A brief description of acidophilic bacteria and their natural environment

Acidophiles are organisms that grow optimally in an environment with a pH significantly lower than 7 (Johnson, Reference Johnson, Gerday and Glansdorff2007). The environment of moderate acidophiles is characterized by a pH in the range of 3–5, while extreme acidophiles develop at a pH less than 3 (Johnson, Reference Johnson, Gerday and Glansdorff2007). The intracellular pH of acidophilic organisms is approximately neutral (Johnson, Reference Johnson, Gerday and Glansdorff2007). The methods developed by acidophiles to maintain this pH are: (a) high impermeability of their cell membranes to hydronium ions (H₃O⁺), as well as (b) positive potential within these membranes (Johnson, Reference Johnson, Gerday and Glansdorff2007). Extreme acidophiles are strictly microorganisms and can be both prokaryotes and eukaryotes (Johnson, Reference Johnson, Gerday and Glansdorff2007), with representatives found in each of the three domains in the phylogenetic tree of life (Johnson, Reference Johnson, Gerday and Glansdorff2007). The group of acidophilic, iron- and sulphur-oxidizing organisms includes microorganisms such as (Johnson, Reference Johnson, Gerday and Glansdorff2007): Leptospirillum spp. (Leptospirillum ferrooxidans, Leptospirillum thermoferrooxidans, Leptospirillum ferriphilum, Leptospirillum ferrodiazotrophum), Acidithiobacillus spp. (Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Acidithiobacillus albertensis, Acidithiobacillus caldus), Hydrogenobaculum acidophilum (Hydrogenobaculum acidophilum, Hydrogenobaculum (H55)), and Thiobacillus spp. (Thiobacillus thioparus, Thiobacillus prosperus, Thiobacillus ferrooxidans). Acidophilic sulfur-oxidative archaea are represented by: Sulfolobus acidocaldarius, Sulfolobus metallicus, Sulfolobus tokodaii, Metallosphaera spp. and Sulfurococcus yellowstonensis.

Characteristics of the Acidithiobacillus ferrooxidans bacteria

Acidithiobacillus ferrooxidans, a genus of Gammaproteobacteria, are known for their role as sulphur and iron oxidants (Schuler and Tsuchiya, Reference Schuler and Tsuchiya1975) and are classified as Gram-negative, chemolithoautotrophs (Wang et al., Reference Wang, Long, Wang and Lau Vetter2024). This means that they can grow on inorganic substances (Lin et al., Reference Lin, Liu, Shan, Zhang and Yang2024). This ability to utilize inorganic substrates in extreme conditions, such as very low pH environments, makes them an important model for studying extraterrestrial life. These bacteria, originally isolated from a sour coal mine and described by Colmer and Hinkle (Reference Colmer and Hinkle1947), gained recognition for their unique morphological and physiological properties. Before the reclassification in 2000 (Kelly and Wood, Reference Kelly and Wood2000), Acidithiobacillus ferrooxidans bacteria were known as Thiobacillus ferrooxidans (Zhang et al., Reference Zhang, Yan, Xing, Chen, Zhang and Wang2018c). In this study 98 strains of them were listed (Table 2). Moreover, micrographs of Acidithiobacillus ferrooxidans, strain DSM 583 are presented (Figs. 1 and 2).

Table 2. Strains of the Acidithiobacillus ferrooxidans bacteria

ATCC, American Type Culture Collection; CCM, Czech Collection of Microorganisms; KCTC, Korean Collection of Type Cultures.

Figure 1. Micrograph of Acidithiobacillus ferrooxidans (DSM 583) at 12 000× magnification.

Figure 2. Micrograph of Acidithiobacillus ferrooxidans (DSM 583) at 8000× magnification.

