Underwater Thermal Vents and the Bacteria that Inhabit the "Unlivable" Environment

     In 1977, underwater thermal vents were discovered. They consist of superheated water that has dissolved minerals and compounds such as H₂S coming out the ground. Before the discovery, it was thought that no life could possibly live in such a harsh environment. Now, the thermal vents are seen as islands for very unique bacteria to grow on. The environment is extremely heated and there is a lot of pressure from the weight of the ocean water above. No sunlight penetrates to the bottom of the ocean floor. The bacteria that live in these environments are therefore chemoautotrophic. Since there is lots of pressure and heat, the bacterium are also classified barophiles and thermophiles. Crabs, clams, mussles, and tube-worms are the main organisms that live near the vents. Chemoautotrophic bacteria live in very high concentrations around the vents. Bacteria tend to live inside of the tube-worms. This seems to be a symbiotic relationship where the worm houses the bacteria and in turn bacteria provide the worm with organic metabolites after their metabolism of H2S (Stover, Dawn).

     Scientists still have a lot to learn about underwater thermal vents. They have recently discovered a previous site that was destroyed by an underwater volcanic eruption, and they are now watching as a new habitat forms around this new thermal vent. Thermal vents are very strange environments. They facilitate very harsh living conditions, and they can end at any time. Some thermal vents might only last a few years; while others are expected to last to tens of thousands of years. Even though it is a challenging lifestyle, thermal vents are one of the most productive habitats in the world. They are able to convert inorganic materials into organic biomass at an unusually fast pace (Scearce C, 2006).

     Studies have shown a great deal about various organisms by studying thermal vents. They used to think that the sun's energy was the only method of converting inorganic material into organic material. They thought that the complex organisms that lived deep underwater got their energy from organic materials that drifted down from the surface. This was disproved when they found that the organic material was consumed by other organisms before it got down that far. They started looking more closely at thermal vents. And they found that the microorganisms that lived that far down where able to use methane and hydrogen sulfide to convert inorganic materials into organic matter (Scearce C, 2006).

Adaptations needed by the bacteria to survive in deep-sea thermal vents

     Thermal vent organisms defy the odds due to the fact that they deny the past theory that all organisms rely on the sun’s energy in order to survive. This point drives the opinion that thermal organisms are some of the most unique organisms in the world. H2S is one of the more toxic substances to living organisms, but bacteria that populate the thermal vents can metabolize this potent molecule. There are also methanotrophs, methanogens, and sulfate-reducing bacteria (Slonczewski, Foster, 2009, p. 808).

     Adaptations for the organisms on the ocean floor have to adapt for more than just metabolism though. The dissolved metals and other nutrients coming out of the vents are heated from magma, which can be up to 1,200 deg. C. The reaction zone where metabolism takes place is going to be around 400 deg. C. This temperature is much too high for most organisms to survive in and that is why these bacteria have adapted to be thermopiles. Thermopiles can live in very high temperature or they may even depend on having the high amount of heat to live (Slonczewski, Foster, 2009, p. 808).

Thermophiles have vital characteristics in order to withstand and utilize high temperature environments. There is a difference in how amino acids fold to create working proteins in thermophiles. It seems that the folds tend to have much more interaction with the other amino acids to have a stronger over-all bond for the proteins. The proteins tend to be much more rigid and compact as well as compared to mesophile, which are less heat tolerant and have more open 3 dimensional shaped proteins and enzymes (England J, Shakhnovich B, Shadhnovich E, 2005).

     One can also wrap their minds around how much weight is bearing down on these bacterium that literally have the whole ocean pressing down upon them. The thermal vent bacteria are also classified as barophiles. It seems that barophiles have adaptations in their cell-membranes to sense change in pressure and to withstand high stress. The bacterium at the ocean floor can live in environments as high as 1,000 atm or 14,000 psi. It is still not completely known how these organisms can withstand such high pressures, but it is vital that their cell membranes maintain fluidity. In order to retain fluidity at such high pressure, barophiles tend to have a large proportion of polyunsaturated fatty acids in their membranes (England J, Shakhnovich B, Shadhnovich E, 2005) 

Specific Bacteria that inhabits the thermal vent environment

     A bacterium that thrives in the high heat and chemical of the thermal chimneys is Nautilia lithotrophica. This Gram-negative bacterium is an anaerobic, sulfur- reducing mixotroph that are able to colonize because of the use of sulfur, carbon dioxide and hydrogen. The large amounts of H₂S allow the bacterium to use H₂ as the electron donor, S as the terminal electron acceptor, and CO₂ as the carbon source. N. lithotrophicagrows in dense population of short, rod shaped cells with polar flagella. It is found to growth the most abundant at 60 C (Alain, K.,  Callac, N.,  Gue´ gan, M., Lesongeur, F., Crassous, P.,  Cambon-Bonavita, M., Querellou, J.,  &  Prieur, D. (June, 2009).

