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.

The organisms in hydrothermal vents, are exposed to wide variations in dissolved inorganic carbon species (DIC) = CO₂ + HCO₃⁻ + CO₃^2– = 2 to 7 mM; pH = 5 to 8 and bottom water (DIC = 2 mM; pH = 8) [²⁵]. In this environment with constantly changing pH and DIC, the CO₂ concentration oscillates between 20 μM and 1 mM [²⁵–²⁸]. Although these habitats are characterized with oscillations in the availability of the inorganic nutrients necessary for chemolithoautotrophic growth, many organisms have adapted to this fluctuation in the concentration of dissolved inorganic carbon. One of the strategies of adaptation, in the presence of low concentrations of dissolved inorganic carbon, is cellular ability to use both HCO₃⁻ and CO₂ [²⁹]. In contrast to plants that are able to fix CO₂ via a Calvin Benson cycle, the organisms of hydrothermal vents use multiple mechanisms for fixation of CO₂ such as Calvin-Benson cycle, reductive tricarboxylic acid cycle (reductive TCA cycle), acetyl-CoA pathway, dicarboxylate/4-hydroxybutyrate cycle and 3-hydroxypropionate/4-hydroxybutyrate cycle.

Based on numerous reports, organisms of deep sea hydrothermal vents use several metabolic pathways for CO₂ fixation. These include the Calvin-Benson cycle, reductive TCA cycle, 3-hydroxypropionate bicycle, acetyl CoA-pathway, dicarboxylate/4-hydroxybutyrate cycle and 3-hydroxypropionate/4-hydroxybutyrate cycle [²⁰–²⁴] (Figure 1).

As shown, several enzymes are associated with CO₂ fixation such as adenosine triphosphate (ATP) citrate lyase, 2-oxoglutarate: ferredoxin oxidoreductase and fumarate reductase, ribulose-1,5-bisphosphate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase, formate dehydrogenase, CO dehydrogenase/acetyl CoA synthase, 4-hydroxybutyryl-CoA dehydratase and acetyl-CoA-propionyl-CoA-carboxylase. In addition, organisms of deep sea vents possess different forms of carbonic anhydrase important for assimilation of CO₂ [³⁰–³⁴].

Organisms that live in the environment of deep sea hydrothermal vents characterized by extreme physico-chemical conditions of temperature, pressure, pH and high concentrations of toxic heavy metals represent one of the most important sources for the development of new biotechnological applications. The biotope of hydrothermal vents harbors various and complex microbial communities adapted to different environmental conditions with unique features and characteristics and consequently these organisms could be used in biotechnology. Concerning carbon dioxide fixation and assimilation, the environment of deep sea hydrothermal vents can provide sources of unique enzymes, genes and metabolic processes important for the development of technologies related to industrial processes for reduction of atmospheric CO₂, biofuels production, materials and chemical synthesis [¹⁷,¹¹⁹–¹²²].

Carbon dioxide is the gas that is the major contributor to the green house effect and as such is largely responsible for global warming [¹²³–¹²⁵]. This gas has been extensively released during the past 100–150 years into the atmosphere due to human activities. Over the past 150 years atmospheric CO₂ concentrations have increased approximately by 30% [¹²⁶]. To overcome the effects of global warming there is an urgent need to reduce the atmospheric CO₂ content. Biotechnological methods have been used to reduce the atmospheric CO₂ content at two levels; the biological fixation using microorganisms, and the capture of carbon dioxide via enzyme (carbonic anhydrase).

Some microalgae like Cyanophyceae (blue-green algae), Chlorophyceae (green algae), Bacillariophyceae (including diatoms) and Chrysophyceae (including golden algae) are known to be very efficient in utilizing atmospheric CO₂ via photosynthesis [¹²⁷,¹²⁸]. Using genetic engineering and technology, new strains of these microalgae have been developed that can tolerate high concentrations of CO₂ [¹²⁷]. In addition, current technologies are being employed to examine the possibility of coupling wastewater treatment with microalgal growth for eventual production of biofuels [¹²⁷]. Recently, a cyanobacterium, Synechococcus elongatus PCC7942 has been genetically engineered to produce isobutyraldehyde and isobutanol directly from CO₂, increasing productivity by overexpression of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) [¹²⁹]. Isobutyraldehyde is a precursor for the synthesis of other chemicals, and isobutanol can be used as a biofuel. However, a bioreactor that is able to achieve maximum productivity and maximum energy efficiency under a given set of operational costs is not yet fabricated [¹²⁷]. A major problem with these reactors is related to low efficiency of carbon fixation using the Calvin cycle native to microalgae [¹³⁰]. In order to develop a new reactor for enhanced microalgal CO₂ fixation, it is necessary to increase the efficiency of the Calvin cycle. Genetic manipulation of RuBisCO might help to develop a new biotechnological system for large-scale carbon dioxide capture. In addition to the Calvin cycle, other CO₂ fixation pathways or carboxylase enzymes could be used. These engineering alternatives for CO₂ fixation strategies might be advantageous as they may avoid the regulatory constraints and substrate limitations of native pathways [¹³⁰]. Moreover, besides microalgae other microbes, i.e., bacteria and archaea, can also contribute to biofuel production and reduction of global warming [¹³¹]. For example, various types of bacteria that use energy obtained from chemical oxidation under dark conditions can be efficient in CO₂ fixation and can reduce CO₂ to fuel. These bacteria possess the genes that encode the key enzymes of ethanol biosynthesis from pyruvate. Several studies have showed that CO₂ may be converted to ethanol by Rhodobacter species under anoxygenic conditions in the light or under dark aerobic growth conditions [¹³²–¹³⁴]. Therefore, microbes from hydrothermal deep sea vents that can fix CO₂ into biomass could be of interest for development of the technologies for the production of biofuel as well as other compounds.

