involved with the operation of shipboard equipment. In this way, scientific expertise can be brought to the exploration team as soon as discoveries are made, and at a fraction of the cost of traditional oceanographic expeditions.

The Okeanos Explorer’s powerful satellite dome enables explorers aboard the ship to have high-bandwidth communication with remote parts of the world. “High-bandwidth” means that a large amount of data can be transmitted from the ship in a short span of time, including three high-definition video feeds as well as real-time voice communication Internet connections. High-definition video transmissions use broadcast industry equipment to deliver high quality video with very little time delay. Even with intensive signal processing and the delays introduced by satellite and land-based links, video travels from the ROV at depths as deep as 4000 m to the ECCs thousands of miles away in an average of 6 seconds.

Voice communications use an Internet-based intercom system which allows all participants, regardless of location, to easily communicate with all other participants. This real-time voice communication is supplemented by a real-time text-based tool called “the Eventlog,” which allows each participant to write their personal observations to a common log. Log entries made by individuals can immediately be seen by all other users in real-time. All users on the ship and at the ECCs are encouraged to participate in the Eventlog, since each individual is potentially able to provide unique observations and insight. The Eventlog software automatically records the date, time of entry, and author of each text observation.

The ability to watch the live events aboard the Okeanos Explorer is not limited to those with access to an ECC. Video streams from the ship are distributed over an advanced computer network called Internet2 that allows users in academic institutions around the globe to simultaneously view the live video. At the beginning of the 2010 INDEX-SATAL Expedition, only computers connected to this advanced network were able to view video streams, but as the excitement built up around the Okeanos Explorer and the expedition, participants began using increasingly creative solutions for developing ad-hoc viewing stations and in some cases mini-ECCs utilizing the standard Internet. These solutions extended telepresence capabilities to smaller academic institutions, public venues, hotel rooms, the cafeteria at the U.S. Embassy in Jakarta, and even at one scientist’s private residence.

Multibeam Sonar

Sonar (which is short for SOund NAvigation and Ranging) systems are used to determine water depth, as well as to locate and identify underwater objects. In use, an acoustic signal or pulse of sound is transmitted into the water by a sort of underwater speaker known as a transducer. The transducer may be mounted on the hull of a ship or may be towed in a container called a towfish. If the seafloor or other object is in the path of the sound pulse, the sound bounces off the object and returns an “echo” to the sonar transducer. The time elapsed between the emission of the sound pulse and the reception of the echo is used to calculate the distance of the object. Some sonar systems also measure the strength of the echo, and this information can be used to make inferences about some of the reflecting object’s characteristics. Hard objects, for example, produce stronger echoes than softer objects. This is a general description of “active sonar.” “Passive sonar” systems do not transmit sound pulses. Instead, they “listen” to sounds emitted from marine animals, ships, and other sources.

Multibeam sonar is one of the most powerful tools available for modern deep-sea exploration. A multibeam system uses multiple transducers pointing at different angles on either side of a ship to create a swath of signals. The time interval between signal transmission and return echo arrival is used to estimate depth over the area of the swath. In some systems, the intensity of the return echo is also used to infer bottom characteristics that can be used for habitat mapping. In addition to high-resolution maps, multibeam data can be used to create three dimensional models or even “fly-through” videos that simulate a trip across the area being mapped. View a fascinating example. This 3D fly-through movie shows the seafloor in the Magic Mountain area of Explorer Ridge (near the coast of Vancouver Island) where there are active hydrothermal vents. To see what the vents look like, you can view other fly-throughs of the same area. Recently, a new generation of multibeam sonar systems has been developed that are able to map features in the water column as well as the seafloor. This ability will potentially allow multibeam sonar to map the location of fishes and marine mammals, as well as a wide range of physical oceanographic processes.

Okeanos Explorer carries a Kongsberg Maritime EM302 deepwater multibeam sonar system. Transducers for the system are installed on the ship’s hull in a custom-designed housing. The system can transmit up to 288 beams, can collect as many as 432 depth measurements in a single swath, and automatically compensates for movements of the ship. The EM302 operates in depths ranging between 10 m and 7,000 m. The width of the swath is about 5.5 times the depth, to a maximum of about 8 km. Depth resolution of the system is 1 cm. At a depth of 4,000 m the system can resolve features with a dimension of approximately 50 m. More information about sonar systems is available here.

