Symbiosis in the
The remarkable density of life at deep-sea hydrothermal vents is explained by die mutually beneficial symbiosis of in vertebrate animals and sulfide-oxidizing bacteria that colonize their cells
by James J. Childress, Horst Felbeck and George N. Somero
Biologists categorize many of the world’s environments as deserts:
regions where the limited availability of some key factor, such as water, sunlight or an essential nutrient, places sharp constraints on the existence of living things. Until recently the deep sea was considered to be such a desert, where the low abundance of organisms stems from the extreme limitation of the food supply.
Yet there is one habitat
in the deep sea where the density of life equals, if it does not surpass, what
is found in any other marine ecosystem. It is the system of hydrothermal vents,
It was at a spreading
ridge in the Pacific Ocean, 320 kilometers northeast of the Galapagos Islands,
that geologists on the research submarine
Such biological density was completely unexpected, and also very puzzling. Any ecosystem depends on the presence of primary producers: autotrophic (self-supporting) organisms that synthesize their own reduced carbon compounds, such as carbohydrates, from carbon dioxide. Green plants that convert carbon dioxide into carbohydrates in the presence of sunlight are called photoautotrophs, and they are the primary producers in most marine and terrestrial ecosystems.
Yet clearly photosynthesis is impossible at the depth of the vents. Not enough light penetrates beyond the upper 200 or 300 meters of the ocean’s surface to support photosynthesis. Below the sunlit surface the density of life falls off rapidly because there is less food. The organisms that live in the deep sea depend on organic matter that drifts down from the sunlit “euphotic” zone to the ocean floor. Most organic matter is synthesized, consumed and recycled in the euphotic zone, and only a small fraction sinks to the deep sea. Yet at the vents life was blooming 2,600 meters below the surface of the ocean. What unique feature of the hydrothermal-vent system could explain the abundance of life at these depths?
It seemed possible that the density of life at
Instead there is a better
explanation for these teeming oases of life at the bottom of the ocean. Water
samples taken from the vent sites and analyzed by John M. Edmond of the Massachusetts
Institute of Technology and his colleagues indicated that these underwater
For years biologists had
known that sulfide-rich habitats, such as terrestrial
Holger W. Jannasch of the Woods Hole Oceanographic Institution and David Karl of the University of Hawaii at Manoa isolated bacteria from the vent waters and did a series of experiments demonstrating that some species of vent bacteria are indeed autotrophic and depend on hydrogen sulfide and other reduced forms of sulfur for their metabolic activities. These sulfide-oxidizing bacteria might form the base of the vent food chain by providing food for the animal species.
Although they are very distantly rebated, green plants and sulfur bacteria are functionally similar in an important way: both are primary producers. Whereas green plants are photoautotrophs, sulfur-oxidizing bacteria capable of using an inorganic energy source to drive carbon dioxide fixation are called chemolithoautotrophs: they are literally consumers of inorganic chemicals that can exist autonomously—without an external source of reduced carbon compounds. Both fix carbon dioxide through a series of biochemical reactions collectively known as the Calvin cycle. The Calvin cycle functions in much the same way for bacteria as it does for green plants; the end product for both is reduced carbon.
It appeared, then, that the sulfur bacteria were in effect serving as the “green plants” of the vents and that their “sunlight” was hydrogen sulfide and other reduced carbon compounds. A major complication of this simple model was soon recognized: one of the dominant vent animals, the tube worm Riftia pachyptila, seemed to lack any means of harvesting the abundant crop of sulfur bacteria proliferating in the waters all around it.
Riftia is a strikingly unusual creature by conventional anatomical standards. It is essentially a closed sac, without a mouth or a digestive system and with no other means of ingesting particulate food. At its anterior tip there is a bright red branchial (gilllike) plume where oxygen, carbon dioxide and hydrogen sulfide are exchanged with the ambient seawater.
