Topics Covered in this Lecture:
- The Discovery of Black Smokers
- The Base of the Food Web: Sulfur-Oxidizing Bacteria
- The Life and Death of Vent Organisms
Suggested Reading: The Octopus 's Garden: Hydrothermal Vents and Other Mysteries of the Deep Sea, Cindy Lee Van Dover, 1996, Addison Wesley PressSlideshow: Plate Tectonics
Man's advances in research and technology over the years have led to some amazing discoveries on our planet. Ventures into new areas have uncovered many things but have left us hungry for more. Our exploration of the moon and regular shuttle flights into Earth orbit has led many of us to believe that all we have left to discover is "out there", beyond the confines of our planet. Yet the oceans continue to dish up new and vast arenas of exploration, continuing to surprise and intrigue us with its strange and "alien" landscapes. The discovery in 1977 of hydrothermal vent communities and giant tube worms emerges as one of the most exciting finds in recent years. Heretofore completely unknown, these communities represent a whole new form of life, making their living on the stuff of the Earth rather than the sun, like most Earth organisms. Over 300 new species have been identified, all, of whom base their survival, in one way or another, on a wily group of bacteria known as chemoautotrophs. Here then is the story of these amazing creatures.
The Discovery of Black Smokers
Amazing as it seems, hydrothermal vents were not discovered until 8 years after Neil Armstrong set foot on the moon (July 20, 1969). You will recall that hydrothermal vents are plumes of hot water that spew from rocks and cracks along the ocean floor, especially in regions of sea-floor spreading, such as oceanic ridges and rift valleys. The formation of hydrothermal vents and the communities of organisms that depend on them are an area of intense and exciting research to this day.
The existence of plumes of "hot water" along the ocean floor had been noted in 1972 (and probably earlier). Water sampling studies along the Galapagos Rift (near the Galapagos Islands off the coast of Peru) indicated some kind of vents. Subsequent explorations by the deep-sea submersible Alvin revealed not only the presence of hydrothermal vents, but also a startling array of giant worms, clams, and mussels. Scientists were baffled: where did all these organisms come from?
Up until this time, it was thought that organisms living in the deep sea depended on a constant "rain" of food from above, i.e. from the lighted regions of the ocean. Everyone knows that all terrestrial life depends on plants, ultimately, feeding on them directly or feeding on organisms that feed on plants, etc. This same "web of life" was thought to exist in the sea, whereby the carcasses or feces of organisms living in the euphotic, or lighted, region of the seas, descended to the bottom where they became a source of food for bottom dwellers. Because this food supply was limited, it was thought that the numbers of organisms living in the deep-sea were limited. Thus, imagine their surprise when scientists saw vast "cities" of organisms living on the ocean floor.
Biologists soon realized that vent organisms relied on a special kind of bacteria, called chemoautotrophs (chemo = source of energy from chemicals; auto = synthesize their own food; trophs = type of feeding). Chemoautotrophic bacteria are able to use the energy of chemicals within the vent water to synthesize the carbon compounds they require to grow and reproduce. In contrast, photoautotrophs, i.e. photosynthetic organisms such as plants and algae, use the energy of sunlight (photo = sunlight) to synthesize the carbon compounds they need to grow and reproduce. We shall examine the relationship between these bacteria and vent organisms in more detail below.
From 1977 through 1979, Alvin made a series of dives to further investigate the nature of these hydrothermal vent communities. The vents were shown to be associated with the formation of new oceanic crust. Water coming from the vents had a temperature of 17 degrees C compared to 2 degrees C for the surrounding bottom water. These vents also issued a rich variety of minerals, presumably from the dissolution of chemicals in rocks deep beneath the vents. As this hot, mineral-rich water comes into contact with the cold bottom water, these minerals precipitate and form deposits on the surrounding rocks.
