Thermophiles: Characteristics, Classification And Environments

The thermophilic are a subtype of extremophiles characterized by high tolerate temperatures between 50 ° C and 75 ° C, either because these values are maintained temperature in these extreme environments, or because often reaching.

Thermophilic organisms are generally bacteria or archaea, however, there are metazoans (eukaryotic organisms that are heterotrophic and tissue), which also develop in hot places.

Marine organisms are also known that, associated in symbiosis with thermophilic bacteria, can adapt to these high temperatures and that have also developed biochemical mechanisms such as modified hemoglobin, high blood volume, among others, that allow them to tolerate the toxicity of sulfides and compounds. sulfur.

Thermophilic prokaryotes are believed to be the first simple cells in the evolution of life and to inhabit places with volcanic activity and geysers in the oceans.

Examples of this type of thermophilic organisms are those that live in the vicinity of hydrothermal vents or vents at the bottom of the oceans, such as methanogenic (methane-producing) bacteria and the annelid Riftia pachyptila.

The main habitats where thermophiles can be found are:

  • Terrestrial hydrothermal environments.
  • Marine hydrothermal environments.
  • Hot deserts.

Characteristics of thermophilic organisms

Temperature: critical abiotic factor for the development of microorganisms

Temperature is one of the key environmental factors that determines the growth and survival of living things. Each species has a range of temperatures within which it can survive, however, it has optimal growth and development at specific temperatures.

The growth rate of each organism versus the temperature can be graphically expressed, obtaining the values ​​corresponding to the important critical temperatures (minimum, optimum and maximum).

Minimum temperatures

At the minimum growth temperatures of an organism, a decrease in the fluidity of the cell membrane occurs and the processes of transport and exchange of materials, such as the entry of nutrients and the exit of toxic substances, can be stopped.

Between the minimum temperature and the optimum temperature, the growth rate of microorganisms increases.

Optimal temperature

At the optimal temperature, metabolic reactions occur with the highest possible efficiency.

Maximum temperature

Above the optimal temperature, a decrease in growth rate occurs to the maximum temperature that each organism can tolerate.

At these high temperatures, structural and functional proteins such as enzymes are denatured and inactivated, as they lose their geometric configuration and particular spatial configuration, the cytoplasmic membrane breaks and thermal lysis or rupture occurs due to the effect of heat.

Each microorganism has its minimum, optimal and maximum temperatures for operation and development. Thermophiles have exceptionally high values ​​at these three temperatures.

Distinguishing features of thermophilic organisms

  • Thermophilic organisms have high growth rates, but short lifetimes.
  • They have a large amount of long-chain saturated fat or lipids in their cell membrane; this type of saturated fat is capable of absorbing heat and turning into a liquid state at high temperatures (melting), without being destroyed.
  • Its structural and functional proteins are very heat stable (thermostable), through covalent bonds and special intermolecular forces called London scattering forces.
  • They also have special enzymes to maintain metabolic functioning at high temperatures.
  • It is known that these thermophilic microorganisms can use the sulfides and sulfur compounds abundant in volcanic areas, as sources of nutrients to convert them into organic matter.

Classification of thermophilic organisms

Thermophilic organisms can be divided into three broad categories:

  • Moderate thermophiles, (optimal between 50-60 ° C).
  • Extreme thermophiles (optimum close to 70 ° C).
  • Hyperthermophiles (optimal close to 80 ° C).

Thermophilic organisms and their environments

Terrestrial hydrothermal environments

Hydrothermal sites are surprisingly common and widely distributed. They can be broadly divided into those that are associated with volcanic areas and those that are not.

Hydrothermal environments with the highest temperatures are generally associated with volcanic features (calderas, faults, plate tectonic boundaries, back arc basins), which allow magma to rise to a depth where it can directly interact with groundwater deep.

Hot spots are also often accompanied by other characteristics that make life difficult to develop, such as extreme pH values, organic matter , chemical composition and salinity.