These microorganisms do not produce spore forms (Quatrini and Johnson, Reference Quatrini and Johnson2019) and are widespread in nature (Zhang et al., Reference Zhang, Yan, Xing, Chen, Zhang and Wang2018c), inhabiting various geoclimactic natural environments characterized by low pH (Quatrini and Johnson, Reference Quatrini and Johnson2019). Across the different strains of Acidithiobacillus ferrooxidans, there are differences in oxidative ability of the same substrates, resistance to heavy metal ions, and optimal pH and temperature for growth (Ageeva et al., Reference Ageeva, Kondrateva and Karavaiko2001). These bacteria can grow in mineral rich, acid environments containing different elements (Table 3). They are found in sulphated soils and rocks as well as in mine waters containing uranium ore (Berthelot et al., Reference Berthelot, Leduc and Ferroni1993), being resistant even to high concentrations of uranium in ore (Dekker et al., Reference Dekker, Arsène-Ploetze and Santini2016). The optimal pH for most strains of Acidithiobacillus ferrooxidans is about 2 (Quatrini and Johnson, Reference Quatrini and Johnson2019). The minimum pH required for growth ranges from 1.3 to 1.5, depending on the strain of this bacterium (Johnson, Reference Johnson, Gerday and Glansdorff2007), while the typical extracellular pH values in which they grow are from 1.5 to 3 (Quatrini and Johnson, Reference Quatrini and Johnson2019). The intracellular pH of Acidithiobacillus ferrooxidans is about 6.5 (Quatrini and Johnson, Reference Quatrini and Johnson2019). Carbon necessary for the biosynthesis of cellular material is obtained by assimilating carbon dioxide (Campodonico et al., Reference Campodonico, Vaisman, Castro, Razmilic, Mercado, Andrews, Feist and Asenjo2016) from the atmosphere in the Calvin–Benson–Bassham cycle (CBB) (Gale and Beck, Reference Gale and Beck1967; Esparza et al., Reference Esparza, Cárdenas, Bowien, Jedlicki and Holmes2010; Quatrini and Johnson, Reference Quatrini and Johnson2019). These bacteria are diazotrophic organisms that can also bind atmospheric nitrogen and assimilate ammonia (Valdés et al., Reference Valdés, Pedroso, Quatrini, Dodson, Tettelin, Blake, Eisen and Holmes2008). As an alternative source for phosphorus, Acidithiobacillus ferrooxidans can use ethyl and methyl phosphonates for their growth (Vera et al., Reference Vera, Pagliai, Guiliani and Jerez2008). They can also proliferate under high pressure (Zhang et al., Reference Zhang, Hedrich, Ostertag-Henning and Schippers2018a). Acidithiobacillus ferrooxidans bacteria play an important ecological role in the bioremediation process due to their ability to decontaminate soil and industrial wastewater from heavy metals (Appia-Ayme et al., Reference Appia-Ayme, Guiliani, Ratouchniak and Bonnefoy1999). As electrotrophs, this bacterium can grow powered by electrons from cathodes and anodes, according to the following reactions (Yamanaka, Reference Yamanaka2008):

(1)$$2{\rm H}^ + { + } 2{\rm e}^-\to {\rm H}_2\quad ( {{\rm with\ the\ cathode}} ) $$

(2)$$2{\rm F}{\rm e}^{2 + }-2{\rm e}^-\to {\rm F}{\rm e}^{3 + }\quad ( {{\rm with\ the\ anode}} ) $$

Table 3. Division of Acidithiobacillus ferrooxidans bacteria into strains via bacterial activity

Acidithiobacillus ferrooxidans can also grow anaerobically (Pronk et al., Reference Pronk, Bruyn, Bos and Kuenen1992; Valdés et al., Reference Valdés, Pedroso, Quatrini, Dodson, Tettelin, Blake, Eisen and Holmes2008). Anaerobic respiration of Acidithiobacillus ferrooxidans bacteria is not only based on Fe³⁺ iron, as they can also use other electron donors derived from elemental sulphur and hydrogen (Ohmura et al., Reference Ohmura, Sasaki, Matsumoto and Saiki2002).