     Closest to the thermal vents you will find massive colonies of tube worms around the thermal vents. These creatures have no eyes, gut, or anus making it fully dependable on a bacteria like Thiomicrospira. Found in the tubes of vestimentiferan in populations as dense as 285 billion. There are six different species of this bacterium, but they all have the same job. This microaerobic chemolithomixotroph uses sulfur as a sole energy source and use oxygen as it's electron acceptor. It uses carbon dioxide, carbohydrates, sugars, and amino acids not only for their growth but also supply nutrients for the tube worm. Thiomicrospira are single, Gram-negative cells that are straight or curved rods with a polar flagellum. (Takai, K., Hirayama, H., Nakagawa, T., Suzuki, Y., Nealson, K.,  & Horikoshi, K. (November, 2004)

     Another common organism that inhabits the environment of the thermal vents are Vesicomyid clams. This is another extremely special organism. Cospeciation is key between the clam an a chemoautrohic bacterira named Calyptogena elongata. The bacterium inhabits the gills of the clam and like many of the bacteria in the thermal vent habitat it uses sulfur as a key energy source and helps break down molecules to produce energy for the clam. It is shown that the Calyptogenaand the Vesicomyid clam have had this cospeciation since the beginning of invertebrate organisms. (PEEK, A.,  FELDMAN, R., LUTZ, R., & VRIJENHOEK, R. (August 1998)

     As you go farther out from the thermal vents you will find Idomarina loihiensis a Gram-negative, rod shaped bacterium with a subpolar flagella that forms a translucent beige or yellow biofilm that covers the bed of the ocean. The older colonies were sticky and viscous. This biofilm is formed because the bacterium produces a fatty acid that are isobranched.  The following constitutive enzyme activities are expressed: alkaline phosphatase, esterase (C 4), esterase lipase (C8 ),  leucine arylamidase,  acid phosphatase and phosphohydrolase. Major fatty acid is 13-methyl tetradecanoic acid. All of these enzymes help a the rapid grow of the colonies. It grows the fastest at 4 C and a thick layer develops after 2 or three days. Donachie, P., Hou, S., Gregory, T.,  Malahoff, A., & Alam, M. (2003)

Picture 5 depicts a Scanning electron micrographs of Idiomarina loihiensis L2-TRT. (a) Cells of 356~1?0 mm, one with a single polar flagellum; (b) an exceptionally long cell ofL2-TRT amid others displaying a regular, slightly ovoid rod morphotype. Bars, 1 mm http://ijs.sgmjournals.org/content/53/6/1873.full.pdf

     Another bacterium that is nitorious for making this thick mat on the the ocean floor is Mariprofundus ferrooxydans. This Fe-oxidizing bacterium is found in both iron-containing flocculent, silica, and sediment samples. They produce a iron oxidize sheaths that cover sea floor rocks, lava crusts, vent sites, and flocculent mats around hydrothermal vents. Flocculent mat material located near hydrothermal environments has been shown to contain hollow tubes of Fe-oxides.  This allows the bacterium to not only oxides the iron given off from the thermal vents, but it also produces nutrients for other organisms and plants in the habitat. Mariprofundus ferroaxydans has a tube like structure. I does not seem to have any flagella and does not have a need for motility. ( Hodges, T., & Olson, J., (2009)

 Picture 6 shows: (A) The Fe-floc mat as it appeared from the Pisces V submersible. (B and C) High-resolution scanning electron microscope images of Fe-floc samples indicated the presence of sheaths and filaments. Bar, 10

m (B) or 1 mm (C). http://aem.asm.org/content/75/6/1650.full.pdf

Metabolism of Bacteria in Thermal Vents 

     Earlier, this paper talked about adaptations that allow bacteria to live in the harsh environment of thermal vents. One of these big adaptations is their metabolism. Since these bacteria live so far under the water they have to rely on sources other than the sun to receive their energy. In this case the primary energy source is hydrogen sulfide. Hydrogen sulfide is toxic since it binds to iron. This stops the trasnport of oxygen by hemoglobin, and binds to the iron at the center of cytochrome molecules. Cytochrome is important since it is very important in aerobic respiration. One of the things that make these bacteria able to metabolize this toxic sulfer is the use of hypotaurine. Hypotaurine is the way that the sulfur gets detoxified so it does not harm the bacteria during metabolism (Yancey P, 2005).

     Since these bacteria live so far under the ocean they have to use the process of chemosynthesis to continue to survive. Chemosynthesis uses the oxidation of inorganic compounds like hydrogen gas, hydrogen sulfide, and methane to produce organic matter. As noted earlier, hydrogen sulfide is the primary energy source of thermal vents. The energy gained from the hyrdrogen sulfide is then used to fix carbon into other organic molecules. Some of the more complex organisms in thermal vents use these bacteria to gain energy. Tube worms, for example, have these bacteria inside of them. They absorb the hydrogen sulfide through their tissues, and then hemoglobin transports it to the bacteria that turn it into organic substances (Yancey P, 2005).

