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Molecular Hydrogen, a Neglected Key Driver of Soil Biogeochemical Processes

Sarah Piché-Choquette, Philippe Constant
Alfons J. M. Stams, Editor
DOI: 10.1128/AEM.02418-18

ABSTRACT

The atmosphere of the early Earth is hypothesized to have been rich in reducing gases such as hydrogen (H2). H2 has been proposed as the first electron donor leading to ATP synthesis due to its ubiquity throughout the biosphere as well as its ability to easily diffuse through microbial cells and its low activation energy requirement. Even today, hydrogenase enzymes enabling the production and oxidation of H2 are found in thousands of genomes spanning the three domains of life across aquatic, terrestrial, and even host-associated ecosystems. Even though H2 has already been proposed as a universal growth and maintenance energy source, its potential contribution as a driver of biogeochemical cycles has received little attention. Here, we bridge this knowledge gap by providing an overview of the classification, distribution, and physiological role of hydrogenases. Distribution of these enzymes in various microbial functional groups and recent experimental evidence are finally integrated to support the hypothesis that H2-oxidizing microbes are keystone species driving C cycling along O2 concentration gradients found in H2-rich soil ecosystems. In conclusion, we suggest focusing on the metabolic flexibility of H2-oxidizing microbes by combining community-level and individual-level approaches aiming to decipher the impact of H2 on C cycling and the C-cycling potential of H2-oxidizing microbes, via both culture-dependent and culture-independent methods, to give us more insight into the role of H2 as a driver of biogeochemical processes.

INTRODUCTION

Several prebiotic chemistry models have hypothesized that the primary atmosphere of the early Earth was rich in reducing gases, such as molecular hydrogen (H2), carbon monoxide (CO), and methane (CH4) (1–4). The ubiquitous availability of H2, its low activation energy requirements, and its ability to diffuse through microbial cells led to the hypothesis that H2 was the first electron donor as a mean to generate an ion gradient and, consequently, ATP synthesis (5, 6). In this regard, the iron-sulfur world hypothesis stipulates that NiFe/FeS active centers similar to the active site of hydrogenase enzymes, catalyzing the interconversion of H2 into protons and electrons, could have emerged a billion years ago and have led to the first biotic sources and sinks of H2 (7). The availability of elevated mixing ratios of H2 in many ecological niches might represent a type of selection pressure maintaining this ancient metabolism within today’s microbes, including environmental isolates (e.g., newly cultivated Verrucomicrobia) (8) and human pathogens (e.g., Helicobacter pylori-associated gastric carcinogenesis) (9). Indeed, hydrogenases are well represented within the tree of life, as they are found in thousands of genomes from over 30 phyla (10, 11) (Fig. 1–3). They support autotrophic and mixotrophic lifestyles in various ecosystems encompassing soil, aquatic, and animal-associated niches (6).

FIG 1
FIG 1

Consensus tree of 25 [Fe]-hydrogenase sequences, with an alignment length of 389 amino acids. Protein sequences were imported from a study by Greening et al. (10), which included nonredundant putative hydrogenase catalytic subunits from cultured and environmental metagenomes sourced from NCBI RefSeq and Joint Genome Institute, Microbial Dark Matter (JGI, MDM) databases. More details are found in the article by Greening et al. (10). See Fig. S1 for complete taxa names and a more detailed phylogenetic tree. Hydrogenase amino acid sequences were aligned with MUSCLE (198) and clustered into phylogenetic trees using the following algorithms: maximum likelihood (Jones-Taylor-Thornton substitution model) using RAxML version 8.2.10 (199), maximum parsimony using PAUP 4.0 (200), and neighbor-joining (Jones-Taylor-Thornton substitution model) using BIONJ (201). CIPRES Science Gateway version 3.3 (202) servers were used for phylogenetic trees construction. Tree branches supported by over 50% of the 1,000 bootstrap replications were represented in the final consensus tree. The consensus tree was built using the “ape” package (203) in R (204). Branch colors represent taxonomic classification at the class level, since all 25 sequences are part of the Euryarchaeota phylum, while pie charts show the relative abundance of each class within Euryarchaeota.

FIG 2
FIG 2

Consensus tree of 1,217 [FeFe]-hydrogenase sequences, with an alignment length of 3,525 amino acids, computed as described in Fig. 1. See Fig. S2 for complete names of taxa and a more detailed phylogenetic tree. Branch colors represent taxonomic classification at the phylum level, while pie charts show the relative abundance of each phylum within hydrogenase subgroups (except for Eukaryota being a superkingdom). Dashed brackets show which tree branches belong to a specific hydrogenases group. Numbers between parentheses depict the amount of sequences within that group.

FIG 3
FIG 3

Consensus tree of 1,988 [NiFe]-hydrogenases sequences, with an alignment length of 1,850 amino acids, computed as described in Fig. 1. See Fig. S3 for complete names of taxa and a more detailed phylogenetic tree. Branch colors represent taxonomic classification at the phylum level, while pie charts show the relative abundance of each phylum within hydrogenase subgroups (except for Eukaryota being a superkingdom). Dashed brackets show which tree branches belong to a specific hydrogenases group. Numbers between parentheses depict the amount of sequences within that group.

In soils, microbial growth is often limited by the scarcity of carbon sources and nutrients (6), which translates to bursts of growth between intervals of starvation and survival. On the other hand, H2 is ubiquitous, requires a low activation energy, and can easily permeate microbial cells (6). The ubiquity of H2 is notably due to its high penetrating power through materials and its continuous abiogenic production in Earth’s upper mantle and biogenic production by fermentation, N2 fixation, and photoproduction (6). H2 oxidation can be combined with the reduction of a wide variety of oxidants, and therefore, it can be performed in most ecosystems. This combination provides a quick energy fix, via H2, to overcome those short-to-long intervals of starvation-survival. Several studies have calculated this minimal energy requirement from thermodynamic and bioenergetic perspectives (12–14).

