Chemosynthetic Oases in the Deeps

Since the late 1970s, scientists have begun to explore the deeps with cameras and scoops on remotely operated submersible vehicles (ROVs). Their discoveries have completely changed our vision of the abyss. Previously it was generally assumed that the deep seas were virtually barren of life and barren of any interest.

To general surprise, scientists have discovered many communities of unknown animals in the deeps, a thinly spread macrofauna benthos, a numerous meiofauna (tiny but visible animals), and a large protist community of forams and radiolarians.

But it is the discovery of deep sea hotspots crowded with life that has captured the public imagination:

		hydrothermal (hot) vents or black smokers with their tubeworms, sulfides chemosynthesis
		Lost City hydrothermal warm vents--methane and hydrogen chemosynthesis
		Seamounts: oases of life on submerged mountains
		cold water coral reefs living in eternal darkness
		Cold Seeps:Chemosynthetic Biologic Communities
		Whale Falls: the century-long communities of whale death

Most importantly , we discovered that photosynthesis is not the only method life has to create chemical energy, otherwise known as food.

The other methods were invented by ancient bacteria, which have invented every great life strategy.

Rather than using sunlight in photosynthesis, these bacteria living in total darkness use sulfur compounds (hydrogen sulfide) dissolved in the hot water from hot vents. They are chemoautotrophs, self-feeders using chemicals instead of light.

Some of these wily microbes use methane instead of sulfides. In this purely chemical process, they create sugars and proteins as end products just as in photosynthesis.

As these early deep ocean explorations continue, we find again and again that Earth is active in ways we had not imagined.

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Hydrothermal Vents

Seafloor is thinner than the continental plates, but more dense and brittle; seafloor crust ruptures at junctions of tectonic plates around the globe. Sometimes these ruptures are dramatic, as at the Mid-Atlantic Rift, plates are slowly moving apart and red-hot magma oozes up to make new seafloor. Hot vents often appear in rocks of ocean ridges.

Here are some 'faces' of the hydrothermal vent community:

hydrothermal vent, or black smoker, near the mid-Atlantic Ridge. Chimneys are made of minerals precipitated from the hot water jetting out.
A multitude of tubeworms and mussels are swarmed by shrimps
Giant chemosynthetic vent clams cluster the fissure in basalt that leaks sulfides.
close view of ventimiferan tubeworms
fish cruises among hot vent chimneys
Riftia tubeworms, some with attached mussels. Both animals live through symbiosis with sulfide-digesting bacteria inside their body tissues.
A host of tubeworms with gills extended are supported by the sulfides pouring from chimneys.
White crabs line the edges of a rift near a hot vent. Geology here is unstable.	Crabs and shrimps graze rocks and the shells of chemosynthetic mussels.
A chemosynthetic tubeworm with very different stalks	octocorals colonize the top of a dead vent chimney
a skate ventures to the busy depths of a hot vent community.	An improbable pyramid of vent mussels attached to one another.

More than 500 new animal species have been found at hot vents since they were first discovered in 1977 off the Galapogos Islands. Picture the cold ocean bed, water slightly above freezing, and a hot spring suddenly spews hot water at 500 degrees C. out of the seafloor. The water is heated by magma just below, often where new seafloor is being formed. These vents are loaded with dissolved minerals, which over time deposit "chimneys" extending up many feet. Some of these minerals are sulfides, which attract bacteria that know the trick of converting sulfur compounds into food. In a few months, very fast for the deeps, a whole community of life comes into being, a kind of hotspot of life. Some recently discovred vent animals were previously unknown, such as the giant tubeworms that receive nutrients directly from bacteria inside them; the worms have no digestive system. Large mussels and clams have also adapted to symbiosis with these chemautotroph bacteria that make food without photosynthesis. A host of white crabs, pink shrimps, white sea stars, sea cucumbers, and occasional bottom-feeding fish round out the vent macrofauna.

The search for new vent fields has become intense. A recent Chinese expedition has found a vent field in the Indian Ocean; European and American scientists have recently found vent fields in the Arctic, in an Iceland fjord, and off the Pacific coast of Costa Rica.

Map of some hydrothermal vent locations image courtesy of Nature

Hot vents have limited lifespans. The first vents ever discovered (in the Galapagos) are now dead and cold--a vast array of gaping clamshells and slumped tubeworms. In the Galapagos, the tectonic plate is moving faster than the magma beneath. How fragile life is and how suddenly life can end.