Below the plume there is a ring of muscle, the vestimentum, that anchors the worm in its white tube. Most of the rest of the animal consists of a thin-walled sac that contains the worm’s internal organs. The largest of them is the trophosome, which occupies most of the body cavity. As its name (“the feeding body”) suggests, the trophosome contributes significantly to the worm’s nutrition—but it lacks a channel through which particulate materials from the outside world can enter the worm. The big question was how, in view of its unusual anatomy, Riftias manages to obtain the nutrients it needs for survival.
Microscopic studies done
by Colleen M. Cavanaugh of
Symbiosis is the co-occurrence of two distinct species in which the life of species is closely interwoven with the life of the other. Symbioses vary from relations that are beneficial to one partner and harmful to the other (parasitism) to relations from which both the partners benefit (mutualism). When one species, known as the symbiont, lives within the body of the other, known as the host, the relation is called endosymbiotic.
The Riftia-bacteria endosymbiosis is mutualistic. The tube worm receives reduced carbon molecules from the bacteria and in return provides the bacteria with the raw materials needed to fuel its chemolithoautotrophic metabolism: carbon dioxide, oxygen and hydrogen sulfide. These essential chemicals are absorbed at the plume and transported to the bacteria in the trophosome by the host’s circulatory system. The worm’s trophosome can be thought of as an internal factory, where the bacteria are line workers producing reduced carbon compounds and passing them to the animal host to serve as its food.
The ability of Riftia to absorb sulfide from vent water and transport it to the bacteria in its trophosome without either poisoning itself or degrading the sulfide presented a major puzzle. Hydrogen sulfide is a highly toxic compound, comparable to cyanide in its ability to block respiration, the process whereby the animal uses oxygen. In most animals sulfide inhibits respiration in two ways: by blocking oxygen’s binding sites on its major carrier, the hemoglobin molecule, and by poisoning an important respiratory enzyme, cytochrome c oxidase. Studies of Riftia showed, however, that sulfide has no effect on oxygen binding and that the worm’s respiration rate is substantial even in the presence of sulfide concentrations lethal to most animals.
We wanted to know how aerobic respiration in Riftia is possible in the presence of high sulfide concentrations. Clearly Riftia had three obstacles to overcome. First, it needed to evolve a special transport system to extract sulfide from vent water. Second, it needed to transport sulfide in its blood without allowing the sulfide to compete with oxygen for binding sites on the hemoglobin molecule or to react with oxygen. (In the presence of oxygen, sulfide is highly unstable, decomposing rapidly to oxidized forms such as thiosulfate and elemental sulfur.) Third, it needed a mechanism to prevent sulfide from diffusing into its cells and poisoning respiration.
We collaborated with Mark A. Powell (who is now
It is large with a molecular weight of as much as two million daltons (human hemoglobin has a molecular weight of 64000 daltons). Rather than being contained within red blood cells, it circulates. freely in the serum. It also has an usually high affinity for oxygen as well as as an unusually high carrying capacity for oxygen. Riftia hemoglobin is thus well adapted for extracting oxygen from the vent waters and transporting it to the cells of the tube worm and to its symbiont.
There is an even more important and striking
difference between the hemoglobin of Riftia and
other hemoglobins: the tube-worm molecule can bind
both oxygen and hydrogen sulfide simultaneously. This discovery, made by Alissa J. Arp (who is now at
After it is unloaded in the cells of the trophosome, the sulfide is oxidized. The oxidation takes place in the bacterial symbionts, as was demonstrated by a special staining procedure that signals the presence of hydrogen sulfide. Further investigation by our group showed that the sulfide oxidation drives the synthesis of ATP and the fixation of carbon dioxide. Moreover, we showed that the activity of the Calvin cycle is approximately the same in the trophosome bacteria as it is in the leaves of a green plant.
Studies of other vent animals reveal that Riftia pachyptila is not the only hydrothermal-vent species that has evolved a symbiotic relation with sulfur bacteria. The large white clam, Calyptogena magnifica, and the mussel, Bathymodiolus thermophilus, also depend on chemosynthetic endosymbionts for food. But these species have evolved quite different approaches to the same problem.