When environmental conditions are not suitable for growth, H2 oxidation could provide energy to undergo anabiosis, i.e., long-term-stationary phase or even dormancy (6, 15). Such a lifestyle might combine starvation-survival with another survival mechanism, such as anhydrobiosis (i.e., desiccation) or cryobiosis (i.e., freezing) (16), not unlike microbes trapped in amber or frozen in permafrost. Metabolically inactive lifestyles drastically reduce energy demands, yet microbial cells still need to prevent amino acid racemization (i.e., spontaneous change of isomeric form) and DNA depurination over extended periods of time by constantly repairing cellular damage or resynthesizing DNA (13, 14, 17, 18), requiring a continuous input of energy. When organic matter and H2 abound, H2-oxidizing microbes (HOM) can switch from short- or long-term dormancy to a more metabolically active lifestyle (6).

While recent studies have shown the ubiquity and importance of H2 in natural and engineered ecosystems, this field of study has been fairly overlooked since few research groups have focused on biotic H2 dynamics at the ecosystem level (19). The aim of this review is to examine and discuss the potential role of H2 as an energy source maintaining microbial viability and activity and, in turn, sustaining soil biogeochemical processes. First, a brief overview of hydrogenase enzymes is provided, including their classification and physiological roles, with emphasis on their kinetic properties and O2 tolerance. The following section describes functional groups producing and oxidizing H2 across oxic and anoxic soil habitats. The last section integrates the biochemical properties of hydrogenases to propose succession patterns of HOM along theoretical gradients of O2 and H2 prevailing in soils, as well as their potential implications in the carbon cycle. This review concludes with research perspectives aiming to evaluate the contribution of HOM in soil biogeochemical processes.

HYDROGENASE CLASSIFICATION: AN OVERVIEW

Hydrogenases are metalloenzymes catalyzing the interconversion of H2 to protons and electrons according to the following reaction: H2 ⇌ 2H+ + 2e− (20). These enzymes are used either to provide energy, to reduce cofactors, or to disperse reducing equivalents generated through anaerobic processes (11). The conformation of electron bridges between structural subunits and the localization of the protein, membrane-bound or soluble, determine their physiological role (20). Their classification into [Fe]-hydrogenases, [FeFe]-hydrogenases, and [NiFe]-hydrogenases is based on differences between their active sites and amino acid sequences (21). These classes probably originated from converging rather than sequential evolution, since their tertiary structure, catalytic activity, and taxonomic distribution are clearly distinct (22, 23). Horizontal gene transfer is also common in hydrogenases due to their modular structure (24–27). Consequently, several microbes, such as Cupriavidus necator (phylum Proteobacteria) and Mycobacterium smegmatis (phylum Actinobacteria), have the genomic capability to express multiple hydrogenases with distinct physiological roles, resulting in an increased metabolic flexibility (28, 29).

(i) [Fe]-hydrogenases.[Fe]-hydrogenases are homodimers consisting of two 38-kDa subunits with a single Fe atom in their catalytic center (20). In contrast with [FeFe]- and [NiFe]-hydrogenases, [Fe]-hydrogenases do not contain Fe-S clusters, are restricted to hydrogenotrophic methanogenic archaea within the Euryarchaeota phylum (30) (Fig. 1; see also Fig. S1 in the supplemental material), and are not inactivated by O2 (31, 32). Under nickel-limiting conditions, [Fe]-hydrogenases work along with F420-dependent methylene tetrahydromethanopterin (methylene-H4MPT) dehydrogenases to redirect electrons originating from H2 oxidation toward H2- and CO2-dependent methanogenesis (33).

(ii) [FeFe]-hydrogenases.[FeFe]-hydrogenases are mono-, di-, tri-, or tetrameric enzymes containing an active center with two Fe atoms, called H-cluster, as well as Fe-S clusters (34). They are strictly anaerobic since their active site is denatured by O2 exposure (20). [FeFe]-hydrogenases almost exclusively produce H2 (28, 35), with a few exceptions, such as in Desulfovibrio vulgaris (phylum Proteobacteria), whose [FeFe]-hydrogenase can oxidize H2 under nickel-limited conditions to circumvent [NiFe]-hydrogenase synthesis (36). [FeFe]-hydrogenases are mostly found in anaerobic Firmicutes and sulfate reducers (Fig. 2 and S2) due to their O2 sensitivity (23, 37, 38). They consist of the only class of hydrogenases found in eukaryotes, namely, in the unicellular green alga Chlamydomonas reinhardtii (phylum Chlorophyta) (39), where they are located within membrane-limited organelles like chloroplasts and hydrogenosomes (40).

(iii) [NiFe]-hydrogenases.[NiFe]-hydrogenases are the most widespread hydrogenase class (11). Several structural and accessory genes are necessary for their maturation (41). While [NiFe]-hydrogenases can be multimeric, they contain at the very least a core heterodimeric component comprising a large α-subunit (ca. 60 kDa) and a small β-subunit (ca. 30 kDa) (20). [NiFe]-hydrogenases represent a heterogenous group of hydrogenases divided into 5 subgroups according to their phylogeny (Fig. 3 and S3) and physiology (20). Among these physiological traits, O2 tolerance and low catalytic affinity are the most common (20). Indeed, only a few subgroups are sensitive to O2, and a single subgroup has a high-affinity toward H2, i.e., it can oxidize atmospheric H2 (0.53 ppmv [42]). This classification scheme supported by concurrence between gene sequences and physiological roles of hydrogenases has been revisited recently using structural gene sequences similarity networks, suggesting a higher number of putative subclasses within each group (10, 43). Such reclassifications will not be covered here owing to the lack of experimental observations supporting their coherence with genetic, biochemical, and physiological features of hydrogenases.