Metabolic processes of the Acidithiobacillus ferrooxidans bacteria

The metabolic processes of Acidithiobacillus ferrooxidans, a resilient bacterium with potential implications beyond Earth, have long been a subject of interest and curiosity among researchers. In addition, exploring the metabolic pathways of this organism could offer valuable insights into how microorganisms adapt metabolically to extraterrestrial environments, thereby positioning Acidithiobacillus ferrooxidans as a model organism for such studies (Janiczek et al., Reference Janiczek, Mandl and Ceskova1998; Nemati et al., Reference Nemati, Harrison, Hansford and Webb1998). These acidophilic extremophiles have gained attention for their metabolic prowess, particularly in the context of industrial biomining and biohydrometallurgical processes (Torma, Reference Torma, Ghose, Fiechter and Blakebrough1977; Osorio et al., Reference Osorio, Mangold, Denis, Ñancucheo, Esparza, Johnson, Bonnefoy, Dopson and Holmes2003; Yamanaka, Reference Yamanaka2008; Ai et al., Reference Ai, Liang, Miao, Chen, Zeng and Qiu2018). Due to their abilities Acidithiobacillus ferrooxidans can be used to desulphurize coal by oxidizing the pyrite contained in the carbonic substance. This process, with the use of bacteria, can be presented as follows (3) (Janiczek et al., Reference Janiczek, Mandl and Ceskova1998; Nemati et al., Reference Nemati, Harrison, Hansford and Webb1998):

(3)$$4{\rm Fe}{\rm S}_2 + 15{\rm O}_2 + 2{\rm H}_2{\rm O}\to 2{\rm F}{\rm e}_2( {{\rm S}{\rm O}_4} ) _3 + 2{\rm H}_2{\rm S}{\rm O}_4$$

The process of oxidation (bioxidation) of ferrous ions and pyrite by the ultimate recipients of the energy released is represented by chemical reactions (4) (Bevilaqua et al. Reference Bevilaqua, Garcia and Tuovinen2010; Jafari et al. Reference Jafari, Shafaei, Abdollahi, Gharabaghi and Chelgani2016) and (5) (Yamanaka, Reference Yamanaka2008; Colmer et al. Reference Colmer, Temple and Hinkle1950):

1. (a) (bio)oxidation of ferrous ion:

   (4)$$2{\rm F}{\rm e}^{2 + } + 2{\rm H}^ + { + } 0.5{\rm O}_2\to 2{\rm F}{\rm e}^{3 + } + {\rm H}_2{\rm O}$$

2. (b) (bio)oxidation of pyrite:

   (5)$${\rm Fe}{\rm S}_2 + 3.5{\rm O}_2 + {\rm H}_2{\rm O}\to {\rm F}{\rm e}^{2 + } + 2{\rm S}{\rm O}_4^{2-} + 2{\rm H}^ + $$

Acidithiobacillus ferrooxidans can also remove hydrogen sulphide (H₂S) from the environment in a two-step process (Halfmeier et al., Reference Halfmeier, Schiifer-Treffenfeldt and Reuss1993, Part 1) presented by reactions (6) and (7) (Barsoukov Reference Barsoukov2018; Bevilaqua et al. Reference Bevilaqua, Acciari, Arena, Benedetti, Fugivara, Tremiliosi Filho and Júnior2009):

(6)$${\rm H}_2{\rm S} + {\rm F}{\rm e}_2( {{\rm S}{\rm O}_4} ) _3\to {\rm S}^0 + 2{\rm FeS}{\rm O}_4 + {\rm H}_2{\rm S}{\rm O}_4$$

(7)$$4{\rm FeS}{\rm O}_4 + {\rm O}_2 + 2{\rm H}_2{\rm S}{\rm O}_4\to 2{\rm F}{\rm e}_2( {{\rm S}{\rm O}_4} ) _3 + 2{\rm H}_2{\rm O}$$

These bacteria can grow on numerous electron donor and acceptor substrates (Yarzábal et al., Reference Yarzábal, Brasseur and Bonnefoy2002a), such as elemental sulphur lumps (Espejo and Romero, Reference Espejo and Romero1987). The elemental oxidation of sulphur (S) by Acidithiobacillus ferrooxidans is illustrated by the following chemical reactions (8) (Janiczek et al., Reference Janiczek, Mandl and Ceskova1998):

