Inexplicable observations
The most exciting find at hydrothermal vents and cold seeps was the lushness of life. It was totally unexpected, very surprising, and a source of mystery. Since there is no sunlight at these depths, how do organisms at cold seeps supply themselves with the energy needed to make food? Most other areas of the deep ocean did not seem to be supporting these densities of life. This was an exciting scientific problem. What triggered the discoverers to think something was wrong with this picture?
Figure 1: Shallow location of the photic zone.
The ultimate source of energy for most life on earth is the sun. In the ocean, life is abundant in sunlit waters of the surface called the photic zone. With increasing depth, it becomes darker and life becomes increasingly scarce, depending on crumbs of food that drop from above. With increasing depth, the crumbs of food become harder and harder to find because most of the crumbs are eaten before having a chance to land on the seafloor. The biological world is very competitive.
Each crumb of organic matter that falls from the surface still carries the sun's energy within. Surface-living planktonic algae may initially capture the sun's energy, to be eaten by a copepod that eventually dies and begins to fall towards the seafloor. The copepod might be eaten by a deep-water fish nourishing the fish indirectly with the sun's energy even if the fish lives below the photic zone.
Although crumbs of organic matter and ultimately sunlight are still supplying deep-sea living communities with energy, enough crumbs do not land on the seafloor to explain the exuberance of life at some hydrothermal vent and cold seep localities. Where is the origin of their secret food supply? This was the big mystery.
Missing pieces
Research on organisms near hydrothermal vents showed that some of these organisms live symbiotically with bacteria capable of chemosynthesis. Some of the better-known organisms that rely on chemosynthetic bacteria include large tubeworms that house bacteria within their guts. Chemosynthetic bacteria tap directly into sources of chemical energy rather than the radiant energy of the sun to fuel their own life processes. This explained the lushness and proximity of vent communities near source of methane and hydrogen sulfide, two chemicals utilized by bacteria. This did not mean that all living things near hydrothermal vents relied only on chemosynthesis, but that part of the food web was dependent on chemosynthesis.
Figure 2: Chemoautotrophic endosymbionts living in one kind of clam.
Image Credit (click)
Since marine scientists first started looking, many chemosynthetic communities have been found around the global oceans. They have been found first near hydrothermal vents and then in areas away from vents. It was important to look and even in recent times, new discoveries are made all of the time.
Although we still do not know much about communities that depend on chemosynthesis, these communities seem less exceptional the more we look. Since chemosynthesis is less rare than we thought, our thinking about early life on earth has undergone a revolution too. Since its now plausible that life could have started in the darkness of the deep sea it allows us to abandon earlier ideas of starting life on land, a scenario that comes with very problematic baggage. One thing connects to the next.
Since cold seeps environments are a relatively recent discovery, even basic observations still need to be collected. Dr. Levin has set us on a course to: 1) find seeps, 2) sample the living things, and 3) sample the environmental conditions. Afterwards we'll complete analyses back at home and draw conclusions about why particular organisms congregate around cold seeps. What will the data really show? Besides finding new organisms, where is the bigger mystery? Let's begin the journey ...
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A C T I V I T I E S
• Activity 1.4a
Word breakdown
Can you figure out the meaning of the term from its root words?
chemoautotrophic endosymbiont
Pick the best meaning for each of the root words.
Chemo:
a. chemical process
b. process triggered by light
c. process involving water
d. process involving energy
Auto:
a. short for an automobile
b. carried out on its own
c. having to do with the self
Trophic:
a. where things live
b. who an organism is related to phylogenetically
c. having to do with what an organism eats
Endo:
a. within
b. outside
Symbiont:
a. an organism living in close relationship with another organism
b. an organism that competes with other organisms
c. an organism that does not interact with other organisms
d. an organism that has evolved from another organism
Now, can you put each of the root meanings together? What is the meaning of chemoautotrophic endosymbiont?
a. an animal that makes its own food and lives on top of another organism
b. bacteria that live in a worm or clam
c. an organism that uses nutrients in water to feed itself inside another organism
d. bacteria that live on top of another organism and feed off of that organism
• Activity 1.4b
Chemosynthesis
Chemosynthesis opens new possibilities about the early origins of life on Earth. With chemosynthesis, life can exist in the oceans before appearing on land.
a. Why do biologists think that life couldn't have started on land?
b. Why is photosynthesis incompatible with beginning life on land?
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