(a) Group 1 [NiFe]-hydrogenases. Group 1 [NiFe]-hydrogenases (membrane-bound hydrogenases [MBH]) are H2 uptake hydrogenases attached to the cytoplasmic membrane (periplasmic side) (20). They are intimately linked to the electron transport chain by supplying the quinone pool with electrons via cytochrome b, leading to chemiosmosis and the subsequent generation of ATP (44). MBH are used for energy generation by coupling the oxidation of H2 with the reduction of several electron acceptors, such as CO2, SO42−, NO3−, Fe(III) oxides, or O2 (44). They are mostly found in Proteobacteria and Firmicutes (62.5% and 17.3%, respectively) (Fig. 3 and S3). Ancestral MBH found in methanogenic archaea and sulfate-reducing bacteria are O2 sensitive (44). In contrast, those associated with the Isp complex, in obligate and facultative autotrophs, and with HybA-MBH, in Gammaproteobacteria, have a low tolerance toward O2 (44). O2-tolerant MBH are used to perform the Knallgas reaction, i.e., oxidation of H2 using O2 as terminal electron acceptor, or other reactions involving high-potential electron acceptors such as nitrogen oxides or bacteriochlorophylls (44).

(b) Group 2 [NiFe]-hydrogenases. Group 2 [NiFe]-hydrogenases consist of cytoplasmic enzymes divided into 2 distinct functional groups, 2a and 2b (20). [NiFe]-hydrogenases encompassing group 2a are soluble uptake hydrogenases oxidizing H2 endogenously produced by N2 fixation (45) or NADH/NADPH oxidation (29) and are mainly found in Cyanobacteria and other N2-fixing bacteria (45, 46) (Fig. 3 and S3). Their role is to channel electrons to the electron transport chain and supply energy to the cell (34). They are often located inside cyanobacterial heterocysts devoid of oxygenic photosynthesis and are upregulated under anoxic conditions due to their O2 sensitivity (47). In contrast, group 2b [NiFe]-hydrogenases are H2-sensing regulatory hydrogenases (RH) acting as two-component signal transduction systems (20). The specific shape and size of the hydrophobic gas channel leading exogenously produced H2 to the active site confer O2 tolerance to the enzyme (48). When H2 is present, RH induce the transcription of group 1 and group 3 uptake hydrogenase operons in Knallgas bacteria (49, 50).

(c) Group 3 [NiFe]-hydrogenases. Group 3 [NiFe]-hydrogenases are cytoplasmic enzymes called bidirectional heteromultimeric [NiFe]-hydrogenases or soluble hydrogenases (SH) (20). Their role is to provide redox balance within the cell in both bacteria and archaea (11). The structural subunits of those enzymes are in close association with protein modules binding soluble cofactors (20). Group 3 [NiFe]-hydrogenases are O2 tolerant due to two extra cyanide ligands bound to the Ni and Fe atoms of their active site (51). These hydrogenases, mainly found in Proteobacteria (37.4%) and Euryarchaeota (27.4%) (Fig. 3 and S3), are further divided into 4 subgroups (20). Group 3a [NiFe]-hydrogenases are heterotrimeric enzymes initially found in methanogenic archaea (52), but more recently in some bacterial phyla (53), and are used to reduce the F420 cofactor. Group 3b [NiFe]-hydrogenases are tetrameric enzymes acting like bifunctional sulfhydrogenases (20). They were initially found in hyperthermophilic archaea yet have also been detected in bacterial phyla more recently (10). They reduce S0 to H2S or oxidize NAD(P)H and produce H2 to disperse reducing equivalents generated through fermentation, or, alternatively, to produce NADPH by using H2 (54). Group 3c [NiFe]-hydrogenases are heterodisulfide reductase-associated hydrogenases that catalyze the reduction of compounds such as methyl viologen and coenzyme M-disulfide bond-coenzyme B (CoM-S-S-CoB) (55) and are mostly found in methanogens and sulfate reducers. Group 3d [NiFe]-hydrogenases consist of a heteromeric hydrogenase and a NADH-dehydrogenase module that binds to NAD(P)H (20). The amount of hydrogenase subunits varies greatly, consisting of, for example, a heterodimeric enzyme in C. necator (56) or a heteropentameric enzyme in Cyanobacteria (57). These hydrogenases are used to balance the NAD+/NADH pool by reducing NAD+ using H2 (20).

(d) Group 4 [NiFe]-hydrogenases. Group 4 [NiFe]-hydrogenases or membrane-associated energy-converting hydrogenase are oxygen-sensitive (58) multimeric enzymes comprising two proteins embedded within the cytoplasmic membrane as well as hydrophilic subunits (20). Group 4 [NiFe]-hydrogenases are mostly found in methanogenic archaea, chemolithoautotrophic bacteria, and hyperthermophilic archaea (Fig. 3 and S3). In facultative chemolithoautotrophs, group 4 hydrogenases disperse reducing equivalents generated through fermentation by coupling the reduction of protons from water with the anaerobic oxidation of C1 compounds like formate or carbon monoxide (59). In acetoclastic methanogens, energy-converting hydrogenases (Ech) perform the H2-dependent reduction of ferredoxin, leading to the generation of a proton gradient through the cytoplasmic membrane and the generation of ATP (60). In methanogens without a cytochrome, group 4 [NiFe]-hydrogenases (Eha and Ehb) reduce ferredoxin and catalyze H2 oxidation, which is then coupled with the generation of a sodium ion motive force (20). Group 4 [NiFe]-hydrogenases also include energy-converting Mbh-type hyperthermophilic enzymes producing H2 in order to reestablish oxidized ferredoxin levels (61).

