Our Mission: Mediterranean Deep Brines
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Microbial ecologist Ginny Edgcomb, Chief Scientist on this cruise, describes the challenges faced by microbes living in DHABs and by the researchers who study them—and how a new sampling device she helped design may overcome some of those obstacles.
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Geobiologist Joan Bernhard talks about a major strategy protists use to survive in DHABs, and how the remotely operated vehicle Jason will help bring back samples of DHAB sediments.
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Join Expedition 14 this fall as we venture into one of the world’s oldest oceans—the Mediterranean Sea—to look for life in some of the most hostile environments on Earth.
Thousands of years ago, the movement of Earth’s crust uncovered ancient buried deposits of salt and exposed them to seawater in several places. The salt began to dissolve into the seawater. Some of this extra-salty water got trapped in depressions on the seafloor and formed brine lakes. Because it’s extra-salty, the brine is denser than normal seawater, so it remains on the bottom and doesn’t mix with seawater.
Over time, these brine lakes have become even more salty. Now they are up to 10 times saltier than normal seawater, salty enough to kill most forms of life and to rapidly corrode the metal parts on oceanographic equipment.
The brine lakes also have no oxygen. Being more than two miles deep, they are totally dark and under pressure about 350 times higher than at the surface. Some are loaded with toxic sulfides. Scientists call them DHABs, which stands for Deep Hypersaline Anoxic Basins. Life on Earth may have begun in similar habitats.
On this 15-day cruise, scientists will use the WHOI ROV (remotely operated vehicle) Jason and a brand-new robotic micro-laboratory to look for life in two DHABs, called Discovery Basin and Urania Basin, in waters off the southern coast of Greece. The research team will bring back samples of water and sediments from the DHABs, from the transition zone between normal seawater and the saltier brine, and from normal areas outside the DHABs. They will be looking specifically for protists , which are mostly single-celled eukaryotic organisms such as the Amoeba and Paramecium you may have studied in school. Eukaryotes are organisms with nuclei and other membrane-bound organelles in their cells. Fungi, plants, and animals (including humans) are eukaryotes, too.
Wherever protists are found in these DHABs, the researchers can begin to ask other questions: How do they survive the harsh conditions? Do they use unusual biochemical processes? Have they developed useful symbiotic relationships with bacteria? What can these organisms tell us about how life might have begun on Earth, and where we might look for life on other planets?
The Briny Deep
Although humans have been navigating the waters of the Mediterranean for thousands of years, DHABs weren’t discovered there until the 1980s. At first, the DHABs were thought to be too hostile for eukaryotes. Then a few years ago, DNA from protists was found in several brine basins. But a big question remains: Were those protists living in the basins, or did they sink into the DHABs and get “pickled” by the heavy brine?
In 2009, the scientists on our cruise found more promising hints that protists are actually living in the DHABs. It’s been hard to find out what’s living in the brine lakes because whenever organisms are collected from such depths, simply bringing them to the surface and exposing them to light, oxygen, and changes in temperature and pressure can affect them so much that it can be impossible to tell what they were like in their natural habitat—or even whether they were alive.
On this cruise, the scientists will use in situ samplers that for the first time will preserve the organisms and do some of the initial processing that would normally be done in the lab, right where the samples are collected. When the samplers and their precious cargo finally return to the ship, the scientists will know that the cells and the molecules inside them have been preserved as they were at the moment they were collected.
Working aboard the R/V Atlantis, we will head south from Piraeus, Greece, to our study areas about 100 miles west of the island of Crete. There, the scientists will lower vehicles and sampling devices more than two miles down toward the brine basins at the bottom of the sea.
First, the scientists will map their targets by sending the ROV Jason down toward each basin. Jason will carry a CTD-O2, an instrument that will allow the researchers to measure salinity, temperature, and oxygen levels at each depth. That will show exactly where the edges of each brine lake are and the depth of the halocline . That’s the transition zone between the brine and the normal seawater. During a cruise here two years ago, the researchers found that the halocline on these basins is only 1 to 2 meters (6 feet) thick. So trying to get a sample of water from it will be like you trying to hit the door of your house from 2 miles away. Their aim has to be perfect!
To keep Jason out of the corrosive brine, they will position it just above the transition zone, right at the edge of a basin. With luck, the team will be able to see the shimmering halocline through the cameras on Jason or its small companion vehicle, Medea. An operator on the ship, working by remote control, will have Jason reach its long arm down through the halocline to gather sediment from the side of each brine basin and from within the halocline itself. Jason will use special collection devices that will immediately preserve the samples so any organisms they contain will still be intact when they reach the surface.
Next, the new robotic micro-lab, SID/ISMS, will be lowered on a cable deep into each brine lake. It will take in large amounts of water—up to 48 samples of 8 liters (2.1 gallons) each at one time—and will filter the water to concentrate any tiny protists living there. Then it will process the samples so they can later be analyzed in several different ways.
When the samples return to the ship, the scientists will use a light microscope to get a first glimpse at the organisms, which are among the most resilient forms of life on Earth—some of which may have never been seen before.