Some of these deposits accumulate as great mounds of material and may attain great heights. In 1979, during an expedition along the tip of Baja California, Alvin discovered 65-foot-high chimney-shaped structure spitting hot black "smoke." Hence was born the name "black smokers" for vents that emitted dark streams of particles. While their chemistry may vary, black smokers often emit particles that are rich in sulfides, lead, cobalt, zinc, copper, and silver. Other types of vents ejecting a different composition of minerals have been found and named, including "white smokers", who spit out streams of gypsum and zinc, rather than sulfides. They also contain less amounts of iron and copper.
One puzzle for scientists to figure out is why the chemistry of hydrothermal vents changes, not only among locations, but over temporal scales as well. Measurements of vent water taken from the time of a sea floor eruption indicate a change in the mineral composition being emitted. Scientists have also found that individual vents or entire vent fields can change anywhere from days to thousands of years.
Despite these puzzles, a great deal has been learned about hydrothermal vents in the past couple decades. Vents have been found at just about every type of sea floor spreading boundary (slow, intermediate, fast); they have been found at "hot spots"; and they have been found in back-arc basins. Seemingly everywhere that some type of mantle-driven activity occurs, scientists have found hydrothermal vents.
Although less than 1% of the sea floor where hydrothermal vents are suspected has been investigated, hundreds of hydrothermal vent fields have been identified around the globe in the past couple decades. Typically, vents within these regions cluster in groups, much like the geyser basins in Yellowstone. The largest vent field, called TAG, (Trans Atlantic Geotraverse, whatever that means) measures as big as a football field. Other vent fields have taken more personal names. The first fields discovered at the Galapagos site were named Rose Garden, Garden of Eden, and East of Eden, after the giant red-fleshed tube worms found there. Along the Mid-Atlantic Ridge, the American west prevails, with names like Lucky Strike, Broken Spur, and Snake Pit. Lucky Strike was named when scientists found a colony of mussels growing on a fresh mound of sulfide. The French and American scientists who found this field also named a couple of individual mounds: Statue of Liberty and Eiffel Tower. Broken Spur refers to the topography of another field, which consists of a series of "spurs" coming off the ridge. The Snake Pit was named for the profusion of white, eel-like fish that live there. One of the black smokers at Snake Pit was named Saracen's Head, after one scientist's favorite British pub.
Other favorites include Clam Acres, Snow Blower, and Genesis. Snow Blower appears to maintain perfect conditions for bacterial growth and the white flaky mats of these bacteria swirl around the vent, much like snow. Genesis was named for a vent that "died" and came back to life again after a few years.
And I would be remiss if I didn't mention another large black smoker, named Godzilla. Godzilla is 13 stories high (about 150 feet) and 40 feet across. It is characterized by large protruding sulfide knobs and shelves that make it look like a giant layered mushroom. Alvin pilots report that hot water often accumulates under these shelves making them particularly hazardous.
To date, the largest black smoker mound yet discovered is located in 3,650 meters (11,972 feet) of water on the eastern side of the Mid-Atlantic Ridge, a few thousand miles directly east of Miami Florida. This circular mound, found in the TAG field, measures more than 160 feet high and 600 feet in diameter. Scientists are studying this mound to gain an understanding of the interaction of crustal rocks and seawater, and to learn how sulfide mounds on land were formed millions of years ago.
Finally, scientists have been able to observe firsthand the formation of hydrothermal vents as lava erupted on the sea floor. Newly cooled lava on the sea floor appears glassy, much like the way glass appears when you melt it in a fire. As Cindy Lee Dover describes it:
The most recent lavas...are glassy, reflective, with a rind of brittle glass that hasn't been in place long enough to weather or to accumulate a covering of dusty sediment rained down from the surface. The terrain in the axial valley can be tortured and rugged, filled with pits and caverns and tall pillars, with rubble, talus*, scree*. It is a landscape of stark beauty.
[*Professor's note: A talus is a sloping pile of rock fragments at the foot of a cliff; a scree is an accumulation of pebbles or rock fragments at the base of a steep slope.]
