Hydrothermal vents
Physical Environment
Hydrothermal vents are where fluids flow from rock fractures in the sea floor at a variety of depths, and are mainly present near volcanic and tectonic plate activity [1] [2] [3]. The extreme environment found here includes pressures up to 420 atm, fluctuating temperatures from a low of 2°C to a high of 400°C, and chemical toxicity [4]. The hydrothermal fluids mainly contain hydrogen sulfide, methane, and heavy metals, resulting in chemotrophy being the sole process for energy at depths where light is absent [2] [3]. Phototrophy is possible for shallow sea hydrothermal vents [2].
Biological Interactions
Many macro organisms have adapted to the environment of hydrothermal vents by creating symbiotic relationships with bacteria [3]. One organism frequently studied is Riftia pachyptila, a giant tubeworm [6]. Bacteria inside the organism process many of the major bioelements including carbon, sulfur, and nitrogen [6]. Carbon dioxide is fixed by the Calvin cycle and then incorporated into host tissues [6]. Sulfide is oxidized and the protons generated as a waste product are disposed of by the tubeworm using ATPase active transport [6]. Nitrate is reduced to ammonia by assimilatory reduction and then incorporated into amino acids in the bacteria, followed by host digestion of bacteria [6].
Another example of symbiosis occurs in the mussel Bathymodiolus azoricus [3]. The mussel’s gills and digestive system are locations of heavy metal accumulation [3]. Bacteria living in hydrothermal vents however have developed metal resistance, with one mechanism involving heavy metal precipitation on the cell wall [3]. A similar method by endosymbionts in the mussel may help with metal detoxification [3].
Microbial Processes
Chemoautotrophy happens by at least two pathways, the reverse TCA cycle, which is more prominent, and the Calvin cycle [7]. The reverse TCA cycle uses four CO2 to make one oxaloacetate, consuming four to five ATPs in the process [7]. This energy requirement is less than half of the Calvin cycle, giving bacteria that use the reverse TCA cycle, such as Epsilonproteobacteria, an advantage [7] [8]. Major enzymes in the reverse TCA cycle include ATP citrate lyase, whose role is to split citrate into oxaloacetate and acetyl-CoA [7]. The reverse reactions of the TCA cycle are energetically unfavourable, but these enzymes make the reactions possible [7].
Carbon fixation requires energy, and a main energy source for chemolithoautotrophs at hydrothermal vents is hydrogen sulfide [8]. There are two pathways for sulfur oxidation [8]. The Sox pathway completely oxidizes hydrogen sulfide to sulfate using a multienzyme complex that includes SoxB, SoxCD, SoxXA, and SoxYZ [8]. In the other method, sulfur is first oxidized to a sulfite intermediate and then goes down the adenosine 5’-phosphosulfate pathway, using the enzymes APS reductase and ATP sulfurylase [8].
Key Microorganisms
Epsilonproteobacteria are the dominant class of bacteria of many hydrothermal vents [10]. There are usually both mesophilic and thermophilic species present due to the temperature gradient [10]. One major genus is the chemolithoautotroph Nautilia which is thermophilic and a sulfur-reducer [2]. H2 and formate are the possible electron donors for Nautilia, and sulfur is the electron acceptor, which produces hydrogen sulfide [2]. Another abundant class of bacteria is Gammaproteobacteria, with the chemolithoautotroph Thiomicrospira being the major genus [2]. Thiomicrospira is a sulfide oxidizer, meaning reduced sulfur compounds are used as the electron donors. CO2 is the carbon source [2].
Some hydrothermal vents have fluids that contain lower levels of hydrogen sulfide and carbon dioxide, which are needed by chemolithoautotrophs [12]. A main class of bacteria present at these vents is Deltaproteobacteria, a heterotroph [12]. Methanotrophs are also common at most hydrothermal vents, and shallow sea hydrothermal vents allow for photosynthetic organisms such as cyanobacteria [2].
Current Research
Organisms adapted many of their biomolecules to survive in the hydrothermal vent environment [4]. Examples are the desensitization of cytochrome C oxidase to hydrogen sulfide, and the production of proteins that can bind heavy metals [4]. These modified molecules such as enzymes and nucleic acids may be of use in the fields of medicine and biotechnology [4]. Also of interest are exopolysaccharides that are produced to protect the cell by acting as a layer between the cell and the surrounding environment [1]. Some have been found to have antiviral and immunomodulatory properties [1].
