The hydrothermal vent microbial community includes all unicellular organisms that live and reproduce in a chemically distinct area around hydrothermal vents. These include organisms in the microbial mat, free floating cells, or bacteria in an endosymbiotic relationship with animals. Chemolithoautotrophic bacteria derive nutrients and energy from the geological activity at Hydrothermal vents to fix carbon into organic forms. Viruses are also a part of the hydrothermal vent microbial community and their influence on the microbial ecology in these ecosystems is a burgeoning field of research.
Hydrothermal vents are located where the tectonic plates are moving apart and spreading. This allows water from the ocean to enter into the crust of the earth where it is heated by the magma. The increasing pressure and temperature forces the water back out of these openings, on the way out, the water accumulates dissolved minerals and chemicals from the rocks that it encounters. There are generally three kinds of vents that occur, and are all characterized by temperature and chemical composition: diffuse vents, white smoker vents, and black smoker vents. Due to the absence of sunlight at these ocean depths, energy is provided by chemosynthesis where symbiotic bacteria and archaea form the bottom of the food chain and are able to support a variety of organisms such as Riftia pachyptila and Alvinella pompejana. These organisms use this symbiotic relationship in order to use and obtain the chemical energy that is released at these hydrothermal vent areas.
Environmental properties Although there is a large variation in temperatures at the surface of the water with seasonal changes, in depths of the thermocline, the temperatures underneath the thermocline and the waters near the deep sea are relatively constant. No changes are caused by seasonal effects or annual changes. These temperatures stay in the range of 0–3 °C with the exception of the waters immediately surrounding the hydrothermal vents, which can get as high as 407 °C. These waters are prevented from boiling due to the high pressure at those depths.
With increasing depth, the high pressure begins to take effect. The pressure increases by about 10 megapascals (MPa) for every kilometre of vertical distance. This means that hydrostatic pressure can reach up to 110 MPa at the depths of the trenches.
Salinity remains relatively constant within the deep seas around the world, at 35 parts per thousand.
Although there is very little light in the hydrothermal vent environment, photosynthetic organisms have been found. However, the energy that the majority of organisms use comes from chemosynthesis. The organisms use the minerals and chemicals that come out of the vents.
Types of hydrothermal vents Vent types correlate with geochemical and physical conditions. Characteristics like temperature, mineralogy, and pH. The most common hydrothermal vents are black smokers and white smokers, the general third type being diffuse vents. Alkaline vents occur as well, along with other forms of hydrothermal vents.
Black smoker Generally release water hotter than the other vents between 300 and 400 °C. Fluids are high in metal sulfides such as iron, manganese, and copper, and zinc. These minerals precipitate when exposed to cold waters, and deposit on the seabed and form chimney-like structures.
White smoker
Vents emit a milky-coloured water between 200 and 330 °C. The milky-colour originates from calcium-rich minerals in ocean crustal rocks, which easily dissolve in cold seawater, when warmed water surfaces again, white plumes are produced. White smoker vents form chimney-like structures from these precipitates.
Diffuse These vents are characterized by clear, low-temperature (< 30 °C) waters. Vent fluids are released and spread out slowly before breaching the sea floor. These vents create temperature and chemical gradients, supporting diverse microbial communities, including sulfur-oxidizing bacteria.
Alkaline Characteristics found in alkaline vents are fluids with a high pH, typically ranging from 9 to 11, and low temperatures ranging from 40 to 90 °C. The fluids are rich in methane (CH4) and hydrogen (H2), which were produced from reactions between upper mantle peridotite and cool seawater. This process is known as serpentinization. Environments hosting ultramafic rocks, such as peridotites, are complex and vast.
Adaptations Extreme conditions in the hydrothermal vent environment mean that microbial communities that inhabit these areas need to adapt to them. Microbes that live here are known to be hyperthermophiles, microorganisms that grow at temperatures above 90 °C. These organisms are found where the fluids from the vents are expelled and mixed with the surrounding water. These hyperthermophilic microbes are thought to contain proteins that have extended stability at higher temperatures due to intramolecular interactions but the exact mechanisms are not clear yet. Along with this, many of the microbes have proteins similar to histones that are bound to the DNA and can offer protection against the high temperatures. Microbes are also found to be in symbiotic relationships with other organisms in the hydrothermal vent environment due to their ability to have a detoxification mechanism which allows them to metabolize the sulfide-rich waters which would otherwise be toxic to the organisms and the microbes.
DNA repair
Microbial communities inhabiting deep-sea hydrothermal vent chimneys appear to be highly enriched in genes that encode enzymes employed in DNA mismatch repair and homologous recombination. This finding suggests that these microbial communities have evolved extensive DNA repair capabilities to cope with the extreme DNA damaging conditions in which they exist.