Inhabitants of terrestrial hydrothermal environments, therefore, survive in the presence of various extreme conditions. These organisms are known as polyextremophiles.

Examples of organisms that inhabit terrestrial hydrothermal environments

Organisms belonging to all three domains (eukaryotic, bacterial, and archaea) have been identified in terrestrial hydrothermal environments. The diversity of these organisms is determined mainly by temperature.

While a diverse range of bacterial species inhabit moderately thermophilic environments, photoautotrophs can come to dominate the microbial community and form macroscopic “mat” or “carpet”-like structures.

These “photosynthetic mats” are present on the surface of most neutral and alkaline hot springs (pH greater than 7.0) at temperatures between 40-71 ° C, with cyanobacteria established as the main dominant producers.

Above 55 ° C, photosynthetic mats are predominantly inhabited by unicellular cyanobacteria such as Synechococcus sp.

Bacteria

Photosynthetic microbial mats can also be predominantly inhabited by bacteria of the genera Chloroflexus and Roseiflexus , both members of the order Chloroflexales.

When associated with cyanobacteria, Chloreflexus and Roseiflexus species grow optimally under photoheterotrophic conditions.

If the pH is acidic, the genera Acidiosphaera, Acidiphilium, Desulfotomaculum, Hydrogenobaculum, Methylokorus, Sulfobacillus Thermoanaerobacter, Thermodesulfobium and Thermodesulfator are common.

In hyperthermophilic sources (between 72-98 ° C) it is known that photosynthesis does not occur, which allows the predominance of chemolytoautotrophic bacteria.

These organisms belong to the phylum Aquificae and are cosmopolitan; they can oxidize hydrogen or molecular sulfur with oxygen as an electron acceptor and fix carbon via the reducing tricarboxylic acid (rTCA) pathway.

Arches

Most of the cultivated and uncultivated archaea identified in neutral and alkaline thermal environments belong to the phylum Crenarchaeota.

Species such as Thermofilum pendens, Thermosphaera aggregans or Stetteria hydrogenophila Nitrosocaldus yellowstonii , proliferate below 77 ° C and Thermoproteus neutrophilus, Vulcanisaeta distributa, Thermofilum pendens , Aeropyruni pernix, Desulfurococcus mobilis and Ignisphaera aggregans, in sources with temperatures higher than 80 °.

In acidic environments, archaea of ​​the genera: Sulfolobus, Sulfurococcus, Metallosphaera, Acidianus, Sulfurisphaera, Picrophilus, Thermoplasma, Thennocladium and Galdivirga are found.

Eukaryotes

Among the eukaryotes from neutral and alkaline sources  , Thermomyces lanuginosus, Scytalidium thermophilum, Echinamoeba thermarum, Marinamoeba thermophilia  and Oramoeba funiarolia can be mentioned.

In acidic sources the genera: Pinnularia, Cyanidioschyzon, Cyanidium  or Galdieria can be found .

Marine hydrothermal environments

With temperatures ranging from 2 ° C to over 400 ° C, pressures in excess of several thousand pounds per square inch (psi), and high concentrations of toxic hydrogen sulfide (pH 2.8), deep-sea hydrothermal vents are possibly the most extreme environments on our planet.

In this ecosystem, microbes serve as the bottom link in the food chain, deriving their energy from geothermal heat and chemicals found deep within the Earth’s interior.

Examples of fauna associated with marine hydrothermal environments

The fauna associated with these sources or vents is very varied, and the relationships between the different taxa are not yet fully understood.

Among the species that have been isolated are both bacteria and archaea. For example, archaea of ​​the genus Methanococcus, Methanopyus, and thermophilic anaerobic bacteria of the genus Caminibacter have been isolated .

Bacteria thrive in biofilms that multiple organisms such as amphipods, copepods, snails, crab shrimp, tubeworms, fish, and octopus feed on.

A common scenario is accumulations of the mussel, Bathymodiolus thermophilus , more than 10 cm in length, clustering in cracks in basaltic lava. These are usually accompanied by numerous galateid crabs ( Munidopsis subsquamosa ).

