Diversity of Culturable Sulfate-Reducing Bacterial Consortia and Species Capable of Hydrocarbon Degradation Isolated from Marine Environments

Abstract

This review examines the role of sulfate-reducing bacteria in the anaerobic degradation of hydrocarbons in marine sediments, where they contribute to the mineralization of organic matter under anoxic conditions. The metabolic diversity of these microorganisms is described, including their ability to degrade various classes of hydrocarbons such as short-chain (C₂–C₅), medium-chain (C₆–C₁₂), and long-chain (C₁₃–C₂₀₊) alkanes, alkenes, and aromatic compounds like naphthalene and phenanthrene. The primary mechanisms involved in the initial activation of these hydrocarbons—fumarate addition and carboxylation—are discussed, along with key enzymes, including alkylsuccinate synthase and benzylsuccinate synthase. Syntrophic interactions are also considered, particularly in which archaea initiate the oxidation of short-chain alkanes (e.g., ethane and butane), with sulfate-reducing bacteria serving as terminal electron acceptors via sulfate reduction. The potential application of these anaerobic processes in bioremediation strategies for oil-contaminated marine sediments is discussed. This microbially mediated degradation may offer a complementary approach to aerobic methods, particularly in oxygen-limited environments. Understanding the activity of sulfate-reducing bacteria activity is relevant to several areas: the development of remediation techniques for anoxic zones, the assessment of methane emissions from marine sediments, the management of microbiologically influenced corrosion, and potential biotechnological applications. Current research directions include the study of syntrophic microbial consortia and the exploration of bioelectrochemical systems.

1. Introduction

The process of sulfate reduction, which involves the conversion of sulfates into sulfides, is a crucial step in the final stage of mineralization of organic matter in marine sediments. As demonstrated by Jørgensen et al. [1], sulfate-reducing bacteria (SRB) play a significant role in regulating the carbon and sulfur cycles in marine ecosystems, being responsible for the oxidation of up to 50% of organic carbon entering the seabed. This process competes with methanogenesis for substrates, thereby regulating the flow of carbon into deeper layers of the geosphere [2]. Comprehensive global surveys have revealed that SRB are distributed worldwide across diverse marine habitats, from shallow coastal waters to the deep layers extending hundreds of meters below the seafloor [3]. The Desulfobacteraceae family stands out as particularly widespread and metabolically versatile. By converting soluble sulfates into insoluble metal sulfides (e.g., pyrite), these microorganisms are key participants in the global sulfur cycle and contribute to the breakdown of complex organic materials that have accumulated in the deep ocean, thereby maintaining ecosystem balance and nutrient recycling [4].

SRB reach their highest activity levels in organic-rich environments, with natural hydrocarbon seeps representing some of the most biogeochemically active habitats in the marine subsurface. Marine hydrocarbon seep sediments are recognized as microbial hotspots, characterized by high rates of sulfate reduction. In these environments, a substantial portion of sulfate reduction is coupled to the oxidation of non-methane hydrocarbons, including both aliphatic and aromatic compounds [5,6]. Sulfate-reducing bacteria capable of degrading alkanes [6] as well as aromatic hydrocarbons such as benzene, toluene, and naphthalene [7,8] have been identified in these environments, underscoring their collective role in anaerobic hydrocarbon degradation. One of the primary classes of organic compounds involved in this process is alkanes, which vary in carbon chain length depending on their source. Short-chain alkanes (C₁–C₄) are highly volatile and dominate natural gas—including subsea gas hydrates—where methane (C₁) is the principal component and C₂–C₄ hydrocarbons occur in minor amounts. Natural gas may be of either biogenic or thermogenic origin. In contrast, crude oil consists predominantly of medium- to long-chain alkanes (C₅–C₂₀₊), which constitute the main components of this hydrocarbon mixture [9]. The entry of alkanes into anaerobic zones occurs both due to natural seepage (cold seeps, hydrothermal fields) and as a result of anthropogenic oil spills, which makes them an important substrate for microbial communities [6].

Traditionally, aerobic oxidation was considered the dominant and most extensively studied pathway for alkane degradation in natural environments. This paradigm originated from the early characterization of aerobic hydrocarbon-degrading microorganisms and the well-understood biochemistry of oxygenases, which long served as the primary model for understanding alkane transformation in nature [10]. The process is initiated by oxygenase enzymes that catalyze the incorporation of molecular oxygen into the hydrocarbon molecule. Long-chain alkanes (C₁₂–C₂₀₊) gradually turn into alcohols, then aldehydes and finally carboxylic acids. These acids then undergo catabolism via the tricarboxylic acid cycle, releasing energy [11].

Nevertheless, anoxic conditions—typical of marine sediments—fundamentally alter the dynamics of hydrocarbon degradation. Under such conditions, SRB emerge as key players, often engaging in syntrophic associations with archaea [12,13,14]. In anaerobic marine environments with high sulfate concentrations, sulfate reduction becomes the dominant pathway for hydrocarbon oxidation, with sulfate-reducing prokaryotes accounting for up to 30% of the total microbial community biomass [15].

