Exploring Vents: Vent Biology

Hydrothermal Vent Biogeography

Since the discovery of animal communities thriving around seafloor hydrothermal vents in 1977, scientists have found that distinct vent animal species reside in different regions along the volcanic 40,000-mile Mid-Ocean Ridge mountain chain that encircles the globe. Scientists are investigating clues to explain how populations are connected, how they diverged and evolved separately.

To date, more than 590 new animal species have been discovered living at vents, but fewer than 50 active vent sites have been investigated in any detail. Scientists currently recognize six major seafloor regions—called biogeographic provinces—with distinct assemblages of animal species.

In the eastern Pacific, tubeworms, clams and mussels dominate vent sites. In contrast, tubeworms are notably absent at vents in the Atlantic. Instead, billions of shrimp swarm at vents along the Mid-Atlantic Ridge, which bisects the Atlantic Ocean floor. There are two biogeographic provinces in the North Atlantic. Different species of shrimp and mussels predominate at vent sites that are at different depths. The deeper ones are south of about 30°N and shallower vent sites occur to the north. Both Pacific and Atlantic vents have mussels, but not the same species.

The fourth province is in the northeast Pacific, off the U.S. Northwest coast, which shares similar types of animals (clams, limpets, and tubeworms) with the eastern Pacific province, but markedly different species of each. In the western Pacific Ocean, at spreading ridges west of the Mariana Islands, vents in the fifth province are populated by barnacles, mussels, and snails that are not seen in either the eastern Pacific or the Atlantic.

VIDEO: Hydrothermal vents host life forms that exist nowhere else on earth, and form in places where there is volcanic activity, such as along the Mid-Ocean Ridge.

Scientists got their first chance to search for vents in the Central Indian Ocean in 2001 and found the sixth province. These vents are dominated by Atlantic-type shrimp, but also had snails and barnacles resembling those in the western Pacific.

Until 2005, all known Atlantic vent sites were north of the equator. Preliminary results from recent discoveries in the Atlantic south of the equator (5°-9°S) suggest these sites host similar but distinct species from known Indian Ocean and East Pacific Rise vents. Thus, the vents in the South Atlantic may represent a seventh biogeographic province.

The southernmost known chemosynthetic community in the Pacific is a vent site near 37°S on the Pacific-Antarctic Ridge. It includes Pacific-“like” fauna (bathymodiolid mussels, vesicomyid clams, and lepetodrillid snails).

Missing Pieces

The largely unexplored oceans in the Southern Hemisphere and the Arctic are critical regions where the missing pieces of the biogeographic puzzle may be found. Strategic exploration for hydrothermal vent and other chemosynthetic fauna in remote regions of the Mid-Ocean Ridge system will lead to discovery of new biogeographic provinces and fundamental insights into evolutionary relationships among the global deep-sea faunas.

All these regions contain the same basic ingredients to support life—chemical nutrients generated by geothermal processes at hydrothermal vent sites. So why do vent fauna differ in the Atlantic and Pacific, or in the eastern and western Pacific? How do we assemble these puzzle pieces to explain the diversity and evolution of vent species throughout the world’s oceans?

Evolutionary biologists are detectives, gathering clues to reconstruct the processes that generated the patterns we see today. Some suspected factors that influence vent biogeographic patterns we observe today include:

Which combinations of these variables limited or encouraged the dispersal of animal populations along the widely scattered, ephemeral patchwork of active vents on mid-ocean ridges? Which sent some populations down divergent evolutionary pathways, led others to extinction, and created fertile niches for yet others?

The answers to these important questions have been aided by breakthroughs in biotechnology that allow rapid gene sequencing. This tool gives scientists powerful new abilities to compare genomes of different species. We can see how closely related they are and examine how far back in time they may have diverged on the evolutionary tree. Determining evolutionary relationships among seafloor species and communities, and their distribution and biodiversity, will help unravel the evolution of life on Earth. Biogeographic studies of global hydrothermal vent animal communities will also guide our search for life on other planetary bodies, where chemosynthesis may also be important to sustaining biological processes.

Subsurface Biosphere

The presence of a microbial-dominated biosphere beneath the Mid-Ocean Ridge seafloor was first suggested as a result of the 1991-1992 seafloor eruption along the East Pacific Rise. When new vent openings were created during the volcanic eruption, vigorous diffuse flow emanated through cracks and crevices laden with microbes and their byproducts. The aftermath of a very recent volcanic eruption was a virtual blizzard of microbial snow. Flocculent white debris composed of sulfur and microbes was lifted more than 100 feet above the seafloor. Vent openings from which microbial material spewed forth were covered with a ~10-centimeter-thick layer of the same white material. The notion of a microbial biosphere quickly emerged. Somewhere beneath the seafloor, there was an expansive and unseen microbial bioreactor, in which microbes utilized inorganic chemicals rather than organic matter for their energy and carbon dioxide as their source of carbon.

Researchers moved to provide evidence of a substantial microbial biosphere—a previously unimagined and potentially large communities of microbial life sustained by volcanic heat and chemical nutrients circulating in oxygen-depleted rocky cracks and crannies below the ocean bottom. Over the past 16 years, research has been aimed at trying to characterize these microbes called hyperthermophiles.

Hyperthermophiles are considered to be resident below the seafloor, to experimentally ascertain the physical limits of life, the depth at which microbes are living, how long can they survive under these conditions, and the adaptations needed to take advantage of energy supplied by the planet, rather than by the sun. While the subsurface biosphere has yet to be sampled directly, emerging technology is being used to sample deeper into earth’s oceanic crust below the seafloor. The potential presence of a biosphere who’s microbial biomass may be greater than all biomass on land has altered our preconceptions and stretched our view of the places and circumstances that can harbor life.

[Further Reading: The subsurface biosphere at mid-ocean ridges (2004) Geophysical Monograph 144, Eds. S.D.Wilcook et al. pp. 399.]