One of the most unusual organisms found is the tubeworm Riftia pachyptila , which can group in large numbers and reach sizes close to 2 meters.

These tubeworms do not have a mouth, stomach, or anus (that is, they do not have a digestive system); they are a completely closed sac, without any opening to the external environment.

The bright red color of the pen at the tip is due to the presence of extracellular hemoglobin. Hydrogen sulfide is transported across the cell membrane associated with the filaments of this plume, and via extracellular hemoglobin reaches a specialized “tissue” called a trophosome, composed entirely of symbiotic chemosynthetic bacteria.

These worms can be said to have an internal “garden” of bacteria that feed on hydrogen sulfide and provide “food” for the worm, an extraordinary adaptation.

Hot deserts

Hot deserts cover between 14 and 20% of the Earth’s surface, approximately 19-25 million km.

The hottest deserts, such as the Sahara of North Africa and the deserts of the southwestern US, Mexico, and Australia, are found throughout the tropics in both the northern and southern hemispheres (between about 10 ° and 30- 40 ° latitude).

Types of deserts

A defining characteristic of a hot desert is aridity. According to the Koppen-Geiger climate classification, deserts are regions with an annual rainfall of less than 250 mm.

However, annual precipitation can be a misleading index, as water loss is a water budget decider.

Thus, the United Nations Environmental Program definition of desert is an annual moisture deficit under normal climatic conditions, where potential evapotranspiration (PET) is five times greater than actual precipitation (P).

High PET is prevalent in hot deserts because, due to lack of cloud cover, solar radiation approaches the maximum in arid regions.

Deserts can be divided into two types according to their level of aridity:

  • Hyper arid: with an aridity index (P / PET) less than 0.05.
  • Aggregates: with an index between 0.05 and 0.2.

Deserts are distinguished from arid semi-arid lands (P / PET 0.2-0.5) and from dry sub-humid lands (0.5-0.65).

Deserts have other important characteristics, such as their strong temperature variations and the high salinity of their soils.

On the other hand, a desert is usually associated with dunes and sand, however, this image only corresponds to 15-20% of all of them; rocky and mountainous landscapes are the most frequent desert environments.

Examples of desert thermophilic organisms

The inhabitants of the deserts, which are thermophiles, have a series of adaptations to face the adversities that arise from the lack of rain, high temperatures, winds, salinity, among others.

Xerophytic plants have developed strategies to avoid perspiration and store as much water as possible. The succulence or thickening of stems and leaves is one of the most used strategies.

It is evident in the Cactaceae family, where the leaves have also been modified in the form of spines, both to avoid evapotranspiration and to repel herbivores.

The genus Lithops or stone plants, native to the Namibian desert, also develops succulence, but in this case the plant grows flush with the ground, camouflaging itself with the surrounding stones.

On the other hand, animals living in these extreme habitats develop all kinds of adaptations, from physiological to ethological. For example, the so-called kangaroo rats present low-volume urination and in small numbers, thus being these animals very efficient in their water-scarce environment.

Another mechanism to reduce water loss is an increase in body temperature; For example, the body temperature of resting camels can increase in the summer from about 34 ° C to over 40 ° C.

Temperature variations are of great importance in water conservation, for the following:

  • Increased body temperature means that heat is stored in the body instead of being dissipated through evaporation of water. Later, at night, the excess heat can be expelled without wasting water.
  • The heat gain from the hot environment decreases, because the temperature gradient is reduced.

Another example is the sand rat ( Psammomys obesus ), which has developed a digestive mechanism that allows them to feed only on desert plants of the Chenopodiaceae family, which contain large amounts of salts in the leaves.

The ethological (behavioral) adaptations of desert animals are numerous, but perhaps the most obvious one implies that the activity-rest cycle is reversed.

In this way, these animals become active at sunset (nocturnal activity) and cease to be active at dawn (daytime rest), thus their active life does not coincide with the hottest hours.

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