(8)$$2{\rm S} + 3{\rm O}_2 + 2{\rm H}_2{\rm O}\to 2{\rm F}{\rm e}_2( {{\rm S}{\rm O}_4} ) _3 + 2{\rm H}_2{\rm S}{\rm O}_4$$

The growth kinetics of Acidithiobacillus ferrooxidans on sulphur was examined in theoretical and experimental terms and used to determine vital microbiological and stoichiometric values. The data obtained for sulphur was then compared with analogous values obtained for metal sulphides (Konishi et al., Reference Konishi, Takasaka and Asai1994). Individual strains of the bacterium show similar behaviour during their action on various sulphide minerals (Harneit et al., Reference Harneit, Göksel, Kock, Klock, Gehrke and Sand2006).

The metabolic activity of Acidithiobacillus ferrooxidans growing on reduced sulphur compounds can be studied by capillary isotachophoresis (Janiczek et al., Reference Janiczek, Mandl and Ceskova1998). The biogeochemical activity of these bacteria can also be monitored via analysis of their cellular ATP (Pakostova et al., Reference Pakostova, Mandl, Pokorna, Diviskova and Lojek2013a).

Various, simple organic compounds (e.g. formic acid – Acidithiobacillus ferrooxidans can grow in formic acid (Pronk et al., Reference Pronk, Meijer, Hazeu, van Dijken, Bos and Kuenen1991), acetic acid, urea and cysteine) on the inhibition of the growth of Acidithiobacillus ferrooxidans during the sulphur and iron oxidation processes was also observed (Tuttle and Dugan, Reference Tuttle and Dugan1976). Synergistic cooperation of Acidithiobacillus ferrooxidans with other types of bacteria was observed for instance in the process of copper bioleaching (Zheng and Li, Reference Zheng and Li2016). A cooperative interaction during sulphur oxidation between Acidithiobacillus ferrooxidans and planktonic cells (which can be characterized as free-floating microorganisms, that inhabit aquatic environments) was also noticed as a result of monitoring which bio-available substrates were delivered to planktonic cells (Pakostova et al., Reference Pakostova, Mandl and Tuovinen2013b). A cooperative interaction during sulphur oxidation between Acidithiobacillus ferrooxidans and planktonic cells was also noticed as a result of monitoring which bio-available substrates were delivered to planktonic cells (Pakostova et al., Reference Pakostova, Mandl and Tuovinen2013b).

The metabolism of microorganisms potentially present in the lower layer of Venus' clouds may be analogous to that of terrestrial microorganisms. The metabolic processes of Acidithiobacillus ferrooxidans indicate that the bacteria could potentially serve as an example of such analogue (Limaye et al., Reference Limaye, Mogul, Smith, Ansari, Słowik and Vaishampayan2018). As one of the possible sources of substrates for metabolism of Acidithiobacillus ferrooxidans, they could be particles of volcanic ash, which under earth conditions, depending on their size, can stay in the air for weeks and even travel thousands of kilometers from the eruption site (Corradini et al., Reference Corradini, Montopoli, Guerrieri, Ricci, Scollo, Merucci, Marzano, Pugnaghi, Prestifilippo, Ventress, Grainger, Carboni, Vulpiani and Coltelli2016). The atmospheric dynamics of Venus cause ash particles to be suspended in its lower cloud layer, which could serve as a potential habitat for microorganisms. It is also conceivable that sand and dust particles may play a similar role (Lorenz, Reference Lorenz2016). These particulate surfaces, which include volcanic ash, provide a conducive environment for the emergence and evolution of protocells. As templates, they facilitate the assembly of essential biomolecules such as lipids, amino acids and nucleotides, thereby promoting gradual protocell formation and development. Furthermore, their porous nature creates a protective and enclosed environment that encourages the progression towards more complex protocell structures (Ferris, Reference Ferris, Abers, Zelt, Taylor and Roecker2006).