Community Composition
Bacterial Diversity The most abundant bacteria in hydrothermal vents are chemolithotrophs. These bacteria use reduced chemical species, most often sulfur, as sources of energy to reduce carbon dioxide to organic carbon. The chemolithotrophic abundance in a hydrothermal vent environment is determined by the available energy sources; different temperature vents have different concentrations of nutrients, suggesting large variation between vents. In general, large microbial populations are found in warm vent water plumes (25 °C), the surfaces exposed to warm vent plumes and in symbiotic tissues within certain vent invertebrates in the vicinity of the vent.
Sulfur-oxidizing These bacteria use various forms of available sulfur (S−2, S0, S2O3−2) in the presence of oxygen. They are the predominant population in the majority of hydrothermal vents because their source of energy is widely available, and chemosynthesis rates increase in aerobic conditions. The bacteria at hydrothermal vents are similar to the types of sulfur bacteria found in other H2S-rich environments - except Thiomicrospira has replaced Thiobacillus. Other common species are Thiothrix and Beggiatoa, which is of particular importance because of its ability to fix nitrogen.
Methane-oxidizing Methane is a substantial source of energy in certain hydrothermal vents, but not others: methane is more abundant in warm vents (25 °C) than hydrogen. Many types of methanotrophic bacteria exist, which require oxygen and fix CH4, CH3NH2, and other C1 compounds, including CO2 and CO, if present in vent water. These types of bacteria are also found in Riftia trophosome, indicating a symbiotic relationship. Here, methane-oxidizing bacteria refers to methanotrophs, which are not the same as methanogens: Methanococcus and Methanocaldococcus jannaschii are examples methanogens, which are found in hydrothermal vents; whereas Methylocystaceae are methanotrophs, which have been discovered in hydrothermal vent communities as well.
Hydrogen-oxidizing Little is known about microbes that use hydrogen as a source of energy, however, studies have shown that they are aerobic, and also symbiotic with Riftia (see below). These bacteria are important in the primary production of organic carbon because the geothermally-produced H2 is taken up for this process. Hydrogen-oxidizing and denitrifying bacteria may be abundant in vents where NO3−-containing bottom seawater mixes with hydrothermal fluid. Desulfonauticus submarinus is a hydrogenotroph that reduces sulfur-compounds in warm vents and has been found in tube worms R. pachyptila and Alvinella pompejana.
Iron- and manganese-oxidizing These bacteria are commonly found in iron and manganese deposits on surfaces exposed intermittently to plumes of hydrothermal and bottom seawater. However, due to the rapid oxidation of Fe2+ in neutral and alkaline waters (i.e. freshwater and seawater), bacteria responsible for the oxidative deposition of iron would be more commonly found in acidic waters. Manganese-oxidizing bacteria would be more abundant in freshwater and seawater compared to iron-oxidizing bacteria due to the higher concentration of available metal.
Ecology
Phyla and genera To illustrate the incredible diversity of hydrothermal vents, the list below is a cumulative representation of bacterial phyla and genera, in alphabetical order. As shown, proteobacteria appears to be the most dominant phyla present in deep-sea vents. Actinomycetota Aquificota Hydrogenobacter and Aquifex Chloroflexota Chlorobiota Chlorobium Deferribacterota Gemmatimonadota Nitrospirota Nitrospinota Leptospirillum ferriphilum Bacillota Acetogen: Clostridium Pseudomonadota Acidithiobacillia Alphaproteobacteria Paracoccus Betaproteobacteria Thiobacillus Sideroxydans lithotrophicus Gammaproteobacteria - major symbionts Allochromatium Thiomicrospira Thioalkalivibrio Methylococcaceae Beggiatoa Thioploca Zetaproteobacteria Mariprofundus ferrooxydans Campylobacterota Sulfurovum lithotrophicum Sulfurimonas paralvinellae Nitratifactor salsuginis Hydrogenimonas thermophila Thiovulum Desulfobacterota - often sulfate-reducing, make up more than 25% of the bacterial community Desulfovibrio Desulfobulbus Desulfuromonas Dissulfuribacter thermophilus - sulfur disproportionation
Symbiotic relationships Symbiotic chemosynthesis is an important process for hydrothermal vent communities. At warm vents, common symbionts for bacteria are deep-sea clams, Calpytogena magnifica, mussels such as Bathyomodiolus thermophilus and pogonophoran tube worms, Riftia pachyptila, and Alvinella pompejana. The trophosome of these animals are specified organs for symbionts that contains valuable molecules for chemosynthesis. These organisms have become so reliant on their symbionts that they have lost all morphological features relating to ingestion and digestion, though the bacteria are provided with H2S and free O2. Additionally, methane-oxidizing bacteria have been isolated from C. magnifica and R. pachyptila, which indicate that methane assimilation may take place within the trophosome of these organisms.
Viruses and deep-sea hydrothermal vents
Viruses are the most abundant life in the ocean, harboring the greatest reservoir of genetic diversity. As their infections are often fatal, they constitute a significant source of mortality and thus have widespread influence on biological oceanographic processes, evolution and biogeochemical cycling within the ocean. Evidence has been found, however, to indicate that viruses found in vent habitats have adopted a more mutualistic than parasitic evolutionary strategy in order to survive the extreme and volatile environment in which they exist.