Some organisms are capable of synthesizing complex organic molecules from simpler inorganic compounds such as carbon dioxide (CO2), minerals and water. On Earth, photosynthetic organisms are able to manufacture complex organic molecules from simple inorganic compounds using the energy from sunlight. These organisms include plants, green algae, some protists (such as phytoplankton), and some bacteria (such as cyanobacteria) [1,2]. Photosynthetic organisms can create their own food and these are called autotrophs, meaning “self-feeding”. Autotrophs also are referred to as primary producers. It has been estimated that their total net primary production on Earth exceeds 104.9 petagrams of carbon per year, and that they play a crucial role in the global carbon cycle [1]. It is important to note that roughly half of this productivity occurs in the oceans and is mainly performed by microscopic organisms called phytoplankton. Although the concentration of carbon dioxide in the atmosphere is low, about 0.039%, this gas is indispensable for terrestrial photosynthesis. However, in some environments, primary production happens though a process called chemosynthesis [3]. Chemosynthetic organisms are autotrophs capable of obtaining energy to make their organic food by oxidizing high-energy inorganic compounds (hydrogen gas, ammonia, nitrates, and sulfides). On Earth, chemosynthetic ecosystems include hot vents, cold seeps, mud volcanoes and sulfidic brine pools. Therefore, autotrophs of different types can produce energy either through photosynthesis or chemosynthesis.
In 1977, marine scientists discovered ecosystems based on chemosynthesis at a depth of 2.5 km around a hot spring on the Galapagos volcanic Rift (spreading ridge) off the coast of Ecuador [4,5]. Since then, several hydrothermal vents rich in living organisms have been discovered and explored along the volcanic ridges in the Atlantic and Pacific oceans. The environments of these hydrothermal vents are considered extreme with unique physical and chemical properties such as elevated pressure (up to 420 atm), high and rapidly changing temperature (from 2–4 °C to 400 °C), acidic pH, toxic heavy metals, hydrogen sulfide and complete absence of light [6,7]. The origin of deep hydrothermal vents is continental drift. The lithosphere is divided into seven major and several minor plates all of which are moving relative to each other, creating cracks and crevices in the ocean floor [8]. These plates are separated by ridges divided into multiple segments separated by fracture zones. The rate of expansion of dorsal segments varies from 1 to 280 mm per year. The fracture zone is characterized by strong volcanic activity. Seawater seeps into these openings and is heated by the molten rock, or magma, which can reach very high temperatures (up to 400 °C) and then this hydrothermal fluid of heated sea water rises back to the surface dissolving large amounts of minerals which provide a source of energy and nutrients to chemoautotrophic organisms [9–17]. Numerous living organisms have been discovered in these hostile environments including microorganisms (Eubacteria and Archaea) and pluricellular organisms such as shrimps, clams and giant mussels, giant tubeworms, crabs and fishes [18]. These organisms have developed different strategies to ensure their adaptation to these extreme environments. In the total absence of photosynthesis, the food chain is based on the primary production of energy and organic molecules by chemolithoautotrophic bacteria, free or more or less associated to some of these organisms such as tubeworms, mussels and clams [6,7,17]. The environment around hydrothermal vents, where different populations of bacteria live, is characterized by large temperature variation. Some microorganisms thrive at the low temperature of 1–4 °C prevailing in the deep sea (cold-adapted psychrophilic bacteria), others, mesophiles, at moderate temperatures (10–60 °C) and finally, some strains, called thermophiles or hyperthermophiles, thrive at around 60 °C and 100 °C, respectively [17]. For example, the archaea Pyrolobus fumarii can grow at 113 °C with an optimal temperature of growth at 106 °C [19]. In contrast to photosynthetic organisms that use solar energy, carbon dioxide, nutrients and water to produce organic materials and thereby biomass, chemiolithoautotrophic bacteria in deep sea hydrothermal vents are able to extract chemical energy starting from the oxidation of reduced mineral compounds present in their habitat. Then, this energy is used to synthesize complex organic molecules from simpler inorganic compounds such as carbon dioxide, nitrate, ammonium and other minerals. The synthesized small organic molecules are available to a number of animal species, which live in an obligate symbiosis with these chemosynthetic bacteria (clams, mussels, gastropods and vestimentiferan tubeworms). The hydrogen sulfide (H2S) and heavy metals (Pb, Cd, Hg, Zn, etc.) which are present in high concentrations in the hydrothermal vents are toxic for living organisms. The organisms of this ecosystem developed efficient mechanisms of defense to protect themselves from these toxic materials [17]. For example, they use unusual enzymes capable of resisting high temperatures and pressures. In addition, the adaptation of these microorganisms implies that there must be many others modifications of their biochemical components such as proteins, membranes and nucleic acids, as well as other physiological modes of adaptation.
Overall, autotrophic organisms (photosynthetic or chemoautotrophic) can use different sources of energy such as light or reduced minerals to synthesize complex organic molecules but possess the common characteristic of being able to incorporate carbon from CO2 into organic compounds. This aspect of carbon fixation has been mentioned in some excellent reviews [20–24]. In this review, we summarize current knowledge about enzymes that are involved in carbon dioxide fixation and assimilation, such as carbonic anhydrase, by organisms associated with deep sea hydrothermal vents. Because of the fact that this environment is characterized not only by diversity in physical and chemical factors but also by microbial and animal biodiversity, suggests that enzymes from these organisms might be of interest in different biotechnological strategies regardless of carbon dioxide capturing.