The genome of the bacterium Acidithiobacillus ferrooxidans

Genomes of selected strains of the Acidithiobacillus ferrooxidans bacteria was completely sequenced (Valdés et al., Reference Valdés, Pedroso, Quatrini, Dodson, Tettelin, Blake, Eisen and Holmes2008; Zhang et al., Reference Zhang, Zhang, Zhao, Ni, Wang and Yan2019b; Kato et al., Reference Kato, Guo, Fujimura, Nakamura, Nishizawa, Kurusu and Ohta2022) in the same manner as the genomes of other acidophilic microorganisms, such as the archaea Thermoplasma acidophilum, Picrophilus torridus, Sulfolobus tokodaii and Ferroplasma acidarmanus (Johnson, Reference Johnson, Gerday and Glansdorff2007). Knowledge of the genome sequence of Acidithiobacillus ferrooxidans can provide information on the physiology and metabolism of these bacteria (Chen et al., Reference Chen, Yan, Wu, Xu, Li, Wang, Liang and Li2015). The genome of Acidithiobacillus ferrooxidans bacteria ranges from 2.89 to 4.18 Mb depending on the genomovar in which it exist (Zhang et al., Reference Zhang, Yan, Xing, Chen, Zhang and Wang2018c) four genomovars have been identified based on the strains of Acidithiobacillus ferrooxidans collected in different parts of the world, some of which were not attached to any of them (Zhang et al., Reference Zhang, Yan, Xing, Chen, Zhang and Wang2018c). So, for example, the ATCC 23270 strain of Acidithiobacillus ferrooxidans is characterized by a genome in the form of a single, circular chromosome containing almost 3 million base pairs (~3 Mb) with almost 60% guanine and cytosine content (Valdés et al., Reference Valdés, Pedroso, Quatrini, Dodson, Tettelin, Blake, Eisen and Holmes2008). Acidithiobacillus ferrooxidans from the DLC-5 strain have a similar genome size (~3 Mb), with a circular chromosome and a similar percentage of guanine and cytosine in the genome (Chen et al., Reference Chen, Yan, Wu, Xu, Li, Wang, Liang and Li2015). The number of proteins in the Acidithiobacillus ferrooxidans (ATCC 23270) genome encoding genes is over three thousand (Valdés et al., Reference Valdés, Pedroso, Quatrini, Dodson, Tettelin, Blake, Eisen and Holmes2008). The functional categories of the genome of Acidithiobacillus ferrooxidans (ATCC 23270) are associated with the cell envelope, transport and binding proteins and the energy of metabolism (Valdés et al., Reference Valdés, Pedroso, Quatrini, Dodson, Tettelin, Blake, Eisen and Holmes2008). In the mobile part of the genome, the presence of integrative conjugation elements (ICE), important in the process of horizontal gene transfer (Bustamante et al., Reference Bustamante, Covarrubias, Levicán, Katz, Tapia, Holmes, Quatrini and Orellana2012), was seen. Phenotypically, many strains of Acidithiobacillus ferrooxidans are similar to each other, but differentiated by the 16S rRNA gene sequence and the overall composition of DNA (Zhang et al., Reference Zhang, Sun, Kou, Zhao, Wei and Xing2018b). The transcriptomic studies, based on DNA microarray techniques, have identified a reference set of genes in the genome of Acidithiobacillus ferrooxidans: map, rpoC, alaS and era. This has allowed for better interpretation of gene-expression profiles contained in this genome (Nieto et al., Reference Nieto, Covarrubias, Jedlicki, Holmes and Quatrini2009). Individual strains of Acidithiobacillus ferrooxidans contain plasmids (Rawlings, Reference Rawlings2005; Chen et al., Reference Chen, Yan, Wu, Xu, Li, Wang, Liang and Li2015), though their presence in the genome of the bacterium was not found at all (Valdés et al., Reference Valdés, Pedroso, Quatrini, Dodson, Tettelin, Blake, Eisen and Holmes2008). Plasmids may presumably regulate the intensity of the oxidation process performed by Acidithiobacillus ferrooxidans (Ageeva et al., Reference Ageeva, Kondrat'eva and Karavaiko2003). Studies carried out on 27 strains of Acidithiobacillus ferrooxidans showed polymorphism occurring in their plasmid profiles (Kondrat'eva et al., Reference Kondrat'eva, Ageeva, Muntyan, Pivovarova and Karavaiko2002). The Acidithiobacillus ferrooxidans genome has genes encoding various forms of the RubisCO enzyme (Dekker et al., Reference Dekker, Arsène-Ploetze and Santini2016). This protein mediates the absorption of atmospheric carbon dioxide (CO₂) (Bracher et al., Reference Bracher, Whitney, Hartl and Hayer-Hartl2017), but it can also contribute to the resistance of Acidithiobacillus ferrooxidans to uranium (Dekker et al., Reference Dekker, Arsène-Ploetze and Santini2016). The oxygen reduction pathway associated with the multicentre iron respiratory chain in the bacterium Acidthiobacillus ferrooxidans (Li et al., Reference Li, Painter, Ban and Blake2015) creates a protein supercomplex (Castelle et al., Reference Castelle, Guiral, Malarte, Ledgham, Leroy, Brugna and Giudici-Orticoni2008). The proteins that create it lie inside and outside the cell membrane as well as in the periplasm, where oxygen reduction also occurs (Kai et al., Reference Kai, Fukumori and Yamanaka1989; Castelle et al., Reference Castelle, Guiral, Malarte, Ledgham, Leroy, Brugna and Giudici-Orticoni2008). In Acidthiobacillus ferrooxidans, in the process of iron oxidation, the main electron acceptor is cytochrome c, which then transfers it to another protein called rusticinin (RCy) (Hazra, Reference Hazra1992). This protein plays an important role during electron transfer in the iron respiration process