Eruptions of lava on the sea floor as it widens appear to form deep cracks into which seawater rushes. Some of these cracks may extend thousands of feet, or more than a mile in some cases, into the lower crust. These cracks evolve into a series of dikes or passageways along which hot, mineral-rich seawater flows towards the surface. It is likely that some kind of circulation system develops where seawater enters cracks in the crust at one location, is transported downwards where it makes contact with hot rocks or even magma, and boils to the surface. Water in these vents becomes superheated (because of the intense pressure) often reaching temperatures of 350 degrees C. As it streams to the surface and mixes with the cold bottom water (at a temperature of 2 degrees C), the vent water quickly cools and dumps its load of minerals. From the opening on the sea floor to a few feet away from the vent, a temperature gradient is formed along which different organisms might live, depending on their temperature preference. At some vents, the exiting water is cooler, around 17 degrees C, which allows organisms to live comfortably within its confines.
The importance of hydrothermal vent circulation is being increasingly realized. Scientists now estimate that the circulation of seawater through the oceanic crust accounts for 34% of the heat input into the global oceans, about 25% of the globe's total heat input. It is also thought that hydrothermal vents may regulate the chemistry of the global oceans and could be responsible for the elemental composition of seawater as a whole. Obviously, the formation of sea floor mineral deposits and the interaction of seawater with crustal rocks has a big influence on the composition of the crust and may be the "icing on the cake" as they say that determines the geological properties of the sea floor.
Of course, the formation and evolution of a hydrothermal vent system is complicated by the fact that nothing is ever "permanent" on the sea floor. Magma is always rising to fill old dikes, and the sea floor is always spreading to form new ones. Such dynamics have sparked the interests of a whole generation of scientists and many prominent programs, such as NOAA's VENTS program, NSF's RIDGE program, and the French-Japanese STARMER program are underway to understand the geological, chemical, and biological processes that occur at hydrothermal vents.
The Base of the Food Web: Sulfur-Oxidizing Bacteria
Compared to the surrounding sea floor, upon which organic matter rains from above, hydrothermal vents boast a community of organisms that is 10,000 to 100,000 times denser. The reason for this: the presence of sulfur-oxidizing bacteria as a food source either directly or through a kind of cooperative "agreement" between the bacteria and a particular vent organism.
Sulfur-oxidizing bacteria and other forms of bacteria that gain energy from the metabolism of inorganic compounds belong to a category of organisms called chemoautotrophs, as mentioned above. These bacteria are able to oxidize (remove electrons) compounds such as hydrogen sulfide and store energy in the form of ATP (adenosine triphosphate), which is the universal "energy" molecule in all organisms, including humans. These bacteria use this energy to transform carbon dioxide into simple sugars and other molecules, just like plants. Thus, these unique bacteria function much in the same way as plants, creating organic matter at the base of the food web for other organisms to use.
While some vent organisms may consume the bacteria directly, a more ingenious method has evolved for many of the vent organisms. A kind of symbiosis, or "living together" has developed, where vent organisms provide a secure "house" for the bacteria, who in turn, provide carbon compounds to the host animal. Symbiosis occurs throughout the animal and plat kingdoms. It is formally defined as a close association between two organisms that benefits one or both partners and may be obligatory (absolutely essential to the survival of one or both partners) or it may be facultative (beneficial, but not required.
These symbioses are critical for hydrothermal vent organisms, allowing them to survive in numbers that far surpass what would otherwise be possible on the sea floor. While symbiotic relationships in coral reefs have been known for some time, the discovery of hydrothermal vents in 1977 has spurred an interest in symbiotic relationships, and led to their recognition in many other types of habitats, such as mangrove swamps, mud flats, and sewage outlets. Shades of Gaia begin to creep in here.
Nowhere is the importance of symbiosis better demonstrated than in the giant tube worm, who has the scientific name Riftia pachyptila. When these worms were collected and examined in greater detail, a most stunning feature was found. These worms have no mouth, no gut, and no anus. They are completely devoid of any digestive system whatsoever. Now how do you get to be four-feet tall without eating?