(e) Group 5 [NiFe]-hydrogenases. Group 5 [NiFe]-hydrogenases are O2-tolerant (62) uptake hydrogenases harboring a high affinity toward H2 enabling them to oxidize atmospheric H2 (63). Their activity was first discovered in spores of Streptomyces spp. (phylum Actinobacteria) (64). These enzymes are likely associated with the cytosolic side of the membrane through an electron acceptor, but conflicting results have been reported (62, 65). Genomic database mining demonstrated that group 5 [NiFe]-hydrogenases are mostly found in Actinobacteria (84.0%, Fig. 3 and S3) (10, 66). The proposed role of these hydrogenases is to provide a growth advantage (67) or a supplementary energy source within a survival-mixotrophic lifestyle (68) to sustain microbial seedbanks in oligotrophic environments (66). However, C. necator has a weakly expressed group 5 [NiFe]-hydrogenase that cannot oxidize atmospheric H2 (62). Indeed, biochemical properties conferring high affinity for H2 are unknown. While there is a high sequence similarity between the active site of group 5 [NiFe]-hydrogenases, such hydrogenases display a wide spectrum of affinities, e.g., Km over 1,000 ppmv in C. necator in contrast to 11 ppmv in Streptomyces sp. strain PCB7 (64). This suggests that either a lateral gene transfer in C. necator (69) or the evolution of unknown biochemical attributes in hydrogenases were found in high-affinity HOM. Alternatively, other factors, such as the currently unknown physiological electron acceptor of group 5 [NiFe]-hydrogenases (65, 70), could be responsible for their affinity toward H2. Their affinity has not been tested extensively in vitro; thus, we cannot assume that C. necator is an exception to the rule. Also, diverging phylogenetic analyses spawned confusion in hydrogenase terminology, yet group 5 (62, 66, 71) and group 1h (10) refer to the same enzymes.

OVERVIEW OF THE H2 BIOGEOCHEMICAL CYCLE

Biological sources and sinks of H2 are common in oxic and anoxic ecosystems. In anoxic ecosystems, such as wetlands, freshwaters under the chemocline, marine sediments, and animal gastrointestinal tracts, H2 is mainly produced as a reaction intermediate of organic matter degradation by organisms from the three domains of life. The main biological sinks of H2 in anoxic ecosystems are acetogens, methanogens, sulfate-reducing microbes, iron oxide-reducing microbes, and nitrate-reducing microbes (35). In oxic environments, such as upland soils, freshwaters above the chemocline, and open oceans, H2 is mainly generated by N2 fixation and is consumed by Knallgas bacteria and high-affinity H2-oxidizing bacteria (35). Due to its energetic potential, H2 is quickly consumed by microbes within the same microenvironment, indicating that H2 production is likely the limiting step of the H2 biogeochemical cycle (33). Microbial functional groups responsible for H2 production and utilization in anoxic and oxic ecosystems will be presented in the following subsections to showcase their vast diversity and their involvement in the C biogeochemical cycle.

(i) H2 production in anoxic ecosystems.In anoxic ecosystems, photoproduction (or biophotolysis), dark fermentation, and photofermentation are responsible for most of the H2 production (35).

(a) Photoproduction. Photoproduction of H2 in aquatic sediments is performed by unicellular algae such as Chlorella fusca (phylum Chlorophyta), Tetradesmus obliquus (formerly Scenedesmus obliquus; phylum Chlorophyta) (72), and Chlamydomonas reinhardtii (73), as well as Cyanobacteria. In soils, several purple nonsulfur bacteria perform photoproduction, including Rhodobacter capsulatus (phylum Proteobacteria) (74) and Rhodopseudomonas rutila (phylum Proteobacteria) (75). This process occurs under oxygen and CO2 limitation (76, 77). Light-excited electrons are transferred across photosystems to reduce ferredoxin, and then ferredoxin is reoxidized in order to produce H2 via an [FeFe]-hydrogenase (78). This process maintains the electron flow within the electron transport chain to resume oxygenic photosynthesis as soon as better conditions arise (77), such as changing tides or the next photoperiod.

(b) Photofermentation and dark fermentation. Unicellular algae such as Fragilariopsis spp. (phylum Bacillariophyta) and Pyramimonas spp. (phylum Chlorophyta) (78), strict anaerobes like Clostridium acetobutylicum (phylum Firmicutes) (79), and even facultative anaerobes like Escherichia coli (phylum Proteobacteria) (80) from both aquatic and terrestrial environments can produce H2 through dark fermentation. Glycolysis, followed by the decarboxylation of pyruvate into acetyl-coenzyme A (acetyl-CoA), reduces ferredoxin/NAD+ into oxidized ferredoxin/NADH, supplying electrons to a hydrogenase and generating H2 by their reoxidation (78, 81, 82). Alternatively, the conversion of CoA and pyruvate into formate and acetyl-CoA followed by the cleavage of formate also leads to the generation of H2 and CO2 (83). Oxidized ferredoxin and NADH are regenerated by discarding reducing equivalents, and glycolysis can generate ATP anew. Photofermentation is performed by purple nonsulfur bacteria (e.g., Rhodospirillum rubrum [phylum Proteobacteria] or Rhodobacter sphaeroides) that use light energy to convert organic substrates, like lactic acid, into H2 and CO2 (83). These processes can be performed sequentially, given that there are dark and light cycles. According to genomic surveys and culture-dependent measurements, in aquatic environments, H2-evolving fermenters include unicellular algae, Firmicutes, Bacteroidetes, Spirochaetes, and Proteobacteria (78, 84–86). In anoxic soils, such as paddy fields, molecular surveys targeting [FeFe]-hydrogenases have shown that Firmicutes (mostly Clostridia), Proteobacteria (mostly Deltaproteobacteria), and Chloroflexi are the most abundant H2 producers (87, 88). Similarly, in a moderately acidic fen slurry enrichment, fermenters were identified as Firmicutes (Clostridia and Negativicutes) and Deltaproteobacteria (89).