Two general biochemical mechanisms have been proposed for the initial activation of alkanes under anoxic conditions: addition to fumarate (yielding alkyl- or benzylsuccinates) and carboxylation. The fumarate addition pathway, mediated by glycyl radical enzymes, appears to be the most widely distributed mechanism among phylogenetically diverse anaerobic hydrocarbon degraders [16]. Analysis of available pure cultures of SRB indicates that they typically metabolize only a limited range of hydrocarbons. This substrate specificity is explained by the evolutionary adaptation of enzyme systems to specific substrate classes and carbon chain lengths, reflected in the structure of the active sites of key enzymes such as alkylsuccinate synthase (Ass) for alkanes and benzylsuccinate synthase (Bss) for aromatic compounds [17,18].

Short-chain hydrocarbons (C₁–C₄), found in high concentrations in hydrothermal vents, volcanic emissions, and oil industry facilities, are of significant scientific interest.

Scientific understanding of anaerobic alkane degradation has advanced considerably since the first demonstration of this process. For many years, the mechanisms by which microorganisms oxidize short-chain alkanes (C₂–C₅) under anoxic conditions remained poorly characterized. A major breakthrough occurred with the isolation of a sulfate-reducing bacterium capable of oxidizing propane and butane, providing the first definitive evidence that such metabolic capacity exists in anaerobic marine environments [12]. Subsequent research elucidated the underlying biochemical mechanism, demonstrating that these bacteria initiate alkane oxidation via fumarate addition, a pathway that yields characteristic alkylsuccinate intermediates [19]. Within natural habitats, these microorganisms often form syntrophic consortia; for example, anaerobic methanotrophic archaea (ANME) coupled with sulfate-reducing bacteria mitigate atmospheric methane emissions by oxidizing the gas in anoxic zones [20].

Different phylogenetic lineages of sulfate-reducing bacteria exhibit pronounced specialization in the degradation of hydrocarbons, with substrate preferences determined primarily by both molecular structure and carbon chain length. Pure culture studies have demonstrated that individual SRB strains typically metabolize a restricted range of hydrocarbons, reflecting the evolutionary adaptation of their enzymatic machinery to specific substrate [21,22,23]. For aliphatic hydrocarbons, specialization is often linked to chain length: distinct species have been isolated that selectively utilize medium-chain alkanes [21], long-chain alkanes [22], or long-chain alkenes as growth substrates [23]. This metabolic specialization is attributed to the structural configuration of key enzymes such as alkylsuccinate synthase, whose active sites have evolved to accommodate hydrocarbons within particular chain length ranges [17]. Beyond chain-length specificity, a parallel enzymatic specialization governs the degradation of aromatic hydrocarbons. Certain SRB lineages are adapted to metabolize compounds such as benzene, toluene, naphthalene, and phenanthrene through distinct mechanisms. For alkylaromatic substrates like toluene, fumarate addition is catalyzed by benzylsuccinate synthase (Bss)—a glycyl radical enzyme phylogenetically related to Ass but adapted to aromatic structures [18]. Unsubstituted polycyclic aromatic hydrocarbons, such as naphthalene and phenanthrene, are typically activated via carboxylation [8,24].

This metabolic versatility is not limited to bacteria, however. The understanding of anaerobic alkane degradation has been further expanded by discoveries involving archaea. Initially, it was assumed that all anaerobic short-chain alkane degraders were bacteria that couple complete oxidation to sulfate reduction [25,26]. However, this paradigm was fundamentally revised by the discovery of archaeal species capable of syntrophically oxidizing alkanes in partnership with thermophilic sulfate-reducing bacteria [27,28], revealing greater phylogenetic and metabolic diversity in anaerobic hydrocarbon degradation than previously recognized.

2. Anaerobic Alkane Degradation by SRB: Mechanisms, Enzymes, and Syntrophic Partnerships

2.1. Alkane Activation Mechanisms in SRB

The initial activation of alkanes during anaerobic degradation is catalyzed by sulfate-reducing bacteria. Studies have demonstrated that n-alkanes are primarily activated via the addition of fumarate, yielding the corresponding alkylsuccinic acids [29,30]. The fumarate addition pathway proceeds via homolytic cleavage of a C–H bond at the subterminal carbon of the alkane, generating an alkyl radical that adds to the double bond of fumarate. This reaction yields (1-methylalkyl)succinates as characteristic intermediates [31,32]. Subsequently, these intermediates undergo decarboxylation and beta-oxidation, ultimately channeling the carbon into central metabolic pathways [5]. Alternatively, carboxylation may serve as an activation mechanism for certain substrates [33]. The choice between these pathways—fumarate addition or carboxylation—depends on both the substrate structure (e.g., branched alkanes are more prone to carboxylation) and the phylogenetic affiliation of the degrading microorganism [34].

