MICROBIAL ECOSYSTEMS IN DEEP SEA VENTS

MICROBIAL ECOSYSTEMS IN DEEP SEA VENTS

Ivan Moix, Aina Perich, Lucas Barrero & Ander Congil

INTRODUCTION

Hydrothermal vents were discovered for the first time in 1977 at depths of 2500 meters on the Galapagos Rift (Van Dover, 2000). These vents are commonly found near volcanically active places, where tectonic plates are moving apart at spreading centers, ocean basins, and hotspots (figure 1).

Figure 1. Global distribution of hydrothermal vent fields.

Source:https://www.pmel.noaa.gov/eoi/PlumeStudies/global-vents/index.html

The chemical components of hydrothermal fluids are a combination of basalt leachates and the reduction of sulphate and bicarbonate in seawater. Different types of compounds and ions are present such as H₂S, CO₂, NH⁺₄, H⁺, Fe²⁺, etc (figure 2). and there are also high concentrations of trace metals, calcium and barium. This mixture allows the life of different types of bacteria due to the great variety of microhabitats that can be formed. The communities that inhabit the hydrothermal vents are mainly composed of bacteria and protozoa, but organisms belonging to the phylums Nematoda, Coelenterata, Mollusca, Vestimentifera, Annelida, Arthropoda, Enteropneusta and fishes can also be found frequently (Van Dover, 2000).

Figure 2. Schematic diagram showing inorganic chemical processes occurring at warm- and hot- water vent sites. Deeply circulating seawater is heated to 350℃ to 400℃ and reacts with crustal basalts, leaching various species into solution. The hot water rises, reaching the sea floor directly in some places and mixing first with cold, downwelling seawater in others. On mixing, iron-copper-zinc sulfide minerals an anhydrite precipitate.

Source: Jannasch & Motti, 1985

There are two types of hydrothermal vents, the black smokers and the white smokers. The first ones can reach temperatures 350ºC high and their chemistry comes from the magma chambers that lie beneath the ocean bottom expansion zones. The effluent of these smokers is typically acidic (ph 2-3). The white ones, on the other hand, are relatively cold (between 50-90ºC) and their chemistry is driven by a metamorphic process that takes place at a low temperature, serpentinization. Their effluent is highly alkaline (ph 9-11), this alkaline enviroment allows to have high concentrations of dissolved H₂ and CH_4, but not dissolved CO₂. The generation of H₂ in these zones occurs thanks to this process and this allows to generate abiogenic CH₄. The abiogenic methane orginates from CO₂ that has leached from an inorganic carbon source in the mantle (Martin, Baross, Kelley and Russell, 2008).

DEEP SEA BACTERIA

The most significant microbial process taking place is the bacterial chemosynthesis (Jannasch & Motti, 1985).  These bacteria assimilate the CO₂ and have the ability to use certain reduced inorganic compounds as energy source. We can find nitrification, fixation of N₂, Fe, H₂ or methanogenesis (CH₄ oxidation), SO₄^2- reduction and autotrophy (Sylvan, Toner, & Edwards, 2012).

One of the bacteria found are the Rhodobacteria. Some are able to oxidize S, reduce the SO₄^2- and oxidize H and CH₄ (table 1). The epsilonproteobacteria are just able to reduce SO₄^2-. The other ones that correspond to the same phylum are the Thiobacteria. In general, Thiobacteria are anaerobic and reduce sulfates and sulfur. These are able to reduce SO₄^2- and use nitrogen oxidation. Rhodobacteria usually are purple bacteria but can also be chemolithotrophic.

Table 1. Potential ecological roles of tags for which obvious metabolism can be inferred.
Source: Sylvan, Toner, & Edwards, 2012

By the way, huge amounts of iron and manganese cover the surface and are used as an energy resource for bacteria by oxidation.

The strange symbiosis between Riftia & chemoautotrophic bacteria 

The most distinctive and conspicuous animal found around the deep-sea hydrothermal vents are the Riftia pachyptila (Childress, Arp, & Fisher, 1984) giant vestimentiferan tubeworm endemic to deep-sea hydrothermal vent areas in the Pacific (López-García, Gaill, & Moreira, 2002). These may reach up to 1,5 m in length (Jones, 1981) and densities may reach 176 individual m² (Hessler and Smithey, 1984). These tubeworms have evolved a symbiotic relationship with these bacteria and use them to rapidly produce large colonies at vent sites for as long as the sites exist (decades). These large worms are known to form a major element of unique biological communities (Jones, 1981). There has always been a great deal of interest to the nature of their food source because, as pogonophorans, they lack a digestive system. Three major routes by which material could be incorporated into tissues have been proposed, but the most favored one currently is the one involving chemoautotrophic symbiotic bacteria located within the worms (figure 3). This occurs by the uptake of reduced inorganic materials such as sulfide, methane and hydrogen from the vent water and their subsequent oxidation by the described bacteria. These bacteria could use the energy from such oxidation to fix carbon, which might be translocated to the worms in a manner analogous to that of animal-algal symbioses (Childress et al., 1984). In contrast to other symbionts coevolving with their hosts, there is compelling evidence that R. pachyptila endosymbiont is newly acquired by each generation (López-García et al., 2002).