The Galápagos Rift Expedition 2011will also use a Kongsberg Simrad EK60 scientific sounder, designed for fishery research that is capable of detecting a sonar echo from a single fish at depths of 1,000 m. The EK60 is also capable of detecting small particles, such as gas bubbles that may be part of plumes (discussed under “CTD,” below) associated with the presence of hydrothermal vents. The EK60 is a split beam sonar, which means the transducer is typically divided into four quadrants. Echoes from each quadrant are compared with echoes from the other quadrants to characterize reflecting objects.


CTD stands for conductivity, temperature, and depth, and refers to a package of electronic instruments that measure these properties. Conductivity is a measure of how well a solution conducts electricity and is directly related to salinity, which is the concentration of salt and other inorganic compounds in seawater. Salinity is one of the most basic measurements used by ocean scientists. When combined with temperature data, salinity measurements can be used to determine seawater density which is a primary driving force for major ocean currents. Often, CTDs are attached to a much larger metal frame called a rosette, which may hold several water sampling bottles used to collect water at different depths, as well as other instruments that can measure additional physical or chemical properties.

In addition to sensors for measuring conductivity, temperature, and water pressure (which is used to calculate depth), for the Galápagos Rift Expedition 2011Expedition Okeanos Explorer’s CTD will also carry an altimeter (to measure distance from the seafloor), a light scattering sensor (LSS), an oxidation-reduction potential (ORP) sensor, and a dissolved oxygen sensor. A LSS measures the concentration of hydrothermal particles in the water. Oxidation-reduction (also called redox) potential is a measure of the tendency of a substance to gain or lose electrons. ORP potential is measured in volts, and increases directly with the tendency of a substance to gain electrons and become reduced. Because chemosynthetic communities are based on chemical substances that can donate electrons, these chemical substances have a tendency to lose electrons. So a drop in ORP may signal the presence of chemosynthetic communities nearby.

Ocean explorers often use CTD measurements to detect evidence of volcanoes, hydrothermal vents, and other deep-sea features that cause changes to the physical and chemical properties of seawater. Masses of changed seawater are called plumes, and are usually found within a few hundred meters of the ocean floor. Since underwater volcanoes and hydrothermal vents may be several thousand meters deep, ocean explorers usually raise and lower a CTD rosette through several hundred meters near the ocean bottom as the ship slowly cruises over the area being surveyed. This repeated up-and-down motion of the towed CTD may resemble the movement of a yo-yo; a resemblance that has led to the nickname “tow-yo” for this type of CTD sampling.

Okeanos Explorer carries a Seabird Electronics Model 9/11+ CTD system mounted on a SBE 32 rosette frame. This rosette includes 24 sampling bottles that can be individually triggered to collect samples at various depths. The SBE 9+ CTD unit has a depth rating of 6,800 meters. The ship also carries an Expendable Bathythermograph (XBT) that is used to measure the velocity of sound in the ocean at various depths. This information is needed by the multibeam sonar system to collect accurate bathymetric data. Visit Sonde and CTD and CTD and Tow Methods  for more information.

Remotely Operated Vehicles (ROVs)

Remotely operated vehicles are unoccupied robots linked to an operator by a group of cables. Underwater ROVs are usually controlled by an operator aboard a surface ship. These robots typically include five basic systems:

  • Power system
  • Propulsion system
  • Communication system
  • Buoyancy control system
  • Information gathering system(s)
In addition, some ROVs are equipped with other tools such as a manipulator or cutting arm, water samplers, and measuring instruments to expand the vehicle’s capabilities.

For the Galápagos Rift Expedition 2011, NOAA Ship Okeanos Explorer will carry Little Hercules, an ROV originally developed by a team of engineers at Dr. Robert Ballard’s Institute for Exploration (IFE) at the University of Rhode Island (URI), for the primary purpose of gathering high quality video imagery.