(c) Nitrogen fixation. Various microbes, such as Clostridium spp. and Bacillus spp. (phylum Firmicutes) as well as methanogenic archaea and Cyanobacteria can fix N2 and consequently produce H2 in anoxic or microoxic environments (90, 91). Since current estimates suggest that most of the biological nitrogen fixation is performed in oxic ecosystems (92), especially in surface waters (93, 94) and in the rhizosphere of leguminous and actinorhizal plants (95), it will be discussed in the next section.

(ii) H2 production in oxic ecosystems.Nitrogen fixation. The main biological process responsible for H2 production in oxic ecosystems is biological N2 fixation (BNF). BNF is performed by the nitrogenase complex according to the following reaction (96–98):N2+8H++8e−→2NH3+H2 (1)

BNF requires a tremendous amount of energy, i.e., a minimum of 16 ATP per fixed N2 molecule (2 ATP per electron transferred [98]); thus, it is strictly regulated and used only in the absence of bioavailable N (99). Since nitrogenases are irreversibly inactivated by O2 (98, 100), microbes can avoid O2 via temporal or spatial separation strategies, as follows: separation of N2 fixation and oxygenic photosynthesis (100), avoidance of O2, high respiration rates, and the use of O2 transporters to buffer local O2 concentrations (98, 101–103). H2 is an obligate by-product of the BNF reaction, requiring approximately 35% of the electron flux used toward BNF (104) and leading to local accumulations in environments such as the legume rhizosphere (105, 106). BNF and hydrogen oxidation are often linked due to their complementary outcomes, since H2 oxidation coupled to O2 reduction recovers part of the ATP used toward N2 fixation and leads to a reduction in O2 pressure and H2 buildups, both of which would prevent nitrogen fixation (46, 107). Furthermore, three BNF strategies are used by microbes, including symbiotic N2 fixation (99, 108), plant-associated N2 fixation (109), and free-living N2 fixation (97, 110). Unlike other nitrogen fixers, most leguminous plants symbionts do not possess the genetic capability to recycle H2 produced through nitrogen fixation (i.e., absence of Hup activity) (111). Studies have hypothesized that the additional energy input provided by H2 exerts a fertilization effect promoting growth of the host plant (112, 113).

(iii) H2 uptake in anoxic ecosystems.In anoxic ecosystems, numerous microbial functional groups use H2 as an electron donor to reduce terminal electron acceptors according to their availability and underlying thermodynamic constraints (33). This section discusses some of them in increasing redox potential order, from acetogenesis to denitrification, although other electron acceptors can be coupled to H2 oxidation, such as MnO2 and CrO4. Many of those microbes are facultative anaerobes, yet these processes are only performed in anoxic ecosystems since the reduction of O2 would be favored thermodynamically over alternative electron acceptors. These functional groups are not mutually exclusive, i.e., numerous bacteria and archaea are able to perform H2 oxidation using more than one electron acceptor.

(a) Acetogens. Acetogenesis is performed by several strict anaerobes from the bacterial and archaeal domains of life (114). Acetogens are found in a wide variety of ecosystems: wetlands (115), the gastrointestinal tract of animals (116, 117), oceans, subsurface sediments (118), hypersaline waters (119), and oxic soils (120). While the latter sounds counterintuitive, several acetogens and methanogens can survive in the presence of O2 due to different mechanisms (121, 122). Most acetogens, namely, Clostridium spp., are comprised within the Firmicutes (123), yet some are also found among other phyla, including the archaeal phylum Bathyarchaeota (114). The acetogenesis reaction (equation 2) follows the Wood-Ljungdahl pathway to convert C1 inorganic compounds (CO2) into C2 organic compounds (acetate) (124):2CO2+4H2⇌CH3COO−+H++2H2O (2)

Acetogens scavenge H2 generated through the degradation of organic matter. Acetogens face fierce competition from methanogens, methanogens having a lower threshold for H2 utilization and a higher energy yield (125, 126). Acetogens can overcome methanogens in some ecosystems, such as in the termite gut (127), since the production of acetate consists of an ecological advantage in its animal host. Acetogens are inherently involved in H2 and C-cycling processes by coupling the utilization of CO2 and H2. In turn, this leads to the generation of acetate, a bioavailable C source for the surrounding microbial community. As strict or facultative anaerobes, acetogens can also use dissolved organic matter (DOM, also known as labile) through fermentation and produce their own H2 and CO2. Additionally, some of them, e.g., Clostridium carboxidivorans and Clostridium drakei (128, 129), are also involved in CO oxidation to CO2, crystalline cellulose and hemicellulose degradation, polymeric organic matter (POM, also known as recalcitrant) turnover (130), and humic acid decomposition (131).