Lost City, A New Kind of Hydrothermal Vent

A new and different kind of hydrothermal vent has been recently described. It is not a “black smoker” whose heat (up to 700 F.) is from magma seeping through the seafloor crust. Instead, this new kind of vent is heated entirely by a chemical reaction between seawater and peridotite rock. The water streaming from the new vents is only hot enough to shimmer (up to 170 F).

The new vent field perches on top of a twelve mile (20 K). long seamount, the Atlantis Massif, near the Atlantic Ridge. Lost City is 2,600 feet (800 m.) below the surface.

Carbonate minerals paint nearby outcrops brilliant white, and form vents ranging in shape from tiny toadstools to a 200 foot column named Poseidon.

A thirty foot carbonate tower at Lost City photo credit University of Washington Click to enlarge	Freshly-deposited carbonate on this ancient tower is bright white. photo credit University of Washington. Click to enlarge
Peak of 'beehive' on a carbonate tower. Photo credit U. of Washington. click to enlarge	One pinnacle of 60 meter (200 foot) Poseidon tower. New carbonate deposits here are fragile and porous. Photo credit U. of Washington. Click to enlarge
Microbe Biofilms from chimney interior. In filament strand on right, each dot (microbe) in strand is one micron. Microphoto credit University of Washington. Click to enlarge	Huge flange on tower side deposits new carbonate as water flows. Photo credit U. of Washington. Click to enlarge
deepwater cup coral Desmophyllium at Lost City. image credit Tim Shank, Woods Hole Click to enlarge	Living biofilm on carbonate photo credit Matt Shrenk, U. of Washington
"antler" formation photo credit NOAA Click to enlarge	two carbonate towers in rov lights photo credit NOAA Click to enlarge
spectacular tower, rov lights photo credit NOAA Click to enlarge	submersible Hercules lights IMAX tower photo credit U. of Washington click to enlarge
A wreckfish patrols the Lost City carbonate spire. Photo credit U. of Washington. click to enlarge

Lost City was discovered by luck in 2000, during an Alvin dive to the Atlantic Massif, and is still being explored. Only some local sediments  have been sampled. Many of those sediments were deposited during the last glacial maximum 20,000 years ago, according to Swiss scientists.

Scientists are impressed with how stable and long-lived the Lost City vent field is, and wonder if this sort of vent-field running for thousands of years might have improved the chances for life to spark and to be sustained until it could take hold. Could this kind of vent field be the cradle of life?

"It's difficult to know if life might have started as a result of one or both kinds of venting," says Deborah Kelley, University of Washington oceanographer, "but chances are good that these systems were involved in sustaining life on and within the seafloor very early in Earth's history."

Biomass at Lost City is primarily microbes, archaea and bacteria. Their numbers are immense. An estimated one billion cells inhabit each gram of the porous rock of Lost City. Some of these feed on hydrogen and produce methane. Others feed on methane. Both kinds produce carbon compounds as by-products; scientists speculate that some of these complex molecules may have given rise to life.

Differences between the warm Lost City vents and  very hot “smoker” vents are striking.

Lost City warm vents	Black Smoker Hot vents
Lost City carbon-dated at 30,000 years. Peridotie rock reacts chemically with alkaline seawater to become serpentine rock--no magma connection.	Smokers are short-lived, depend on volcanic activity. Hot spots in the magma shift locations as the seafloor plate moves. The first smoker system discovered in the 1970s is now lifeless.
microbes live in very alkaline water	microbes live in very acid water
chemosynthetic microbes feed on methane, hydrogen compounds	chemosynthetic microbes feed on sulphur, hydrogen compounds
diverse meiofauna, tiny and transparent, hard to see	diverse meiofauna, overshadowed by large animals, hard to see
small but diverse macrofauna; deep corals, fish, crustaceans	huge diverse macrofauna; tubeworms, mussels, shrimps and crabs.
microbes live on and inside chimneys in biofilms	microbes live in tubeworms, mussels, clams and in chimneys
Lost City chimneys are white to gray carbonates and tall, up to 200 feet (60 m.), and thousands of years old..	Smoker Chimneys are dark and made of metal-rich mineral compounds, much shorter ,30 feet, comparatively brief lifespans.

Lost City was discovered by chances in 2000, during an Alvin dive, and is still being explored. Only some local sediments  have been sampled. Many of those were deposited during the last glacial maximum 20,000 years ago, according to Swiss scientists.

Scientists are impressed with how stable and long-lived the Lost City vent field is, and wonder if this sort of vent-field running for thousands of years might improve the chances for life to spark and to be sustained until it could take hold.

"It's difficult to know if life might have started as a result of one or both kinds of venting," says Deborah Kelley, University of Washington oceanographer, "but chances are good that these systems were involved in sustaining life on and within the seafloor very early in Earth's history."