of Acidthiobacillus ferrooxidans (Blake and Shute, Reference Blake and Shute1987; Ronk et al., Reference Ronk, Shively, Shute and Blake1991; Djebli et al., Reference Djebli, Proctor, Blake and Shoham1992; Hazra, Reference Hazra1992; Yarzábal et al., Reference Yarzábal, Duquesne and Bonnefoy2003). Rusticyanin is characterized by a very high oxidation–reduction (redox) potential (Barrett et al., Reference Barrett, Harvey, Sundararajan, Surendran, Hall, Ellis, Hough, Strange, Hillier and Hasnain2006). Its amino acid sequence was determined by micro-sequencing and mass spectrometry techniques based on the structural characteristics of tryptic peptides and Asp-N endoproteinase (Ronk et al., Reference Ronk, Shively, Shute and Blake1991). The operon which encodes the specific types of cytochromes c, which are proteins involved in energetic metabolism in bacteria, in the process of electron transfer (Appia-Ayme et al., Reference Appia-Ayme, Guiliani, Ratouchniak and Bonnefoy1999) during Fe²⁺ oxidation (Valkova-Valchanova and Chan, Reference Valkova-Valchanova and Chan1994) was also characterized. Computer analysis of the genomes of selected strains of Acidithiobacillus ferrooxidans showed that the synthesis of cytochrome c in these cells is dependent on the type of electron donor, which is associated with the type of medium (iron or sulphur) on which the mentioned strains grew. Acidithiobacillus ferrooxidans bacterial cells growing on iron (Fe²⁺) were characterized by a higher number of cytochrome c than those growing on sulphur (S⁰) (Yarzábal et al., Reference Yarzábal, Brasseur and Bonnefoy2002a). Identification and characterization of these cytochromes found them to be localized specifically to the outer cell membrane (Yarzábal et al., Reference Yarzábal, Brasseur, Ratouchniak, Lund, Lemesle-Meunier, DeMoss and Bonnefoy2002b). The studies on the molecular mechanisms of iron oxidation (Fe²⁺ to Fe³⁺), from which Acidthiobacillus ferrooxidans draws energy to survive, have identified the tce gene cluster responsible for coding cytochrome c and cytochrome c₅₅₂ (type cytochrome c₄) (Ai et al., Reference Ai, Liang, Miao, Chen, Zeng and Qiu2018). To investigate the reaction kinetics of electron transport between rusticyanine and cytochrome c₄ (CYC₄₁) present in Acidthiobacillus ferrooxidans, the stopped-flow spectrophotometric method and the electron paramagnetic resonance (EPR) technique were used. To determine the crystalline structure of cytochrome c₄, the multiwavelength anomalous diffraction (MAD) method, a type of X-ray crystallography, can be used (Abergel et al., Reference Abergel, Nitschke, Malarte, Bruschi, Claverie and Giudici-Orticoni2003). The energetics of metabolism of Acidthiobacillus ferrooxidans was also analysed in the context of genes encoding electron transfer proteins based on biochemical and genetic data (Appia-Ayme et al., Reference Appia-Ayme, Guiliani, Ratouchniak and Bonnefoy1999). An analysis of the physicochemical properties of cytochrome c₄ was performed, obtaining EPR spectra and absorption spectra of amino acid composition, both in the context of the strains of Acidthiobacillus ferrooxidans from which they were obtained (Giudici-Orticoni et al., Reference Giudici-Orticoni, Leroy, Nitschke and Bruschi2000). The cytochrome c₄ of this bacterium in the absorption spectrum is characterized by a Soret peak at wavelength λ = 411 nm in the oxidized state and λ = 417 nm, λ = 523 nm and λ = 552 nm in the reduced state (Cavazza et al., Reference Cavazza, Giudici-Orticoni, Nitschke, Appia, Bonnefoy and Bruschi1996). Toxin–antitoxin (TA) systems, which work by inhibiting the activity of toxic substances, are also found within the genetic material of Aciditiobacillus ferrooxidans (Bustamante et al., Reference Bustamante, Tello and Orellana2014). There are three types of TA systems (I, II and III) and they occur in the genomes of almost all bacteria, playing an important role in their survival under stress conditions (Yamaguchi et al., Reference Yamaguchi, Park and Inouye2011). TA systems whose Acidithiobacillus ferrooxidans bacteria may have a high content (28–29), e.g. type II, consist of pairs of genes responsible for toxin (stable) and antitoxin (unstable) coding (Bustamante et al., Reference Bustamante, Tello and Orellana2014). Studies (Bustamante et al., Reference Bustamante, Tello and Orellana2014) have shown that some of the chromosomally coded TA systems are part of the mobile genome of Acidithiobacillus ferrooxidans. The ‘genomic islands’ present in the genome of Acidithiobacillus ferrooxidans, which increase their resistance to copper, may also contribute to the possible competitive advantage of these microorganisms (Orellana and Jerez, Reference Orellana and Jerez2011). Acidithiobacillus ferrooxidans have two glutamyl-tRNA synthetases that can indirectly regulate haem (Valdés et al., Reference Valdés, Pedroso, Quatrini, Dodson, Tettelin, Blake, Eisen and Holmes2008). Oxidative functions in the genome of Acidithiobacillus ferrooxidans are controlled by the petI and rus operons (Valdés et al., Reference Valdés, Pedroso, Quatrini, Dodson, Tettelin, Blake, Eisen and Holmes2008). In two different strains of Acidithiobacillus ferrooxidans, ATCC 33020 and ATCC 23270, rus operons are organized in a similar manner (Quatrini et al., Reference Quatrini, Appia-Ayme, Denis, Jedlicki, Holmes and Bonnefoy2009).

