Chemotrophy Along Seafloor Hydrothermal Vents

by Pamela Moriarty

Life Along the Hydrothermal Vents

Earth’s oceans are populated by microorganisms which perform a wide range of important services. Three oceans, the Atlantic, Pacific and Indian, contain hydrothermal vents along their floor. Hydrothermal vents are formed where ocean plates pull apart and hot, mineral rich water and lava escape. Living along or in hydrothermal vents can pose great challenges to microorganisms. Most deep-sea hydrothermal vents contain low amounts of Mg2+, SO42-, and total S, while being abundant in silica, metals and dissolved gases. Seawater circulation allows for ion exchange and helps carry certain molecules, such as NH4+, Fe2+, Mn2+, H2S, H2, CO, and CH4, to the deep sea. However, this leads to more challenges for microorganisms inhabiting the vents, as the high concentrations of Fe2+, and NH4+ can affect metabolism (Karl 1995). The high pressure along the entire sea floor also presents challenges. Finally, temperature is extremely variable, supporting both psychrophiles living below 10⁰C and superthermophiles living above 115⁰C (Van Dover 2000). The wide range of environments occurring by the hydrothermal vents on the ocean floor present a set of unique challenges to microorganisms. We are just beginning to understand the variety of adaptations they have evolved to meet these challenges. While some environmental conditions remain the same from vent to vent, there are also major differences between vents in potential carbon and energy sources available . In addition, there are nutrient gradients within vent ecosystems due to circulation patterns. Each vent typically contains four distinct microbial habitats: 1) emitted vent fluid that contain free-living microbial populations, 2) rock, chimney, sediment and animal surfaces exposed to vent fluids that contain free-living microbial populations, 3) vent fauna and microorganisms in endo and exosymbiotic relationships and 4) the hydrothermal vent plumes. (Karl 1995).

The metabolic processes used by microorganisms in these environments span a wide range of electron donors and acceptors. Many, if not most, microbes along the hydrothermal vents practice mixotrophy, using combinations of different metabolic pathways based on what is available in the highest concentrations. Potential aerobic chemosynthetic processes include oxidation of sulfur compounds including sulfide, thiosulfate, hydrogen sulfide and elemental sulfur, as well as iron, manganese, carbon-compound and hydrogen oxidation and nitrification, while anaerobic chemosynthetic possibilities include sulfur or sulfate reduction, denitrification and methanogenesis. In the environment, aerobic chemosynthesis uses O2 as the electron acceptor, which has relatively large energy yields, allowing for the production of large amounts of biomass. Under anaerobic conditions, H2 is typically the electron donor. Hydrogen oxidation also yields high amounts of energy, but along the hydrothermal vents the low amounts of hydrogen available mean that little biomass can be produced through anaerobic chemosynthesis. Most likely, sulfide oxidation accounts for most of the energy available in this environment. (Van Dover 2000).

In addition to chemotrophy, two alternative hypotheses of energy production have been proposed. The first is organic thermogenesis, in which organic simple molecules such as thiocyanate and glycine are generated abiotically when sea water and hot rock react at large depths. Some sulfur compounds can also be synthesized from formaldehyde and sulfur that are in hydrothermal vent fluids. Other organic thermogenesis pathways include the possibility of producing urea, alkanes and alkenes abiotically. Another proposed hypothesis is detrital thermal alteration, which occurs when refractory organic carbon is thermally degraded to petroleum hydrocarbons and volatile fatty acids, which can be used by heterotrophic microbes. While these have not been extensively studied, they are interesting alternatives for energy production. (Van Dover 2000).

Microorganisms inhabiting hydrothermal vents have many possible biotechnological applications. One possibility is using microbes for bioremediation of waste sulfides and returning the resulting sulfate to the oceans and using the bacterial biomass as food for aquaculture or fermenting it into synfuels. Hyperthermophilic bacteria may also provide enzymes useful in a variety of molecular techniques including PCR, amylases, glucosidases and proteases as they are stable at a wide range of temperatures and may also be resistant to detergents and organic solvents. (Van Dover 2000).

It was long thought that there was no life along deep-sea hydrothermal vents because organisms would not be able to produce enough energy to live. Obviously this is now known to be incorrect, as microbes there have been found to be diverse and abundant, and larger organisms, including mussels, snails, sponges and shrimp have been found, though there are likely thousands more microorganisms that are unculturable that we have not yet discovered or been able to identify. Oxidation and reduction of sulfur are thought to be the prevalent metabolic pathways for life along the hydrothermal vents. Methylotrophy and methanogenesis also play extremely important roles, while iron, manganese, hydrogen and ammonia oxidation are just beginning to be studied in depth.

Oxidation and Reduction of Sulfur Compounds

Include some current research in each topic, with at least one figure showing data.

Methylotrophy and Methanogenesis

Include some current research in each topic, with at least one figure showing data.

Less Common Pathways- Iron, Manganese, Hydrogen and Ammonia Oxidation

Include some current research in each topic, with at least one figure showing data.

Conclusion

Overall paper length should be 3,000 words, with at least 3 figures.

References

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Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2009, Kenyon College.

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