Little Herc is operated in tandem with a camera platform that carries 2,400 watts of lighting provided by HMI (hydrargyrum medium-arc iodide) arc lamps. This lighting illuminates the total darkness of the deep ocean, helps guide Little Hercules, and provides lighting for the high-definition video images of the ROV at work. The camera platform is named Seirios, after the name of the brightest star in the night sky (also called the Dog Star, sometimes spelled “Sirius”). Little Herc is attached to Seirios by a 30meter tether, while the camera platform is attached to the Okeanos Explorer’s traction winch by a 17 mm electromechanical cable. Little Herc’s systems include:

  • Power – Primary power is from the Okeanos Explorer, supplied to the ROV by wires in the tether cable that connects Little Herc to the ship. A transformer inside a watertight container, called a power bottle, converts the ship’s current to voltages needed by the thrusters and other equipment aboard the ROV.
  • Propulsion – Little Herc has four motors called thrusters. Two of these are oriented horizontally to provide forward, backward, and rotational motion. The other two thrusters are mounted so that they form a V when viewed from the front. This is called a vertran configuration, and allows the ROV to move up, down, and laterally. The thrusters are controlled by electronic motor controllers that regulate the speed and direction of the motors’ rotation.
  • Communication – Video and sensor data from Little Herc are sent to the Okeanos Explorer’s Control Room on fiber optic cables included in the tether that connects the ROV to the ship. Inputs from the sensors are routed through a multiplexer that makes it possible to send many separate signals through a small number of cables.
  • Buoyancy control – The yellow floatation package on Little Herc is made of syntactic foam made from tiny hollow glass balls that will not collapse under the pressure of the deep-ocean environment.
  • Information gathering – Little Herc carries a variety of sensors used for navigation as well as to collect scientific data. These include depth and altitude sensors, an Ultra Short Baseline Tracking System, full color imaging sonar, and a Seabird SBE 49 FastCAT CTD. Video equipment includes two Insite Pacific single CCD high-resolution miniature color video cameras, one Insite Pacific triple CCD high-definition Zeus Plus video camera, two Deep Sea Power and Light 250 watt LED matrix lights, and two Deep Sea Power and Light 400 watt HMI arc lamps.
Overall specifications for Little Hercules are:
  • Length: Five feet
  • Width: Three feet
  • Height: Three feet
  • Weight: 1,000 lbs
  • Depth Rating: 4,000 m
View a compilation of video clips collected by Little Hercules during the INDEX-SATAL 2010 expedition. Also see more information about other ROVs.

More About Deep-Sea Chemosynthetic Communities

When scientists discovered thriving biological communities in the deep ocean, they were completely surprised, because it had been assumed that food energy resources would be scarce in an environment without sunlight to support photosynthesis. Researchers soon discovered that the organisms responsible for this biological abundance do not need photosynthesis, but instead are able to obtain energy from chemical reactions through processes known as chemosynthesis. Photosynthesis and chemosynthesis both require a source of energy that is transferred through a series of chemical reactions into organic molecules that living organisms may use as food. In photosynthesis, light provides this energy. In chemosynthesis, the energy comes from other chemical reactions (for more details, see More About Chemosynthesis and Reducing Habitats, below).

Following the discovery of the first hydrothermal vents in the Galapagos, ocean explorers have discovered other types of chemosynthetic communities. Cold seeps are areas where hydrocarbon gases (such as methane) or oil seep out of sediments and are commonly found along continental margins. Like hydrothermal vents, cold seeps are home to many species of organisms that have not been found anywhere else on Earth. Other chemosynthetic communities are found in parts of the ocean where oxygen has been depleted by large amounts of decaying organic matter. Under these conditions, hydrogen sulfide builds up and can support communities of organisms similar to those found near hydrothermal vents and cold seeps. These communities are found around dead whales (called whale falls) and large masses of dead kelp or trees.

Although there are similarities between chemosynthetic communities associated with hydrothermal vents, cold seeps, and large organic falls, each of these habitats has hundreds of endemic species (endemic means that these species aren’t found anywhere else). Even along the global ridge crest, which is almost continuous along its entire length, at least six different groups of endemic fauna have been recognized (Van Dover et al., 2002). The reasons for the diversity are not clear. One hypothesis is that these systems are usually separated by large areas in which conditions are not suitable for chemosynthetic communities, and this isolation contributes to differences between communities. Another possibility is that different chemosynthetic processes are involved depending upon which chemicals are present, and these differences involve different species.