Due to their genetic properties, Acidithiobacillus ferrooxidans bacteria can be an important component of the ecosystem formed in terrestrial volcanic sediments, as are Acidithiobacillus ferrooxidans bacteria representing the NFP31 strain (Kato et al., Reference Kato, Guo, Fujimura, Nakamura, Nishizawa, Kurusu and Ohta2022). The presence of genes in the genome of Acidithiobacillus ferrooxidans bacteria, as in the case of one of its strains YNTRS-40, is involved in the oxidation of both sulphur and iron and related to the process of adaptation to the environment, and also determines the good growth of these bacteria in an environment containing heavy metals and with a very low pH. These genes also allow bacteria to bio-extract metals and remove sulphur from gases (Zhang et al., Reference Zhang, Zhang, Zhao, Ni, Wang and Yan2019b). Genetic modification, through genetic engineering, of the bacterium Acidithiobacillus ferrooxidans can contribute to the enhancement of the bioleaching process in order to recover more metals (Jung et al., Reference Jung, Inaba, Jiang, West and Banta2022). Perhaps genome-altering processes in microorganisms potentially existing in Venus clouds, which may be possible analogues of the Earth's Acidithiobacillus ferrooxidans, with similar effects to genetically engineered ones, would allow said organisms to adapt to the extreme conditions in the lower layers of Venus' clouds and to possibly survive there to this day.

