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Life in Total Darkness–Investigating Underwater Cave Ecosystems

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How does life exist in total darkness, in a habitat with little oxygen or food?

For more than 30 years, scientists have known that remarkably complex ecosystems thrive within underwater coastal caves, habitats that naturally contain no light and very little food or oxygen. Yet, almost nothing is known about the ecology of these systems.

Photo showing the cave passage and diver, with green tint from the water and strong shadows from the light source
Above: Cave passage and diver (Bil Philips, cave explorer) in Ox Bel Ha Cave System of the northeastern Yucatan Peninsula. Photo credit: HP Hartmann. [larger version]

John Pohlman of the USGS Woods Hole Coastal and Marine Science Center and David Brankovits, a post-doctoral scholar with the USGS and the Woods Hole Oceanographic Institution (WHOI), are providing answers to these questions. The two are using sophisticated cave diving techniques and sampling devices created at the USGS to learn more about what types of life exist in underwater caves along limestone and volcanic coastlines. With this information, they are developing an understanding of the processes that allow life to flourish within the submerged darkness.

Photo of the 4 member dive team in a verdant setting Photo of a dive team member setting up a computer in the jungle
Above: Ox Bel Ha Cave Project Field Team Members (left to right) David Brankovits (USGS/WHOI), Jake Emmert (Moody Gardens), John Pohlman (USGS), and Francisco Bautista De La Cruz (Speleotech). Photo credit: Jacob Pohlman. [larger version]

  Above: Deploying equipment in a cenote (entrance of the cave). Photo credit: John Pohlman. [larger version]

So far, their research has been conducted along the Yucatan Peninsula of Mexico, where over 1,000 km (about 621 miles) of cave passages within the limestone coastline have been mapped. By analyzing the chemical and isotopic content of water samples and animals collected from within the caves, they have shown that dissolved methane gas and other dissolved organic materials that trickle in from the jungle floor are an important component of the cave-adapted animal’s diet. These dissolved materials though, are not directly accessible to the animals living there—the microbes in particular—unless they mix with oxygen. John and David have shown that oxygen enters the system from beneath, with the seawater, and from above through sinkholes that connect to the caves. Once the dissolved materials have mixed with oxygen, bacteria are able to grow. In turn, higher-level organisms, like crustaceans, feed on the bacteria, which form the basis of the food web for the Yucatan caves.

Summary diagram showing the team's findings from the Ox Bel Ha Cave System
Above: A summary diagram of their findings from the Ox Bel Ha Cave System of the northeastern Yucatan Peninsula. Photo credit: USGS. [larger version]

Recently, John and David had the unique opportunity to conduct a similar study in the Atlantida Tunnel, the world’s longest submerged basaltic lava tube that extends beneath the coastline of the Island of Lanzarote in the Canary Islands. Using the same approach as in the Yucatan Peninsula, they collected water samples and animals that they are now analyzing for their stable isotopic composition to construct a model of the food web for lava-tube caves. However, because the Canary Islands are dry and lack dense tropical vegetation found in the Yucatan, they hypothesize the microbial loop in the Atlantida Tunnel is supported by material that originates in the ocean and washes into the lava tube. The study is being conducted in collaboration with Lanzarote and Chinijo Islands Unesco Global Geopark and Dr. Alejandro Martínez García (Water Research Institute IRSA-CNR).

Photo of a scientist collecting water samples in Molnar Janos Cave, Hungary, Budapes
Above: David Brankovits collecting water samples in Molnar Janos Cave, Hungary, Budapest (note: this was not the location of the study, but they used the same water-collecting tools.) Photo credit: Zsolt Sasdi. [larger version]

These are very delicate ecosystems; slight changes to the environment can have devastating consequences to the animals inhabiting the area. Thus, as John puts it, “the cave-adapted animals are the metaphorical canaries in the coal mine for the condition of water quality.” In other words, these sensitive life forms serve as early warning indicators for habitat degradation caused by pollution, land-use change, and the effects of climate change. Similar environments are found within the Florida platform and the Hawaiian Islands, where sea-level rise and human activity are impacting the condition of the groundwater. John and David hope to take what they have learned from the pristine settings in Yucatan and the Canary Islands and apply these principles to U.S. coastal margins to understand how to protect and preserve these unusual, but globally distributed, ecosystems.

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Above: David Brankovits collecting cave-adapted animals for the study in the Yucatan Peninsula. Photo credit: Balazs Lerner.[larger version]

  Above: David Brankovits and Tom IIiffe entering a cenote in the Yucatan Peninsula. Photo credit: Sergio Benitez.[larger version]

Interested in learning more? View the YouTube video “This Cryptic Underwater Maze Holds Life That Survives on Methane.” Also, their research from the Yucatan Peninsula was recently published in the journal Nature Communications.

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USGS Scientists Participate in Genomic Aerobiology Workshop
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June 2003
St. Pete Hosts Leaky Coastal Margins/Karst Interest Group Workshops
Mar. 2001

Related Websites
This Cryptic Underwater Maze Holds Life That Survives on Methane
Methane- and dissolved organic carbon-fueled microbial loop supports a tropical subterranean estuary ecosystem
Nature Communications
Mexico’s Yucatan Peninsula Reveals a Cryptic Methane-Fueled Ecosystem in Flooded Caves
The Transport of Nonindigenous Microorganisms Into Caves by Human Visitation: A Case Study at Carlsbad Caverns National Park
Geomicrobiology Journal

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in this issue:

Cover Story USGS Images Gas Hydrates with New Seismic Data on U.S. Mid-Atlantic Margin

News Brief
News Briefs

Experiment Shows “Turbidity Currents” Involve Seafloor Movement

Field Work
Life in Total Darkness— Underwater Cave Ecosystems

Recent Fieldwork

The Development and Demise of Florida’s Coral Reefs

Oct. - Nov. Publications

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