Carbonic anhydrases (CAs), the enzymes that catalyze the conversion of CO₂ to bicarbonate and the selective conversion of CO₂ to a liquid phase, can separate the CO₂ from other gases. Therefore, as a potential catalyst, CA could be used in capture of CO₂ from combustion fuel gas streams [¹³⁵,¹³⁶]. Different laboratory-scale reactors have been developed to evaluate the capture of carbon dioxide from a gas into a liquid. The capture efficiencies could be enhanced by adding base (e.g., sodium hydroxide) to form bicarbonate or carbonate, which could be further transformed into insoluble CaCO₃ by adding precipitating cations, like Ca²⁺ [¹³⁷]. CaCO₃ is a thermodynamically stable mineral found in all parts of the world, and is the main component of marine shells, snails, pearls, and eggshells. Sharma et al. [¹³⁸] screened diverse groups of bacteria and found the best activity for CO₂ conversion was obtained with a 29 kDa CA extracted from Enterobacter taylorae. Bhattacharya et al. [¹³⁹] have developed a spray reactor coated with immobilized CA for CO₂ capture and storage. They obtained a decrease in CO₂ of almost 70%, and observed stability of CA at 40 °C. Novozymes Inc has a patent application for the cloning and purification of CA for CO₂ storage [¹⁴⁰]. The cloning of CA from Methanosarcina thermophila (Archaea) was performed using the bacterium Bacillus halodurans, and expressed enzymes were then purified by chromatography. Carbon Sciences Inc. has developed a method for synthetic precipitation of calcium carbonate (PCC) that can be used for various applications, e.g., paper, medicine and plastics production, and in a technology to transform CO₂ emissions into the basic fuel building blocks required to produce gasoline, diesel, and jet fuel and other fuels. CO₂ Solution Inc. has developed a method by which CO₂ emissions from cement factories can be captured and converted into bicarbonate ions. These ions are then used to produce limestone, a raw material that can be reintroduced into the cement manufacturing process [¹⁴¹]. However, existing CAs are expensive due to high manufacturing costs, low activity and stability. The majority of enzymes exhibit very low, or no, activity when the temperature exceeds 50 °C [¹³⁴]. Most industrial processes to eliminate CO₂ occur at elevated temperatures, and immobilization techniques to retain biocatalyst activity will need to be performed at relatively higher temperatures [¹⁴²]. In the environments of deep sea hydrothermal vents, many microorganisms have adapted to high temperatures, toxic substances such as H₂S and heavy metals. For these reason, biomolecules from these organisms might be of great value in different biotechnological strategies [¹⁷]. Therefore, the exploration of carbonic anhydrases for carbon capture from these environments could be attractive for use in new biotechnological applications.

Deep sea hydrothermal vents are isolated habitats that contain many unique organisms of the three domains of life; archaea, bacteria and eukarya. Most microbial communities in these habitats have the capability to fix inorganic carbon dioxide. Five CO₂ fixation pathways have been documented as important in hydrothermal habitats; the Calvin-Benson cycle, reductive tricarboxylic acid cycle, reductive acetyl-CoA pathway, dicarboxylate/4-hydroxybutyrate cycle and 3-hydroxypropionate/4-hydroxybutyrate cycle. Four different forms of RuBisCO, designated as I, II, III and IV, operate in different microbial communities associated with deep sea hydrothermal vents. The rTCA cycle is found in the Epsilonproteobacteria and Aquificales and the reductive acetyl-CoA pathway in the methanogens microorganisms. It appears that the 3-HP/4-hydroxybutyrate is potentially an important carbon fixation pathway for archaeal communities in deep-sea hydrothermal vent environments. In addition to these pathways for the direct fixation of carbon dioxide, carbonic anhydrase catalyzes the interconversion of CO₂ and HCO₃⁻, and facilitates inorganic carbon dioxide uptake, fixation and assimilation. The bicarbonate formed by CA is an essential growth factor for microorganisms and is a metabolic precursor for many other compounds.

Human activities have significantly increased the atmospheric carbon dioxide concentration and this is an important cause of global warming. Therefore, it is of interest to find technologies for carbon dioxide capture. These technologies, combined with other efforts, could help stabilize greenhouse gas concentrations in the atmosphere and mitigate climate change. Biological CO₂ fixation has attracted much attention as an alternative strategy. It can be done by plants and by photosynthetic and chemosynthetic microorganisms. These biological technologies could also be attractive for production of biofuels or other industrial products. A variety of technological solutions have been proposed for CO₂ sequestration systems. In addition, a number of technologies are currently employed or under development to separate carbon dioxide from mixed byproduct streams of large stationary anthropogenic sources. Therefore, a variety of reactors containing an enzyme such as carbonic anhydrase have been designed to extract CO₂ from mixed gas.

In order to develop and improve new technologies, it is important to search and explore enzymes from different sources. The organisms of deep sea hydrothermal vents are well adapted to fix carbon dioxide in an unusual range of temperatures, pressure condition, pH and metal toxicity. So, organisms from the environment could be used for engineering microbes to solve the various technology options for carbon capture and storage.

We thank the anonymous reviewer for thoroughly reading the paper and providing thoughtful comments. We would like to thank Ron Verrall and Ronald Steer for the critical review of this manuscript. This review is dedicated to Lucienne Arbos Bloch, Carlo Cereceda and Jean Laporte.