(b) Methanogens. Methanogenesis is only performed by methanogenic archaea, which are common in wetlands, in the intestinal tract of animals, in extreme environments, such as hot springs and hydrothermal vents (132), and even in oxic soils (120). Methanogens consist of the only functional group harboring [Fe]-hydrogenases, which enables them to use H2 as a reducing agent for the ferredoxin-dependent reduction of CO2 to CH4, according to the following simplified reaction (133, 134):CO2+4H2→CH4+2H2O (3)

Methanogens thrive in ecosystems or layers within those ecosystems where electron acceptors with higher redox potential, such as sulfate and nitrate, are depleted due to the low redox potential of CO2 (33). They scavenge CO2 and H2 derived from the degradation of organic matter, which they can ferment on their own. Hydrogenotrophic methanogens using electron bifurcation are considered obligate autotrophs (135). Methylotrophic methanogens using oxidative phosphorylation consume a wider variety of substrates (136). Recent studies, however, have shown that hydrogenotrophic methanogens such as the marine methanogen Methanococcus maripaludis (phylum Euryarchaeota) (137) might in fact have a greater flexibility than initially thought by using a combination of CO and formate rather than only CO2 and H2. The assumption that methanogens are strict anaerobes has also been challenged with the discovery of “Candidatus Methanothrix paradoxum” (phylum Euryarchaeota) (nonhydrogenotrophic) (138). The involvement of methanogens in the C cycle also includes their ability to reduce humic acids, e.g., in Methanosarcina barkeri (phylum Euryarchaeota) (139), which temporarily decreases their methanogenesis potential. Studies even suggest that anaerobic methanotrophic archaea (ANME) could be methanogens as well (140, 141).

(c) Sulfate-reducing bacteria and archaea. Sulfate-reducing bacteria (SRB), such as Desulfovibrio spp. and Desulfotomaculum spp. (phylum Firmicutes), as well as sulfate-reducing archaea (SRA), such as Archaeoglobus spp. (phylum Euryarchaeota), are anaerobic microbes coupling the oxidation of H2 (or organic compounds) to the reduction of sulfate (SO42−) into sulfide compounds (142, 143).

While they are found in oxic environments, such as coastal sediments (144), sulfate-reducing microbes are more common in marine and freshwater sediments, deep subsurface environments, and the gastrointestinal tract of animals (143). They are also found in acid mine drainages and alkaline environments, such as soda lakes (143). H2-utilizing sulfate reducers can outcompete acetogens and methanogens within the same ecosystem due to their higher growth kinetics (143). Species of Desulfitobacterium (phylum Firmicutes) are also able to perform the reduction of humic acids and polyaromatic compounds (145, 146). Studies have also shown their potential ability to fix CO2 (147, 148) and oxidize CO (149). Sulfate-reducing microbes are thought to be the main drivers of carbon cycling in marine sediments (143). H2-oxidizing SRB also act as syntrophs in consortia performing the anaerobic oxidation of CH4 (150).

(d) Iron oxide reducers. Iron oxide-reducing bacteria and archaea [Fe(III) reducers] are found in soils, acid mine effluents (151), aquatic sediments (152), hot springs (153), and subsurface ecosystems. Since Fe(III) is insoluble in water, microbes need direct contact with Fe(III) to reduce it (154). Microbes such as Shewanella spp. (phylum Proteobacteria), Geobacter spp. (phylum Proteobacteria), and Geothrix spp. (phylum Acidobacteria) can produce chelators to solubilize Fe(III) in water or even produce electrically conductive bacterial nanowires to reach it (154). They use Fe(III) as an electron acceptor for the oxidation of organic matter or H2 (155–157). Many Fe(III) reducers use other electron acceptors, such as O2, SO42−, NO3−, or even CO and CO2 (139, 155). Those Fe(III) reducers include bacteria, such as Geobacter spp. and Shewanella spp. (152, 155), but also methanogens (139) and other archaea, like Pyrobaculum islandicum (phylum Crenarchaeota) (158). Fe(III)-reducing processes are more efficient at scavenging H2 than are sulfate-reducing processes (159). Several Fe(III) reducers can use humic substances as electron acceptors to anaerobically oxidize organic compounds (e.g., acetate produced by acetogens) as well as H2 to support their growth (160). Such electron transfers also chelate Fe(III) oxides and make them bioavailable to Fe(III) reducers such as Geobacter metallireducens, which also contribute to pollutant degradation (e.g., benzene) (160).

(e) Denitrifying bacteria and archaea. Denitrifying bacteria and archaea couple the reduction of nitrate (NO3−) and subsequently reduced nitrogen compounds to the oxidation of H2 (or organic compounds) (161). Denitrification is performed in aquatic and terrestrial ecosystems mostly by heterotrophic microbes, such as Paracoccus denitrificans (phylum Proteobacteria), but is also quite common in autotrophic microbes, such as Thiobacillus denitrificans (phylum Proteobacteria), or within the Hydrogenophilales order (phylum Proteobacteria) (162–164). It is the most energetically favored anaerobic respiration process and is used by a wide variety of microbes (10, 33). Since it is performed in numerous sequential reduction reactions, typically NO3− → NO2− → NO → N2O → N2, many denitrifying microbes do not possess the whole metabolic machinery to perform complete denitrification. Within a single ecosystem, different microbial strains often complement their denitrifying capabilities for more efficient energy retrieval and avoidance of toxic N intermediates (165). Pseudomonas spp. (phylum Proteobacteria) can perform denitrification as well as POM degradation, including degradation of lignin (166) and aromatic compounds (167). Like SRB, some denitrifying bacteria are also syntrophs within AOM consortia (168).