Researchers are intrigued by Lost City partly because it may give us insight into the conditions that might foster life on other planets.

Note: Several of the above photos were first published in Oceanography, Vol 18, No.3, Sept. 2005 to illustrate Mantle to Microbes, by Deborah S. Kelley.
Download .pdf file1.67 MB

 

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Cold Seeps

Cold seeps are places on the deep ocean floor where hydrogen sulfide and methane seep or flow from bottom sediments. They support chemosynthetic communities based on bacterial food production much like the hot vent communities. But cold seeps are often small in area, and the numbers of organisms comparatively small. They are called "cold" in contrast to hydrothermal vents, which spew super-heated water.

Cold seeps were first discovered and named by the Monterey Bay Aquarium Research Institute. But those same researchers now find that many chemosynthetic biological communities (CBCs) exist without any seepage at all. MBARI's key finding is that CBCs develop where previously buried sediments have been exposed, by erosion or slides. Animals in seep communities such as tubeworms grow "roots" that penetrate sediments and rock fissures in a way analogous to plant roots. CBCs are small communities, but much more common than their larger hot vent relatives. Since many CBCs do not depend on active seeping, they may have longer lifespans than hot vents.

CBCs are based in chemosynthesis, with tubeworms, mussels and clams the root inhabitants, but often include anemones and other invertebrates that do not depend on sulfide- or methane-feeding bacteria.

In CBCs, tubeworms grow very slowly and live long, like most abyssal animals. In contrast, hot vent tubeworms grow rapidly and die young.

Here are some 'faces' of Cold Seeps , aka CBCs:

chemosynthetic clams characterize cold seeps	CBC hotspot: tubeworms, mussels, crabs and eels make a tangle of life.
tubeworms thrive next to methane hydrates deep in the Gulf of Mexico photo credit Ian MacDonald	mussels, tubeworms, crab at a cold seep
this abyssal hermit crab hosts bacterial "fur" on its pincers
a unique species of polychaete worm lives in burrows in methane "ice" (methane hydrate) outcrop in the abyss of the Gulf of Mexico. Photo credit Ian MacDonald
crabs foraging among thriving chemosynthetic mussels and tubeworms

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Whale Falls

Whale falls are simply the carcasses of dead whales that have fallen to the seafloor, often into abyssal waters, where cold and pressure slow down decay. A dead whale weighs somewhere between 60,000 and 320,000 pounds, which is an enormous influx of organic matter into a relatively food-challenged ecosystem.

In 1987, the submersible Alvin spotted a blue whale skeleton on the seafloor that was seventy feet of bones carpeted with life from worms to bacteria.

A whalefall bonanza quickly becomes a banquet for scavengers such as hagfish, sleeper sharks, eels, squat lobsters and other crustaceans. These feed for generations on the whale's soft tissues. Meanwhile, a substantial life community grows up in the sediments near the carcass; they are enriched by its decay. After that, there are the whale's huge bones, which, unlike the bones of most skeletons, are rich in fats and oils. Whale bones are rich in sulfur compounds, which attract sulfur-digesting bacteria and their symbiotic partners, such as Osedax worms which "burrow" into the bones with rootlike structures and wave their red gill plumes into the water for oxygen. By this late stage of decomposition, even sea anemones can make a good living at the whale fall. The community becomes complex and can persist for a century or so. Some 190 species of animals have been found on a single whale skeleton. Life is slow-paced in the cold depths of the sea, and so is decay. Meanwhile, generations of mussels, clams and osedax worms thrive on and near the bones.

Craig Smith of the University of Hawaii is the leading researcher of whale falls. He has arranged to be notified when a dead whale is washed up on a Hawaii beach; he then assembles a crew to tow the whale out to sea and sink it. This is difficult, since decay gases make the whale bouyant, so they weight it with large quantities of scrap iron until it sinks. A buoy is attached so they can return periodically for research.

Some 'faces' of the whale fall community:

montage of a whale fall rich with life: sea cucumbers, crabs and myriad Osedax worms waving red gill plumes. Photo courtesy MBARI
many eels wind through the ribs and vertebrae of a fallen whale. Photo courtesy NOAA
Photo courtesy Greg Rouse
An Osedax worm removed from the bone. Gills on top reach out of the bone, The green 'roots' are at the bottom, and the white ovaries are at the center.	The submersible's arm retrieves an old fragment of whale bone that supports a thriving host of Osedax worms
The fauna on and around the whale skull include an octopus, two kinds of crabs, and sea anemones, plus an assortment of meiofauna we can't see, plus the biiions of procaryotes that are the source of this self-sustaining food web.