In Calyptogena the bacteria are not in an internal organ but in the gills, where they can readily obtain oxygen and carbon dioxide from the respiratory water flow. The basic metabolic plan is the same, however: the bacteria oxidize sulfide and supply the clam with fixed carbon compounds. Like other invertebrates harboring sulfur bacteria as endosymbionts, Calyptogena has a greatly reduced ability to feed on and digest particulate foods.
Still, we were puzzled at first by the data from water and blood samples. The clams must be concentrating sulfide in their blood, because the sulfide level there is orders of magnitude higher than the concentration in the ambient water. Because of the way the clams are oriented in the vent waters, however, the sulfide concentration in the water bathing their gills is low. From what source, then, do these giant clams obtain sulfide to feed their endosymbionts? The path turns out to be indirect. Apparently the clams absorb sulfide through their large, elongated feet, which extend into the hydrothermal vents, where the concentrations of sulfide are highest. Once it is absorbed through the clam’s feet, sulfide is transported by the blood to the bacteria in the gills.
Transport of sulfide in the blood of Calyptogena is a very different process from the one described for Riflia. The clam’s hemoglobin (which is incorporated within the animal’s red blood cells) is irreversibly poisoned by sulfide, and so it cannot serve as a sulfide-transport protein. Calyptogena has overcome the problem of sulfide poisoning by evolving a special sulfide-transporting protein. It is an extremely large protein and it circulates in the serum rather than in the red cells. The protein has a dual function: it protects the animal’s hemoglobin and cytochrome c oxidase from sulfide poisoning and also protects sulfide against oxidation on the way to the gills. The binding of sulfide to the protein is reversible: it is off-loaded to bacteria in the gill (by mechanisms not yet understood), where it is oxidized to provide energy for the Calvin cycle.
Much less is known about symbiosis in Bathymodiolus thermophilus. Like the giant clam, the mussel has bacterial symbionts in its gills, but the pathways of sulfide transport and metabolism are not yet understood. Kenneth L. Smith, Jr., of the Scripps Institution of Oceanography has, however, carried out an interesting series of experiments demonstrating that the relation between these deep-sea mussels and the vent water is obligatory. When he moved mussels away from the immediate vicinity of a vent to a more peripheral region, they showed clear signs of starvation. Such starvation is a naturally occurring process at hydrothermal-vent sites. Individual vent sites are active for only a few decades at most. Large numbers of dead mussels and clams litter sites where water flow has ceased, indicating that for these animals survival is not possible without a supply of sulfide.
serves as another indication that symbiosis is effective in meeting the
nutritional requirements of the animal hosts. The rapid turnover of life at
these vent sites is reflected in the accelerated growth and the quick
attainment of reproductive age that are observed in these animals. Work done by
Richard A. Lutz of
The tube worm, the clam and the mussel, the largest and most dominant numerically of the vent species, owe their ecological success to their symbiosis with sulfur-metabolizing bacteria. Many of the smaller and less conspicuous vent animals lack symbionts. They obtain their nutrients either by filtering particulate food, such as bacteria, from the water or by feeding on the animals that do contain symbionts. We have observed vent crabs, for example, in the act of feeding on the respiratory plume of Riftia.
The symbiont-free animals are interesting objects to study in
their own right. Because sulfide is readily absorbed across the surface of an
invertebrate’s body, the symbiont-free animals,
like the symbiont-containing species, have had to
evolve mechanisms to prevent sulfide poisoning. With the collaboration of
Russell Vetter of Scripps and Mark Wells of
The animal’s heart rate and the beating of its scaphognathites (appendages that drive the flow of respiratory water) did not change when we raised sulfide concentrations well above normal levels in ambient seawater, and the level of sulfide in the crab’s blood increased only minimally. Because we could not isolate from the crab’s blood a sulfide-binding protein comparable to the tube worm’s hemoglobin, we assumed this species must have evolved some different strategy for detoxifying sulfide. We found that the crab can detoxify the sulfide it absorbs by oxidizing sulfide to thiosulfate, a much less toxic form of sulfur. The process is carried out in the crab’s hepatopancreas, a tissue similar in function to the vertebrate liver.