(iv) H2 uptake in oxic ecosystems.In aerated soils, the oxidation of elevated H2 mixing ratios is performed by Knallgas bacteria, whereas high-affinity H2-oxidizing bacteria (HA-HOB) are able to oxidize much lower H2 mixing ratios, such as atmospheric H2 (ca. 0.53 ppmv [42]).

(a) Knallgas bacteria. Knallgas bacteria are aquatic and terrestrial obligate or facultative chemolithoautotrophs that use O2 as a final electron acceptor to oxidize H2 according to the Knallgas reaction:H2+O2⇌H2O (4)

Model Knallgas microbes include C. necator (formerly Ralstonia eutropha), which possesses 4 [NiFe]-hydrogenases from groups 1, 2, 3, and 5 (28, 62, 169), yet not all Knallgas bacteria have several hydrogenases. While C. necator possesses a group 5 [NiFe]-hydrogenase, it cannot oxidize atmospheric H2 mixing ratios (62) and is thus not a HA-HOB, as opposed to Mycobacterium smegmatis (65). When H2 concentrations are sufficiently high, C. necator fixes CO2 and oxidizes H2 as its sole energy source using its group 1 [NiFe]-hydrogenase (51). The group 3 [NiFe]-hydrogenase is rather used to maintain redox balance within the cell by coupling H2 oxidation to NAD+ reduction (28). To avoid wasting energy by using group 1 and 3 hydrogenases constitutively when H2 mixing ratios are low, C. necator also produces a group 2 hydrogenase acting as a sensor regulating the transcription of structural and auxiliary genes of the two other hydrogenases (28). Finally, the group 5 hydrogenase generates ATP at a lower rate than the MBH (group 1) but is entirely insensitive to O2 (62). Other Knallgas bacteria have similar H2 uptake mechanisms and encompass various phylogenetic backgrounds (e.g., Proteobacteria, Firmicutes, Cyanobacteria, Chloroflexi, and Acidobacteria). Furthermore, many N2-fixing microbes, namely, the actinorhizal plant symbionts Frankia spp. (phylum Actinobacteria) and Cyanobacteria, can oxidize H2 produced through N2 fixation (170). Various Knallgas bacteria are also known plant growth-promoting rhizobacteria (PGPR) (112, 171), while many more perform crucial C-cycling steps. Several Knallgas bacteria can break down polymeric compounds. For instance, Bacillus pseudofirmus and Bacillus thermotolerans can reduce humic acids when oxidizing Fe(III) oxides (172, 173), while Burkholderia spp. (phylum Proteobacteria) can break down lignin (166). Furthermore, many methanotrophs, such as Methylosinus trichosporium (phylum Proteobacteria) and Methylacidiphilum fumariolicum (phylum Verrucomicrobia), can oxidize H2 and CH4 under oxic conditions (174, 175) and are thus also Knallgas bacteria. Various Knallgas bacteria, such as Hydrogenomonas facilis (phylum Proteobacteria) (176) and C. necator (177), can also fix CO2 or oxidize CO (178).

(b) Atmospheric H2 oxidation. The oxidation of atmospheric H2 has long been considered to originate from abiotic enzymes due to the failure to isolate microbes responsible for this process at low H2 partial pressure (179, 180). Bacteria responsible for the oxidation of H2 at low mixing ratios have only been recently isolated (64), hence their small representation in the scientific literature. Recent genomic database surveys (10) have shown that most microbes possessing group 5 [NiFe]-hydrogenases are Actinomycetes, such as Streptomyces spp. and Mycobacterium spp. Only a few high-affinity H2-oxidizing microbes have been isolated and characterized so far, including Rhodococcus equi (phylum Actinobacteria) (181), various Streptomyces spp. (64, 68, 181), and Mycobacterium smegmatis (65). In this regard, the role of group 5 [NiFe]-hydrogenases within the cell is not understood. HA-HOB are mostly found in soils, and they represent a major (70%) sink of atmospheric H2 (182). While atmospheric H2 provides substantial amounts of energy, it is insufficient for bacterial growth (63). So far, HA-HOB have been shown to use H2 as an additional energy source for enhanced growth and survival (67, 181) or within a mixotrophic lifestyle called survival mixotrophy (68). HA-HOB are also involved in C cycling. For instance, Rhodococcus jostii and Rhodococcus equi can degrade lignin and several other POM compounds (166, 183). Streptomyces species are also known to perform the breakdown of humic acids and even coal, thus contributing to POM cycling (184–186). Other Actinobacteria, such as Mycobacterium smegmatis, can also oxidize atmospheric CO and H2 (187). Candidate taxa might also have the genetic potential to oxidize atmospheric H2 and perform CO2 fixation (188).

JUXTAPOSITION OF THE H2 AND C BIOGEOCHEMICAL CYCLES IN SOILS ALONG THEORETICAL O2 GRADIENTS

A wide variety of HOM can hydrolyze DOM but also take part in the breakdown of POM, including cellulose, hemicellulose, lignin, humic substances, and polycyclic aromatic compounds (189), and also catalyze the oxidoreduction of CO, CO2, and CH4 (Fig. 4). HOM are able to switch between different modes of nutrition, both in terms of carbon and energy sources (6), which confers them great metabolic versatility when subject to various C inputs. Many of these C-cycling processes also lead to the generation of C sources usable by the broader microbial community, such as acetate. Moreover, in microenvironments rich in H2, e.g., the legume rhizosphere, termite gut, and serpentinizing systems, metabolites released by HOM might also prime the decomposition and turnover of organic matter, namely, in soils. Recent studies have shown that the exposure of soil to elevated H2 concentrations representative of those found in nature (i.e., 10,000 ppmv) increased the amount of C sources used, as well as the intensity of their consumption within a defined time period (190, 191). However, it is not known if HOM involved in H2 oxidation are responsible for this priming-effect-like response due to their mixotrophic growth strategy or if positive interactions between HOM and heterotrophic non-HOM led to it. With this in mind, it could be expected that other C-cycling processes performed by HOM, such as decomposition, humification, and hydrolysis, could also increase in H2-rich ecosystems (Fig. 4).

