Who turned up the heat?

An Introduction to Hydrothermal Vent Communities!!

-Kaitlin Johnson

  
     If you've ever watched Planet Earth, you know our planet is filled with strange habitats that defy the imagination. The deep ocean is prime territory for these extraordinary environments. Just going a few miles below the glimmering surface can transport you to a habitat as unfamiliar as the surface of Mars.

Hydrothermal vent with black smokers

Of these unique habitats, one that I personally find bizarre yet intriguing, is that of one consisting of hydrothermal vent communities.  A hydrothermal vent can be described as a crack in the ocean floor that spews scalding hot water and toxic chemicals. Temperatures around these vents have been recorded as high as 400° Celsius, or 752° Fahrenheit!  Several chemicals such as hydrogen sulfide, sodium, calcium, silicon, chloride, and manganese have been found to pour out of these crevices in the ocean floor.  
Around the hydrothermal vents you will find long chimney-like structures referred to as black smokers. These hollow columns are composed of mineral deposits and emit a substance that looks like black smoke. However, this "black smoke effect" is created by the extreme hot water from the Earth's core mixing with the cold deep ocean water temperatures. Combining these two different temperature extremes causes the metal sulfides in the water to precipitate out. The precipitates then sink back down to the ocean floor landing upon one another and build up the chimney, layer by layer. 

3-Passenger Alvin submarine shown with lead-acid
 batteries illuminating the dark ocean floor

The discovery of hydrothermal vents is a prime example of one of those serendipitous findings. Back in 1977, Dr. Robert Ballard and his team of oceanographers were in the submarine Alvin, looking into mid ocean ridge topography and hydrothermal activity on the ocean floor when they detected a spike in temperatures, and decided to further investigate. They were expecting to only find abnormally heated water and were pleasantly surprised when they came face to face with the first ever recorded hydrothermal vents. The most astonishing part of their discovery however, is that these hydrothermal vents support a myriad of organisms. The benthos (bottom of the ocean) is often thought of as a lifeless wasteland, but their discovery showed that a rich community of organisms was possible, even in the extreme environments found miles beneath the surface of the ocean. Listen to Dr. Ballard describe the experience first hand. 

Hydrothermal vent crabs located atop tube worms

One of the first questions to arise from this discovery of new life was obviously; how are these organisms surviving without sunlight? 

The answer is relatively simple and lies within the small microbial organisms known as chemolithoautotrophic bacteria. I know this word may sound intimidating but broken down its not so bad. Combining "chemo" (chemical), "litho" (rock), "auto" (self) and "trophy" (food) means that chemolithoautotrophs are bacterial organisms that use chemicals from the Earth's core, rather than sunlight, to obtain energy and nutrients. They are able to do this by oxidizing the dissolved sulfide leaking from these vents in the ocean floor, a process commonly referred to as chemosynthesis. 

Photosynthesis in comparison to Chemosynthesis

         
        The presence and accumulation of chemolithoautotrophic bacteria, then, allows for the biological colonization of other unique organisms. Some of the organisms such as crabs, limpets, bivalves, anemones, amphipods, and scale worms feed directly on these bacteria while others have adapted more of a symbiotic relationship with the bacterial microbes.

            A prime example of a mutualistic relationship can be seen in the giant tubeworm (Riftia pachptila). These tubeworms possess a specialized organ known as the trophosome that houses billions of these chemolithoautotrophic bacteria. These giant tubeworms can grow up to 8ft long but they possess no actual mouth, gut, or eyes. Instead, they rely solely on the bacteria within them to convert (or, "fix") carbon dioxide into nutrition and energy. The bacteria, in turn, rely on the tubeworms' hemoglobin to transport oxygen and hydrogen sulfide to the trophosome where the bacteria can then oxidize the hydrogen sulfide into nutrients for themselves. 

Giant tubeworms (Riftia pachptila) inhabiting
a hydrothermal vent

        Other symbiotic relationships can be seen in the mollusks, Bathymodiolus thermopiles (Hydrothermal vent mussel) and Calyptogena magnifier (Hydrothermal vent clam) which use the bacteria in the same way as the tube worms. However, rather than housing the bacteria within a trophosome, the bacteria colonize the mollusk' gills. This symbiotic relationship is made even more unique by the fact that these organisms are able to pass on their endosymbionts to their offspring.

        The unique adaptations of organisms in the hydrothermal vent community are not necessarily limited to the particular way they are colonized by the foundational chemolithoautotriphic bacteria. There are also miniature shrimp (Rimicaris exoculata) thriving in these communities that lack eyes as we would normally think of them. Instead, they possess extremely sensitive photoreceptors on their heads that scientists believe have been adapted to detect infrared rather than the visible light spectrum. The shrimp likely use these specialized photoreceptors to detect heat or dim light coming from the vents, a possibly vital adaptation to have in order for these organisms to detect the location of these hydrothermal vents. 

Continued Biological Colonization. . .

Once crabs, limpets, bivalves, shrimps, and tube worms have established residency in these hydrothermal vent communities, higher level organisms can then join the ranks and feed upon these lower trophic level organisms. A few of these organisms include the hydrothermal vent eelpout fish (Thermarces cerberus) which mainly preys upon crabs and is known for having an increased metabolic rate, and is thought to be caused by the increased temperatures found around these vents. Another is the hydrothermal vent octopus (Vulcanoctopus hydrothermalis) which preys on many different crustacean species. Currently there is very little known about how this octopus is able to survive in such an extreme environment, but what scientists do know is that this organism is able to tolerate a high level of normally toxic metals in its system, and that it possesses a semi-translucent skin over its eyes, thought to be an adaptive trait for protecting the eye, optic nerve, and optic lobe from the harsh environmental conditions associated with hydrothermal vents. 

A Hydrothermal vent eelpout swimming
amongst Giant tubeworms.

A Hydrothermal vent octopus

This blog post has only begun to scratch the surface of the many unique organisms that have adapted the ability to live and thrive in these harsh hydrothermal vent communities. More than 300 species have now been identified living in these habitats. Scientists are continuing to study these communities in hopes that broadening our knowledge could lead to vital advancements in other fields. For example, the Polymerase Chain Reaction (PCR), an essential tool in most cellular biology research, relies on a protein (Taq polymerase) that was initially identified in the thermophilic
bacterium Thermus aquatics. These bacterium have evolved in such a way that their enzymes and proteins can still function at extreme temperatures, which is why Taq polymerase is able to function at the high temperatures required by the PCR reaction without denaturing.

In conclusion, it is possible that further research and phylogenetic analyses of these extreme heat adapted organisms could lead to more scientific advancements not only in genetics, but possibly in many other fields as well.


References

Campbell, B. J., C. Jeanthon, J. E. Kostka, G. W. Luther, S. C. Cary. 2001. Growth and phylogenetic properties of novel bacteria belonging to the epsilon subdivision of the Proteobacteria enriched from Alvinella pompejana and deep-sea hydrothermal vents. Environmental Microbiology 67: 4566-4572.

Little, C.S., T. Danelian, R.J. Herrington, and R.M. Haymon. 2004. Early jurassic hydrothermal vent community from the Franciscan Complex California. Journal of Paleontology 78: 542-559.

Nelson, K., C. R. Fisher. 2000. Absence of cospeciation in deep-sea vestimentiferan tube worms and their bacterial endosymbionts. Symbiosis 28 (1): 1-15.

Voight, J.R. 2005. Hydrothermal vent octopuses of Vulcanoctopus of hydrothermalis feed on bathypelagic amphipods of Halice hesmonectes. Journal of the Marine Biological Association of the united Kingdom 85: 985-988.