How the Thermal Vent Bacteria Work Together Relating to Metabolic Activity

     Bacteria that live off of thermal vents are reliant off of one another. They work together by providing each other with food. The thermal vent provides CO₂, H₂S, and H₂ and O₂ is dissolved in the water. The sulfate-oxidizing bacteria will metabolize the H₂S and oxygen to create S⁰ and water. SO + 2O₂ + H₂O reacts to create SO^2- and 2H. Sulfate-reducing bacteria rely on the sulfate-oxidizing bacteria's products to use as food. Sulfate-reducing bacteria "digest" SO₄^2- and 4H₂ to create H₂S, 2H₂O, and 2OH^-.  Methanogens in the thermal vent environment take CO₂ and 4H₂ and metabolize those materials to create CH₄ and water. can use the methane created from methanogens along with 2O₂ as an energy source to produce CO₂ and water (Slonczewski, Foster, 2009, p. 808).   An example of a symbiotic relationship is the tube worm.  These tube worms thrive in the thermal vent environment. The tube worms do not feed themselves. Instead, they rely on their vacated digestive tract to house sulfide-oxidizing bacterium and fluids rich in H₂S.  Since the worm houses the bacteria, they in turn they provide the worm with organic metabolites by oxidizing sulfur from H₂S (England J, Shakhnovich B, Shadhnovich E, 2005).  As stated above, cospeciation is key between the clam and a chemoautrohic bacterira.  The bacteria inhabits the gills of the clam and uses sulfur as a key energy source and helps break down molecules to produce energy for the clam (PEEK, A.,  FELDMAN, R., LUTZ, R., & VRIJENHOEK, R. (August 1998).  As one can see, one bacteria species cannot survive alone in this harsh environment. It takes the "teamwork" of multiple species of bacteria for survival to be possible.

Citations:

 Donachie, P., Hou, S., Gregory, T.,  Malahoff, A., & Alam, M. (2003) Idiomarina loihiensis sp. nov., a halophilic c-Proteobacterium from the Lo¯ ‘ihi submarine volcano, Hawai‘i. Retrieved from : http://ijs.sgmjournals.org/content/53/6/1873.full.pdf

 Hodges, T., & Olson, J., (2009) Molecular Comparison of Bacterial Volcanoes along the Kermadec Arc Flocculent Mats Associated with Submarine Communities within Iron-Containing. Retrieved from: http://aem.asm.org/content/75/6/1650.full.pdf

Yancey, Paul H.. Journal of Experimental Biology, Aug2005, Vol. 208 Issue 15, p2819-2830

PEEK, A.,  FELDMAN, R., LUTZ, R., & VRIJENHOEK, R. (August 1998) Cospeciation of chemoautotrophic bacteria and deep sea clams.  Retrieved: http://www.pnas.org/content/95/17/9962.full.pdf

Takai, K., Hirayama, H., Nakagawa, T., Suzuki, Y., Nealson, K.,  & Horikoshi, K. (November, 2004) Thiomicrospira thermophila sp. nov., a novel microaerobic, thermotolerant, sulfur-oxidizing chemolithomixotroph isolated from a deep-sea hydrothermal fumarole in the TOTO caldera, Mariana Arc, Western Pacific. Retrieved from: http://ijs.sgmjournals.org/content/54/6/2325.full.pdf+html

Alain, K.,  Callac, N.,  Gue´ gan, M., Lesongeur, F., Crassous, P.,  Cambon-Bonavita, M., Querellou, J.,  &  Prieur, D. (June, 2009) Nautilia abyssi sp. nov., a thermophilic, chemolithoautotrophic, sulfur-reducing bacterium isolated from an East Pacific Rise hydrothermal vent. Retrieved from: http://ijs.sgmjournals.org/content/59/6/1310.full.pdf

Scearce, C. (May, 2006). Hydrothermal vent communities. Retrieved from http://www.csa.com/discoveryguides/vent/review.pdf

Stover, Dawn. Creatures of the Thermal Vents. Popular Science, Ocean Planet Smithsonian.

http://seawifs.gsfc.nasa.gov/OCEAN_PLANET/HTML/ps_vents.html

Slonczewski, J. and Foster J. (2009). Microbiology, An Evolving Science. W.W. Norton and Company, Inc.

Jeremy L. England, Boris E. Shakhnovich, and Eugene I. Shakhnovich. (2005). Natural selection of more designable folds: A mechanism for thermophilic adaptation. http://www.pnas.org/content/100/15/8727.full