Deep-sea hydrothermal vents are found along mid-ocean ridges and back-arc basins in all of the world’s oceans. Since 1977, when the first deep-sea vent was discovered near the Galapagos Islands, scientists have identified hundreds of vent fields and over 500 species of animals that are new to science. Many vents remain to be discovered, especially in Polar regions and remote parts of the Southern Hemisphere.
Most vents occur where volcanic activity heats fluids beneath the seafloor. These fluids rise through rock and sediment and emerge as underwater geysers and hot springs. Some vents form around volcanic “hot spots.” Others are strung out along mid-ocean ridges–chains of volcanoes that form where ocean crust is splitting apart. Along active, fast-spreading ridges, vents may occur every few kilometers. At less active ridges, vents may be spaced hundreds of kilometers apart.
The fluids spewing from hydrothermal vents are typically rich in sulfides of heavy metals such as iron, manganese, copper, and lead (some vent fluids contain carbonates or hydrocarbons rather than sulfides). When these superheated fluids come in contact with near-freezing seawater, the minerals crystalize, forming mounds, spires, and chimneys that rise tens of meters above the surrounding seafloor.
Active vents are colonized by a variety of chemosynthetic microbes that consume sulfides, hydrocarbons and even hydrogen. These microbes are the primary source of nutrition for animals that live around the vents. Some vent animals consume the microbes directly. Others incorporate microbes into or on their bodies as symbionts.
Common animals at vents include tubeworms, mussels, clams, barnacles, shrimps, limpets, and snails. All vent animals have special adaptations that help them survive and even thrive in the challenging physical and chemical conditions found near vents.
Many vent species are like terrestrial weeds—fast growing and good at dispersing to new sites. Like weeds, they reproduce prolifically, releasing large numbers of eggs or larvae that are carried far and wide by ocean currents. Being weedy helps them colonize habitats that might last only a few years to a few decades.
The animal species found at a particular vent depend on a wide variety of factors, including water depth, temperature, chemical conditions, and access to currents that transport larvae from one vent field to another. One of the biggest challenges for vent biologists is figuring out how local physical, chemical, or evolutionary conditions affect which animals are able to colonize a particular vent field.
From a biogeographic standpoint, deep-sea hydrothermal vents occur as small island-like habitats scattered widely across the deep seafloor. Individual vents often have low diversity, supporting a limited number of species. But they can also have high abundance, hosting large, dense populations of animals within these key species.
Most of the animals that thrive at vents are uniquely adapted to this type of environment, and cannot survive in other habitats. Nonetheless many vent species are widely distributed, occurring at vent fields that are hundreds or thousands of kilometers apart. The species ranges of vent animals are often bounded by geologic features or divergent ocean currents that prevent larvae from drifting from one vent field to another.
Because vent communities occur in active volcanic areas, they are often affected (both positively and negatively) by seafloor volcanic activity. A seafloor lava flow can “pave over” an entire vent community, wiping out all animal life. But the same eruption may also disperse larvae far and wide, and can create new underwater hot springs that may eventually be colonized by vent animals.
Even if they are not “paved over,” many vents remain active for only 10 or 20 years before their plumbing becomes clogged with mineral deposits. When the flow of heated fluids decreases or the chemistry changes, the vent animals can no longer obtain nutrition, and they gradually die off.
Human activities such as seafloor mining may have effects similar to a major lava flow, wiping out animal life at a vent. Seafloor mining can also create plumes of toxic sulfides that scavenge oxygen and affect animals some distance from the mining area.
When a vent community is destroyed, whether by geological or human activities, its ability to recover depends on whether there is a continuing flow of hydrothermal fluids and whether larvae of vent animals can drift from other hydrothermal vents and recolonize the site.
If a vent is isolated by undersea ridges or divergent currents, recolonization may be quite slow. On the other hand, if there are other active vents nearby and mineral-rich fluids continue to flow out of the seafloor, a site might be recolonized by highly mobile larvae within a few years. Vent species with less mobile larvae might take longer—up to a decade or more—to recolonize a vent.
Deep-sea hydrothermal vents may seem remote and disconnected from human activities, but we are almost certainly having effects on them. As evolutionary biologist Robert Vrijenhoek pointed out in a 2009 essay, “The same anthropogenic factors that affect surface islands worldwide (exploitation, habitat disruption, invasive species, and diseases) will also affect deep-sea hydrothermal vents.”
Vent communities will likely be affected by large-scale human-induced changes in the ocean. Increasing concentrations of carbon dioxide and global warming could lead to changes in ocean currents and seawater chemistry that can affect the delicate balance between oxygenated and anoxic water that hydrothermal vent animals require to survive.
Even research or “ecotourism” at vent sites can have detrimental effects—most human-occupied vehicles drop dive weights on the seafloor when they begin their ascent back to the surface. Vrijenhoek said, “There are heavily visited places on the Mid Atlantic Ridge and in the Guaymas Basin… where I saw more dive weights than animals.” Submersibles can also carry hitchhiking animals, microbes, and possibly diseases from one vent to another. At least one research paper described a species of limpet that was apparently carried from one vent field to another by a research vehicle.
In addition to expanding our knowledge about evolution, the limits and resilience of life on Earth, deep-sea vent communities may also provide tangible benefits for humankind. In the mid 1990s, one of the first commercial enzymes used to amplify DNA was derived from microbes living at hydrothermal vents in the Guaymas Basin of the Gulf of California.
Over the last two decades, marine biologists have worked with medical researchers to find out how vent animals can thrive in environments that are, as Vrijenhoek put it, “ripping with polymetallic sulfides that are known to be toxic, and polyaromatic hydrocarbons that are known to be carcinogenic.” Despite years of research into the abilities of vent animals to detoxify these materials, the mysteries persist. Researchers are still actively searching for natural products in vent animals that might be useful in killing cancer cells.
In 2009, the Guaymas Basin became one of a growing number of hydrothermal vent fields to be declared “marine protected areas.” Other protected vent fields are located on the Endeavor Ridge, off Canada, and the Azores. These fields all lie within the territorial waters of individual countries. Protecting vents in international waters has proven to be much more challenging.
At this stage, the biodiversity of deep-sea hydrothermal vents is relatively well understood. We know much less about the ability of vent communities to withstand human impacts… not to mention our ability to prevent or mitigate these impacts. Over the next few decades, we may find answers to these questions, for better or worse.