whether, in the symbiont-containing animals, each
host was colonized by only a single type of bacterial symbiont. Recent studies of the sequences of a
particular ribonucleic acid (the 165 RNA) of bacterial ribosomes,
done by Daniel Distel and our group in collaboration
with Norman R. Pace of
Our discoveries at deep-sea vents have stimulated surveys of a wide range of sulfide-rich environments, including mangrove swamps, petroleum seeps, sewage outfall zones and marshes. The surveys have revealed that the kinds of sulfur-based symbioses first discovered in the deep sea are widespread, and that various other energy-rich inorganic molecules, such as methane, can be exploited in analogous symbioses between other invertebrates and bacteria. These diverse symbioses have been described in disparate animal groups, including some smaller tube-worm relatives of Riftia, clams of a number of different families, various mussels and several groups of small marine worms.
As our studies progress we expect find further instances in which animals worm rely on chemolithoautotrophic symbionts , thereby gaining the ability to tolerate and exploit habitats the animals could not colonize successfully with out bacterial partners
SULFIDE IS HIGHLY TOXIC to most animals (a). It poisons respiration at two levels in the blood, where it binds to hemoglobin, and in cells, where it inhibits the respiratory to enzyme cytochrome c oxidase (not shown). Animals associated with sulfide-rich hydro- thermal vents have evolved different strategies to avoid sulfide poisoning. The tube Riftia pachyptila (b) has a separate binding site on its hemoglobin molecule for sulfide
and so can transport oxygen and sulfide simultaneously in its bloodstream. The vent clam onts (]alyptogena niagnifica (c) has a special transport protein to carry sulfide to bacteria in crate its gills. The vent crab Bythograca therinydron (d) lacks endosymbiotic bacteria; it is able -to detoxify sulfide by oxidizing it to nontoxic thiosulmate in its liverlike hepatopancrens..
PHOTOSYNTHESIS and chemosynthesis are compared. The energy sources differ, but the conversion process and end products are the same. In photosynthesis light is absorbed by the chloroplasts of plants and drives carbon fixation by the Calvin cycle, a process yielding sugars, fats and amino acids that enter the food chain, passing from herbivores to carnivores. In chemosynthesis energy is provided by hydrogen sulfide issuing from vents in the ocean floor. It is taken up by free-living bacteria and also absorbed by vent animals such as the tube worm, which transport it to endosymbiotic bacteria. In the bacteria it is oxidized, providing energy for the Calvin cycle. The end products enter the food chain, passing directly from lower-order carnivores to higher.order ones.
SYMBIOSIS IN THE DEEP SEA/questions with reading sheet
1. Why was the deep sea considered to have a low abundance of organisms?
2. Where are the hydrothermal vents situated? (overall)
3. What organisms were discovered near the vent
by the crew of the
4. Why is photosynthesis impossible at the vent?
5. What information did the water samples shed on the mystery of why life was around the vents?
6. What do
7. Define chemolithoautroph.
8. What product is produced by both plants and these bacteria?
9. What is strange about the anamotical studies of Riftia?
10. What is the trophosome responsible for?
11. What did examination of the trophosome reveal?
12. What type of symbiotic relationship exists between the bacteria and worm?
13. How do both parties of this relationship benefit?
14. What effect did sulfide have on oxygen bonding in the worm?
15. What 3 obstacles did Riftia have to overcome to carry on respiration?
16. What is responsible for the deep red color of the bronchial plume?
17. How is Riftia hemoglobin different than human hemoglobin?
18. What happens to the sulfides after it is unloaded to the cells of the trophosome?
19. What other vent animals are chemosynthetic endosymbionts?
20. Where is sulfide absorbed in the clam (Calyptogena)?
21. What does the accelerated growth rates of the organisms around the vent indicate?
22. How does the vent crab detoxify the sulfide it absorbs?