Figure 3. Illustration of the symbiosis between Riftia and the chemoautotrophic bacteria that take place in deep sea vents.

Source: http://oceanringoffire.weebly.com/uploads/5/1/5/8/51580217/4047688_orig.jpg

Photosynthesis at depths of 2500 meters, how is it possible? 

However, not all bacteria from deep-sea vents are chemotrophic. A few years ago, researchers described the isolation and cultivation of a previously unknown green sulfur bacterial species from a deep-sea hydrothermal vent at the Pacific Ocean. They concluded that the isolated microorganism could not come from contamination of the samples since they had been taken directly from the effluent plume above the orifice of the vent at more than 2300 m depth using a 1-liter capacity Niskin sampler from a submersible. Besides, the nearest place that could provide solar light and H2S to support the anaerobic photosynthetic growth of green sulfur bacteria is 2250 km distant from the vent, on the coast of Costa Rica.

Green sulfur bacteria are anaerobes that require light for growth by the oxidation of sulfur compounds to reduce CO2 to organic carbon. The most surprising thing is that in these deeps, the only source of light is geothermal radiation. The absence of isorenieratene carotenoids and the presence of BChl c in GSB1 are in accordance with the geothermal light wavelengths that have been measured at these vents (Beatty et al., 2005). GSB1 is a nonmotile bacteria who live on the vent. The growth of this bacteria requires anaerobiosis, light, H2S or elemental S, and CO2. On black smoker chimneys, the total photon flux was estimated to be of the same order of magnitude as the solar photon availability for a green sulfur bacterium living at 80 m depth in the Black Sea. The in-situ cell division time of the Black Sea bacterium was calculated to be 2.8 years (Beatty et al., 2005). This gives us an idea of the harsh conditions in which these deep-sea organisms live. The importance of this lies in that discovery expands the range of environments that could harbor life forms which use light energy to drive endergonic biochemical reactions.

CONCLUSION

Although it could be thought that in environments like the deep-sea vents there would be no life, the microorganisms return to demonstrate their great ability to adapt to environments and extreme situations. Bacteria are the backbone of this ecosystem since they transform inorganic matter and sulphated compounds into organic material that can be assimilated by other organisms (worms, invertebrates…). To sum up, life in these ecosystems would be impossible without the action of these microorganisms.

BIBLIOGRAPHY

Beatty, J. T., Overmann, J., Lince, M. T., Manske, A. K., Lang, A. S., Blankenship, R. E., Plumley, F. G. (2005). An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent. Proceedings of the National Academy of Sciences, 102(26), 9306–9310. https://doi.org/10.1073/pnas.0503674102

Childress, J. J., Arp, A. J., & Fisher, C. R. (1984). Metabolic and blood characteristics of the hydrothermal vent tube-worm Riftia pachyptila. Marine Biology, 83(2), 109–124.

Hessler, R. R. and W. Smithey: The distribution and community structure of megafauna at the Galapagos rift hydrothermal vents. NATO Conf. Ser. (mar. Sciences) 12, 735-770 (1984)

Jannasch, H. W., & Motti, M. J. (August 1985). Geomicrobiology of Deep-Sea Hydrothermal Vents. Science, 229, 717-725

Jones, M. L. (1981). Riftia pachyptila Jones: Observations on the Vestimentiferan Worm from the Galapagos Rift. Science (New York, N.Y.), 213(May), 333–336. https://doi.org/10.1126/science.213.4505.333

López-García, P., Gaill, F., & Moreira, D. (2002). Wide bacterial diversity associated with tubes of the vent worm Riftia pachyptila. Environmental Microbiology, 4(4), 204–215. https://doi.org/10.1046/j.1462-2920.2002.00286.x

Martin, W., Baross, J., Kelley, D., & Russell, M. J. (2008). Hydrothermal vents and the origin of life. Nature Reviews Microbiology, 6, 805. Retrieved from https://doi.org/10.1038/nrmicro1991

Sylvan, J. B., Toner, B. M., & Edwards, K. J. (January 2012). Life and Death of Deep-Sea Vents: Bacterial Diversity and Ecosystem Succession on Inactive Hydrothermal Sulfides. mBIO, 3 (1).

Van Dover, C. (2000). The Ecology of Deep-sea Hydrothermal Vents. Princeton University Press. Retrieved from https://books.google.es/books?id=uaXuCVuYVDUC