In chemosynthetic communities where hydrogen sulfide is present, large tubeworms (phylum Annelida) known as vestimentiferans are often found, sometimes growing in clusters of millions of individuals. Vestimentifera have been regarded previously as a distinct phylum or a group within the phylum Pogonophora. Recent molecular evidence suggests that Vestimentifera and Pogonophora should be considered part of the polychaete family Siboglinidae within the phylum Annelida. These unusual animals do not have a mouth, stomach, or gut. Instead, they have a large organ called a trophosome that contains chemosynthetic bacteria. Vestimentiferans have tentacles that extend into the water. The tentacles are bright red due to the presence of hemoglobin which can absorb hydrogen sulfide and oxygen, and provides a way to transport these chemicals to bacteria in the trophosome. The bacteria produce organic molecules that provide nutrition to the tubeworm. A similar symbiotic relationship is found in clams and mussels that have chemosynthetic bacteria living in their gills. Bacteria are also found living independently from other organisms in large bacterial mats. A variety of other organisms are also found in chemosynthetic communities, and probably use tubeworms, mussels, and bacterial mats as sources of food. These include snails, eels, sea stars, crabs, lobsters, isopods, sea cucumbers, and fishes. Specific relationships among these organisms have not been well-studied.

More About Chemosynthesis and Reducing Habitats

Photosynthesis and chemosynthesis are similar in that they are both processes that provide energy needed to form molecules that can serve as energy sources (food) for living organisms. This energy is captured and stored in other molecules, and is moved from one molecule to another molecule when electrons are transferred between the molecules. These transfers take place in a series of reactions called an electron transport chain.

Photosynthesis begins when a photon of radiant energy from the sun strikes a molecule of chlorophyll or other photosynthetic pigment. This energy causes the photosynthetic pigment molecule to release an electron that is captured by another organic molecule. This molecule releases an electron to a third organic molecule, and the process continues through several other molecules. Each time an electron is transferred, the molecule that gains the electron also gains energy; and the molecule that loses the electron loses energy. The last molecule in the chain keeps the electron, and has a higher energy content than before it received the electron. Some energy is lost each time an electron moves from one molecule to another, so the last molecule in the chain contains less energy than the energy of the photon that started the process. The photosynthetic pigment regains its lost electron from a water molecule through a series of reactions called photolysis that release oxygen gas as a by-product. The energy produced by the series of electron transfers is used by photosynthetic organisms to convert carbon dioxide into organic molecules (such as sugars) that can serve as energy sources (food) needed by living organisms.

In chemosynthesis, the electron transport chain begins with the loss of an electron from a substance such as hydrogen sulfide or methane. As in photosynthesis, the electron is passed along an electron transport chain, and the energy produced by a series of electron transfers is used by chemosynthetic organisms to convert carbon dioxide into molecules that can serve as energy sources (food) for living organisms. Known chemosynthetic organisms from deep-sea environments are bacteria or Archaea, and these microbes often are the basis of complex food webs.

When an atom or molecule loses an electron it is said to be oxidized, and when an atom or molecule gains an electron it is said to be reduced. A reducing substance is a substance that reduces; in other words, it donates electrons. Similarly, an oxidizing substance is a substance that oxidizes; that is, it receives electrons. A reaction in which one or more electrons are transferred between two molecules is called a redox reaction. Note that the terms oxidation, reduction, and redox may also be used in slightly different ways for some types of chemistry, but these distinctions are not important for this discussion.

Here is an example of a simple redox reaction:

H2 + F2 > 2 HF

A molecule of hydrogen gas reacts with a molecule of fluorine gas to form hydrogen fluoride.

The reaction may also be written as an oxidation reaction and a reduction reaction:

H2 > 2 H+ + 2 e- (the oxidation reaction)


F2 + 2 e- > 2 F- (the reduction reaction)

where e- stands for an electron

Chemosynthesis depends upon the availability of reducing substances such as hydrogen sulfide or methane that can donate electrons. Habitats in which these substances occur are called reducing habitats.