Acidithiobacillus ferrooxidans as possible analogues to microorganisms that potentially live in the lower part of the clouds of Venus

Conditions on Venus

Venus is one of the more geologically rugged planets of the solar system that has a very similar radius and mass to Earth (Taylor et al., Reference Taylor, Svedhem and Head2018). The atmosphere of this planet consists mainly of carbon dioxide (CO₂) – 96% and nitrogen (N₂) – 3.5%. It also contains carbon monoxide (CO) – 0.004%, noble gases: argon – 0.007% and neon – 0.0005%, sulphur dioxide (SO₂) – 150 ppm, water vapour – 30 ppm, carbonyl sulphide (carbon oxysulphide) – 4 ppm and some traces of hydrogen chloride – 0.5 ppm, hydrogen fluoride – 0.005 ppm, hydroxyl and atomic oxygen and hydrogen (Taylor et al., Reference Taylor, Svedhem and Head2018). In the lower part of the clouds on Venus, around 47.5–50.5 km from the surface, very different conditions are observed. The pressure is approximately 1 atm (~1 bar), and the average temperature is around 60°C (Limaye et al., Reference Limaye, Mogul, Smith, Ansari, Słowik and Vaishampayan2018). In Venusian clouds there is also aerosolized hydrated sulphuric acid with a concentration in the range of 75–98%, increasing with the height of the clouds from a level of 48–65 km above the surface of Venus. The particles of this aerosol are characterized by different diameters in the range of three compartments (modes): 0.4–0.6 μm (mode 1), 2–2.8 μm (modes 2 and 2′), and 7.3–8 μm (mode 3). A small number of relatively large particles with a diameter of about 35 μm are also observed (Limaye et al., Reference Limaye, Mogul, Smith, Ansari, Słowik and Vaishampayan2018). In the sulphuric acid aerosol in the lower part of the Venusian clouds, there are about 50 particles cm⁻³ with an approximate diameter in the range of 2–8 μm and about 600 particles cm⁻³ with an approximate diameter of 0.4 μm. Taking into account the environmental conditions of the planet Venus, in particular the presence of sulphuric acid in the aerosol and the associated acidity in the lower cloud layer, and the fact that Acidithiobacillus ferrooxidans are characterized by tolerance to the extremely acidic conditions prevailing on Earth, it can be assumed that the atmosphere of Venus is relatively adaptable to these organisms. Additionally, the fact that Acidithiobacillus ferrooxidans thrives at pH as low as 1.3 suggests that it could potentially survive and even metabolize inorganic sulphur compounds found in Venus' clouds (Quatrini and Johnson, Reference Quatrini and Johnson2019). Moreover, the average temperature in these cloud layers of around 60°C coincides well with the upper limits of the temperature range that some strains of this bacterium can withstand (Limaye et al., Reference Limaye, Mogul, Smith, Ansari, Słowik and Vaishampayan2018), further supporting the hypothesis that such extremophiles could possibly adapt to the harsh environments (Table 4).

Table 4. Characteristics of the Acidithiobacillus ferrooxidans bacteria and comparative analysis with Venus's lower cloud layer conditions