This mystery of the deep was solved by a graduate student at Harvard University, Colleen Cavanaugh, who relates the story of jumping up in class and shouting that she figured out how the tubeworms make their living. Inside the body of the tubeworm is an organ called a trophosome. The trophosome is highly vascularized and contains specialized cells packed full of chemoautotrophic sulfur bacteria. By means that are not entirely understood, the tubeworm provides all the chemicals necessary for the bacteria to make food, including sulfur, oxygen, and carbon dioxide, and the bacteria manufacture sugars or some other form of energy-rich molecules that provide nutrition to the tubeworm. Most certainly, the blood-red hemoglobin that fill the tubeworm's cardiovascular system and is so highly visible as the red gill-like polyps that extend from its tube is important in the transport of sulfur and oxygen. How this is accomplished and how nutrition is provided to the tubeworm from the bacteria is less well understood.
Other vent organisms, such as the "giant" white clams (as big as dinner plates) and mussels, use similar symbiotic mechanisms to obtain their nutrition. Other organisms, such as the blind Atlantic vent shrimp, Rimacaris (Rima meaning rift and caris meaning shrimp) feed on the sulfur bacteria directly. Similarly, other worms and polychaetes have been observed with bacteria in their gut, suggesting they feed directly on the mats of sulfur bacteria. Larger organisms, such as some types of crabs and fishes, are most likely opportunistic feeders living on other living or dead vent organisms. Thus, a food web is established, consisting of primary producers (chemoautotrophic sulfur bacteria), the secondary producers (tubeworms, mussels, clams, shrimp), and predators (fishes) or detritivores (crabs).
Because of the difficulty of observing these organisms or even raising them in the lab, progress in understanding how they feed, how they metabolize their food, and what role they play in the food web has been slow. Nonetheless, the existence of vent organisms and their unique way of solving life's problem of growing, surviving, and reproducing has opened up a whole new way of looking at life. The primary dependence of these organisms on symbiotic relationships has led many scientists to re-evaluate the importance of these associations in many other communities of organisms. Thus, the discovery of vent organisms has changed our perspective of marine ecology in all areas of the sea.
I have to quote Cindy Lee Van Dover one more time because she does such a good job at capturing the importance and enthusiasm these new life forms bring to oceanography. She writes:
The stunning implication is that submarine hydrothermal systems, fueled by the heat of volcanic processes, can support life in the absence of sunlight. Vent water may be the ultimate soup in the sorcerer's kettle. The water has a primeval chemistry that has prevailed along submarine mountain ranges since the breakup of Gondwanaland. There is growing interest in and legitimization of theories that the chemical and thermal conditions found in some vent waters may permit the synthesis of organic compounds. Deep-sea vents may have been the site where life originated on this planet.
The Life and Death of Vent Organisms
Life on a hydrothermal vent can be a tricky matter. Surrounded in complete darkness, miles beneath the sea, under intense pressure, and barely keeping your body temperature above freezing, is just the start of it. Not only is the sea floor moving beneath you continuously, but the very rocks on which you build your house may be subject to volcanic eruption at any moment. A 1991 Alvin expedition to the East Pacific Rise witnessed the creation of a new hydrothermal vent. So hot was the vent water -- 403 degrees C -- that many of the previous inhabitants, like the giant tube worms, had been baked to death. This event earned the name of Tube worm Barbecue for the site. Such is the life of an organism living on a hydrothermal vent.
The eruption of lava onto the sea floor and the creation of a new black smoker represents the start of a new phase in the life cycle of vent communities. From the opportunistic early colonists to the mature "climax" species, the organisms that make up the vent community change as the vent environment changes, not only in the geological and chemical sense, but also in the biological sense. This process whereby distinct sets of species are associated with different phases in the colonization of a vent community is known as succession. Strictly defined, succession is the change in species composition and community structure with time. Perhaps nowhere is succession more evident to us than near volcanoes such as Mt. St. Helens in Washington or Mauna Loa in Hawaii. A visitor here can see the dramatic changes in plant and animal life that occur from the time of the eruption to a few years afterwards. This same process takes place on the ocean floor.