Alkaline hydrothermal vents are hypothesized to potentially be the origin of life on Earth [13]. Arguments for this belief include that at hydrothermal vents, cell-like vesicles form, and nucleic acid building blocks such as ribose, deoxyribose, and nitrogenous bases are made abiotically [13]. Hydrothermal hydrogen is an energy source, and ATP synthesis can be driven by chemiosmosis because of the gradients in temperature, redox, and pH [13]. The existence of sulfur-modified DNA and the 1,6-anhydro bond of peptidoglycan have also been named as molecular fossils that will provide clues to how life originated from these vents [13].
References
(1) Maugeri, T., Bianconi, G., Canganella, F., Danovaro, R., Gugliandolo, C., Italiano, F., Lentini, V., Manini, E. and Nicolaus, B. “Shallow hydrothermal vents in the southern Tyrrhenian Sea.” Chemistry in ecology, 2010, DOI: 10.1080/02757541003693250
(2) Zhang, Y., Zhao, Z., Chen, C., Tang, K., Su, J. and Jiao, N. “Sulfur metabolizing microbes dominate microbial communities in Andesite-hosted shallow-sea hydrothermal systems.” PloS one, 2012, DOI: 10.1371/journal.pone.0044593
(3) Kadar, E., Costa, V., Santos, R. and Powell, J. “Tissue partitioning of micro-essential metals in the vent bivalve Bathymodiolus azoricus and associated organisms (endosymbiont bacteria and a parasite polychaete) from geochemically distinct vents of the Mid-Atlantic Ridge.” Journal of sea research, 2006, DOI: 10.1016/j.seares.2006.01.002
(4) Minic, Z., Serre V. and Herve, G. “Adaptation of organisms to extreme conditions of deep-sea hydrothermal vents.” Comptes rendus biologies, 2006, DOI: 10.1016/j.crvi.2006.02.001
(5) Than, K. “Black smoke rising.” National geographic, 2012, http://news.nationalgeographic.com/news/2012/05/pictures/120523-new-hydrothermal-vents-deep-sea-mexico-mbari-oceans-science/
(6) Van Dover, C. and Lutz, R. “Experimental ecology at deep-sea hydrothermal vents: a perspective.” Journal of experimental marine biology and ecology, 2004, DOI: 10.1016/j.jembe.2003.12.024
(7) Campbell, B. and Cary, S. “Abundance of reverse tricarboxylic acid cycle genes in free-living microorganisms at deep-sea hydrothermal vents.” Applied and environmental microbiology, 2004, DOI: 10.1128/AEM.70.10.6282-6289.2004
(8) Hugler, M., Gartner, A. and Imhoff, J. “Functional genes as markers for sulfur cycling and CO2 fixation in microbial communities of hydrothermal vents of the Logatchev field.” FEMS microbiology ecology, 2010, DOI: 10.1111/j.1574.6941.2010.00919x
(9) Sauve, V., Bruno S., Berks, B. and Hemmings, A. “The SoxYZ complex carries sulfur cycle intermediates on a peptide swinging arm.” J. Biol. Chem., 2007, DOI: 10.1074/jbc.M701602200
(10) Flores, G., Shakya, M., Meneghin, J., Yang, Z., Seewald, J., Geoffwheat, C., Podar, M. and Reysenbach, A. “Inter-field variability in the microbial communities of hydrothermal vent deposits from a back-arc basin.” Geobiology, 2012, DOI: 10.1111/j.1472-4669.2012.00325.x
(11) Perez-Rodriguez, I., Ricci, J., Voordeckers, J., Starovoytov, V. and Vetriani, C. “Nautilia nitratireducens sp. nov., a thermophilic, anaerobic, chemosynthetic, nitrate-ammonifying bacterium isolated from a deep-sea hydrothermal vent.” International journal of systematic and evolutionary microbiology, 2010, DOI: 10.1099/ijs.0.013904-0
(12) Nakagawa, S., Takai, K., Inagaki, F., Chiba, H., Ishibashi, J., Kataoka, S., Hirayama, H., Nunoura, T., Horikoshi, K. and Sako, Y. “Variability in microbial community and venting chemistry in a sediment-hosted backarc hydrothermal system: impacts of subseafloor phase-separation.” FEMS microbiology ecology, 2005, DOI: 10.1016/j.femsec.2005.03.007
(13) Dai, J. “Novel molecular fossils of bacteria: insights into hydrothermal origin of life.” Journal of theoretical biology, 2012, DOI: 10.1016/j.jtbi.2012.06.041