FIG 4
FIG 4

Juxtaposition of the carbon cycle and main H2-oxidizing functional groups. HOM contribute to all key steps of the C cycle. All numbers below microbial functional groups consist of reactions performed by these groups. While the oxidation of CH4 can be performed in anoxic ecosystems, the anaerobic oxidation of CH4 (AOM) is not performed by HOM, only by their syntrophic bacterial partners (i.e., sulfate- or nitrate-reducing bacteria). H2-utilizing processes occur in a thermodynamically favored fashion according to available substrates, where O2 is used first, followed by nitrate, iron oxides, sulfate, and carbon dioxide, as shown in the gradient at the bottom of the figure. The only key C-cycling process missing in oxic ecosystems is methanogenesis, yet residual CH4 diffuses from anoxic to oxic layers in various ecosystems. An exception to this is that nonhydrogenotrophic methanogens are also active in oxic layers (138), thus providing CH4 directly to oxic ecosystems.

As for trace gases, a study has shown that atmospheric CO and H2 deposition velocities are positively correlated in both arable and forest soils (192, 193). While the initial hypothesis was that diffusion was ultimately responsible for this response, a more recent study has shown that H2, CO, and CH4 oxidation potential by microbes covaries as well. Indeed, along an increasing gradient of H2 exposure from 0.5 to 10,000 ppmv H2, low-affinity H2 oxidation increased, net CO2 production decreased, and high-affinity H2, CO, and CH4 oxidation decreased (194). CO2 production results were not published but were measured at the same time and with the same instrument as the other trace gases (194). Taken together, those studies imply that ecosystems rich in H2 could be hubs of DOM and POM cycling; HOM oxidize H2 preferentially if available at elevated mixing ratios instead of using other trace gases.

While potential activity does not guarantee that in situ measurements will show the same behavior, examples of H2-driven C cycling exist. Two studies have shown that CO2 dynamics in soil systems continuously fed with air or air supplemented with H2 change from net CO2 production to net CO2 fixation during H2 exposure (195, 196). A recent metagenomic survey has hypothesized similar outcomes in Antarctic desert soils (188). Fixed CO2 would increase the input of organic carbon in the system, which can then be distributed to the rest of the microbial community after cell death or production of secondary metabolites. Responsible strains in these studies have not been identified, yet several Knallgas bacteria are known to oxidize H2 and fix CO2. While these hypotheses are applied to H2-oxidizing CO2-fixing microbes, it is expected that H2-oxidizing methanotrophs could also favor certain metabolic pathways in the presence of H2 by decreasing CH4 oxidation and thus increasing net C input by methanogens (Fig. 4) (194).

While HOM are involved in various C cycling processes, their relative contribution to each of them is unknown. Two different types of approaches could be used to increase our understanding of H2-C dynamics, community-level and individual-level approaches. As for community-level approaches, a good starting point would be to expose soil (i.e., microcosms, mesocosms, or even fields) to a baseline mixing ratio of H2 (e.g., 0.53 as control [42]), as well as to lower and higher mixing ratios representative of natural conditions. Soil H2 exposure could be provided by synthetic gas mixtures or through the manipulation of N2-fixing H2-producing rhizobia (with or without Hup activity as isogenic control and treatment strains, respectively) recruited by legume plants. This would enable a direct comparison between H2 availability and C turnover. As for individual-level approaches, three experiments could be performed, as follows: culturing novel HOM, single-cell sequencing of novel uncultured HOM, or analyzing metagenome-assembled genomes (MAGs) of uncultured HOM. Traditional cultivation would allow for a more direct testing of HOM C-cycling capabilities, while culture-independent methods would give us more insight into their overall versatility. Furthermore, several H2 and C-cycling processes seem to covary in soil, such as atmospheric CH4 and elevated H2 oxidation, yet it is not known if these processes are performed by the same microbes or simply within the same ecological niches. The three proposed individual-level methods could help solve this conundrum, all the while reducing current biases in genomic databases. For example, some taxa have more sequenced representatives and are consequently more represented in databases regardless of their involvement in the cycle (e.g., Actinomycetales and high-affinity H2 oxidation). Further work in these directions would shed light on the importance of the overlap between the C and H2 cycle at the species and ecosystem level as well as provide further insights into the overall biogeochemical potential of HOM. In turn, this knowledge could be used to develop techniques that take advantage of H2 hot spots in soils, e.g., for potential increases in C turnover or for agricultural benefits, namely, in legume rhizosphere rich in H2 (197). Last, this review exemplifies the idea that HOM, as a functional group, is a reductionist concept. That is, HOM should not be reduced to their H2-oxidizing function; they are highly diverse, and many of them perform crucial ecosystem functions that could be more important at the ecosystem level than their individual energy-retrieving potential.

FOOTNOTES

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KEYWORDS

H2 oxidation
anaerobic processes
biogeochemical processes
carbon cycle
environmental microbiology
hydrogen
soil

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