The first "colonists" of a new hydrothermal vent are the sulfur bacteria. This should be obvious since all vent organisms ultimately depend on these bacteria, as explained above. Scientists who have witnessed such events describe the "massive blinding snowstorm of bacteria" and the absence of any other living organisms. But this scenario soon changes.
The colonization of a new hydrothermal vent happens quickly as mobile organisms, such as crabs, move into the site. Reports of hundreds of juvenile crabs scattered over a new vent site are not uncommon. These particular crabs feed on the mats of sulfur bacteria using their feathery appendages to "sweep" bacteria into their mouths.
Another early colonizer is the Jericho worm. Jericho worms are also tube worms, but much smaller and with circular concentric tubes. Colonies of Jericho worms were observed within a year after the "blinding snowstorm", which had been reduced to mats of bacteria on the sea floor. How these worms colonize so rapidly is not known, but its likely that the adults have some mechanism for making sure that larvae are ready at a moment's notice. There has been one report of Jericho worm larvae finding a new vent, but whether the larvae drift for some time until they find a vent, or whether the adults have some means of "sensing" the birth of a vent, whereupon they release larvae, is purely speculative.
Although not mentioned anywhere that I know, amphipods (you would recognize them as beach hoppers) are another animal that likely colonize new vents within the first year. Amphipods are ubiquitous throughout the ocean world, and early photos of the deep-sea floor show them crawling like flies over the carcasses of fish left there by scientists. Swarms of amphipods, which are prodigious swimmers, have been observed "hovering", like insects, within the flow of vent water. Likely, they feed on the bacteria which spew from the vent.
After some time, probably within a couple years or so, the new vent is colonized by more familiar inhabitants, such as the giant tube worms. Tube worms release eggs and sperm, typical of most marine invertebrates, which form a planktonic larvae. The larvae drift until they find a suitable habitat, most likely indicated by some chemical cue which causes them to settle and start building their tubes. The life span of the larvae, its feeding mechanism (if any), and how it obtains its "starter" set of bacteria are not known. However, on this latter point, researchers working with bioluminescent squids and fishes have found special organs and special chemicals that attract and promote the growth of bioluminescent bacteria. It is possible that a similar process occurs for the tube worm bacteria.
The colonization of tube worms also brings a group of organisms that live in and around the tubes. One such group, the siphonostomes, are tiny flea-like copepods who live directly on the tubes of the giant worms. It appears that these copepods feed directly on the flesh of the tubeworm. Limpets, small molluscs whose shells look like tiny, flattened hardhats, also live on and around the tubeworms. Limpets in the rocky intertidal typically feed on bacteria, scraping them off rocks with their "toothpick-like" radulas. Most likely, they feed in a similar manner on the vents. In turn, these limpets are a favorite food source for squat lobsters, who have been known to eat forty or more limpets in one sitting.
A number of other tubeworms and "feather-dusters" also inhabit the vents. One such worm, called Alvinella, lives in the hottest environment of any known animal. These animals prefer to build their tubes where the water is 30 - 40 degrees C (that's over 100 degrees F) These worms, which are fat and red and about 6 inches long, have a "starburst" plume used to gather food. They also have a white "fur" of bacteria that covers their bodies, possibly in a symbiotic relationship that de-toxifies the water for the worm, which contains heavy amounts of hydrogen sulfide, which can be toxic above a certain concentration.
Clams and mussels also make their way into the vent community, and may represent the "climax" community in the succession of organisms that inhabit the vents. Mussels appear to have supplanted the tube worms at Rose Garden. Three years after its discovery, scientists returned to find mounds of mussels covering the vents, and very few tubeworms, for which Rose Garden was named. Mussels on rocky shores are quite dominant and represent the "climax" stage for these communities. Their strong byssal threads -- the chitinous threads that attach them to rocks both on coastlines and in the deep sea -- enable them to withstand considerable forces, and it takes a lot to shake them loose. These threads also form a home for a number of smaller invertebrates, both on the shore and in the deep sea. Like the giant tubeworm, the mussels have symbiotic bacteria, but these bacteria are contained in the mussel's gills. The vent mussels also have a reduced but functional digestive system, which may afford them faster growth rates and, hence, a competitive advantage over the tubeworms. The vent mussels, unlike the shore variety, also have the ability to climb, which allows them to find the perfect spot along a vent, even if the water flow or chemistry changes. Thus, mussels have a couple tools that let them dominate the vent community.
The giant clams are another inhabitant of the later successional stage of a vent community. Big as dinner plates, these clams lie in the cracks of rocks from which vent water flows. Van Dover describes them as "white necklaces about lumps of lava in the heart of the field." Apparently, these organisms also rely on endosymbionts to obtain their nutrition, but not much else is known.
Where clams, mussels, and tubeworms dominate the vents of the Pacific, another group of organisms, the "blind" shrimp, dominate the vents of the mid-Atlantic Ridge. Discovered in 1985 crowding around an active black smoker, these shrimp have no eyes, and cover the sulfide mounds like vast colonies of ants. Whereas symbiosis is the norm for the Pacific tubeworms and bivalves, the Atlantic vent shrimp appear to scrape bacteria from the debris surrounding a vent. The way they appear to attack a vent is epic, and given that such a large organism has to make it living scraping the rocks, it makes sense that they take this duty so seriously. I envision great armies of blind shrimp roaming from vent to vent, removing its matted colonies of bacteria to a mere smattering, then moving on to another, unharvested vent. Thus they travel vent to vent in search of a tiny morsel to live another day.
This curious behavior led Van Dover to study these organisms in greater detail. While preserved specimens appeared to be completely devoid of eyes, she noticed in photographs a pair of reflective spots on the back of the shrimp. She speculated that they might be a type of "eyespot" and set out to determine whether these spots contained pigments that might be light-sensitive. Indeed, she found a visual pigment much like the rhodopsin that occurs in many invertebrates and vertebrates. Yet, the big question was, what were they looking at? Living in the pitch dark blackness of the sea, why did they need eyespots?
Given the behavior of these shrimp, the idea began to emerge that vents emit some kind of light radiation that the shrimp can see. Van Dover and her colleagues rigged a camera to detect this "heat radiation" and found, much to their delight, that vents glow! Your book illustrates this quite nicely, showing how a vent emits a glow in a wavelength that could be detected by a shrimp. It's quite likely that scientists would not have discovered this glow without being curious about the shrimp's eyespots, and thus, another fascinating area of research was born. What we thought were "blind" shrimp weren't so blind after all, and, from the shrimp's point of view, it was the scientists who were blind really! The important point is that these shrimp have evolved a very sophisticated sensor for finding vents and this is what has allowed them to be so successful in making a living off the vents.
As with all good things, life at the vent must come to an end. In the ever-moving, ever-changing cycle of events on the sea floor, hydrothermal vents reach their maturity and fade away. As the dikes and passageways through which the seawater circulates become clogged or altered by the spreading of the sea floor, some hydrothermal vents are turned off. Without their precious supply of sulfides and warm water, bacteria and animals surrounding the vent must move or die. One of the more somber scenes described by Van Dover concerns a site called Clambake, a literal graveyard for giant clams. Empty and dissolving clam shells litter the terrain of a dried up vent, left to "rust" in the cold, black waters of the heartless abyss.
Ultimately, vents are either buried by fresh lava or by sediments, as a rain of particles accumulates over time. From the Tubeworm Barbecue to Clambake, the ghost towns of hydrothermal vents are created, moved aside, and eventually buried. But with the slow "death march" of old vents comes the birth of new ones, and the inexorable cycle of life begins again. Life springs eternal in the hot mineral geysers of the deep dark sea.
from The Remarkable Ocean World