Notably, carboxylation of aliphatic hydrocarbons has not been documented in sulfate-reducing bacteria isolated from marine environments. To date, the only sulfate-reducing bacterial culture known to initiate anaerobic alkane degradation via carboxylation is Desulfococcus oleovorans Hxd3 [18,35]. This strain, isolated from the saline water phase of an oil–water separator in a northern German oil field, activates alkanes (C₁₂ to C₂₀) by adding a carboxyl group at the C3 position—a mechanism distinct from the fumarate addition pathway.

2.2. Key Enzymes for Anaerobic Alkane Degradation and Their Distribution

The enzymes catalyzing fumarate addition belong to the glycyl radical protein family: methylalkylsuccinate synthase (Mas) and alkylsuccinate synthase (Ass). The active component in these enzymes is the MasD domain, which shares structural similarity with the AssA domain [36].

The study by Callaghan et al. [33] on the sulfate-reducing bacterium Desulfatibacillum alkenivorans AK-01 demonstrated that the enzyme Ass, encoded by the assA1/assA2 genes, is responsible for the anaerobic breakdown of aliphatic hydrocarbons (linear alkanes and alkenes) via fumarate addition. In turn, Grundmann et al. [36] identified in the Aromatoleum sp. strain HxN1 the enzyme Mas as the key activating enzyme for anaerobic alkane degradation. It is important to note that both microorganisms are alkane degraders, despite belonging to different physiological groups (sulfate-reducing and denitrifying bacteria). The presence of functionally similar alkane-activating enzymes (Ass and Mas) in phylogenetically distant bacterial lineages is likely the result of horizontal gene transfer, as suggested by Khelifi et al. [37]. This mechanism of exchanging genes encoding key catabolic enzymes may explain the widespread distribution of anaerobic hydrocarbon degradation among phylogenetically diverse groups of prokaryotes [38].

Subsequent environmental surveys have confirmed that genes encoding these enzymes (masD/assA) are widely distributed among sulfate-reducing bacteria in marine sediments and serve as key functional markers for anaerobic alkane degradation in these ecosystems [39].

2.3. Syntrophic Interactions Between SRB and Archaea

In addition to their direct role in oil biodegradation, SRB can also form partnerships with other microorganisms involved in this process. Laso–Pérez et al. [27] and Chen et al. [13] reported the presence of archaea from the candidate genera Syntrophoarchaeum and Argoarchaeum during anaerobic oxidation of short-chain alkanes like butane and ethane. These archaea express modified versions of the enzyme M-reductase (Mcr), which is used to activate butane into butyl-CoM or ethane into ethyl-CoM. This anaerobic activation process by archaea is similar to the initial stage of methane anaerobic oxidation [40]. The butyl-CoM product is further converted to butyryl-CoA, which undergoes traditional β-oxidation to acetyl-CoA. Such syntrophic interactions involving butane oxidation have been observed under thermophilic conditions (and require cooperation with sulfate-reducing partners, such as Desulfofervidus auxilia) [27]. In contrast, archaea of the genus Argoarchaeum, involved in ethane oxidation, do not form physical aggregates with bacterial partners [13]. They coexist with bacteria from the SEEP-SRB1 lineage but occur predominantly as single cells. These archaea lack genetic markers indicative of sulfate reduction capacity, so the mechanism of electron transfer resulting from ethane oxidation remains unclear. It is hypothesized that in such systems, direct interspecies electron transfer may occur via nanowires or conductive mineral surfaces [41].

2.4. Phylogenetic Diversity of Hydrocarbon-Degrading Marine SRB

The majority of characterized hydrocarbon-degrading SRB belong to the class Deltaproteobacteria, while members of the phylum Firmicutes (order Clostridiales) are encountered far less frequently. Many cultivated SRB are affiliated with the Desulfosarcina/Desulfococcus group [12,22,42], and isolates have also been obtained from the genera Desulfotignum [43] and Desulfatiglans [7,44,45,46]. Known SRB specialized in aliphatic hydrocarbon degradation include Desulfatibacillum alkenivorans AK-01, which degrades n-alkanes (C₁₃–C₁₈) [47,48]; the marine Desulfobacteraceae clades SCA-SRB and LCA-SRB, targeting butane and dodecane, respectively [38]; and Desulfosporosinus shakirovi SRJS8ᵀ, which utilizes crude oil alkanes [49]. Representative examples of sulfate-reducing bacteria capable of degrading aromatic hydrocarbons, along with their degradation pathways and supporting references, are provided in the Table S1.

Among the uncultured lineages revealed by molecular surveys, members of the SEEP-SRB groups have been consistently detected in hydrocarbon seep sediments, yet direct evidence of their metabolic activity in situ has long been lacking. Using an innovative combination of CARD-FISH, nanoSIMS, and mathematical modeling, Kleindienst et al. [6] directly quantified in situ metabolic rates of uncultivated SEEP-SRB lineages in hydrocarbon seep sediments. Their study targeted three key groups within the Desulfobacterota: SEEP-SRB1 (Desulfobacterales), involved in complex hydrocarbon degradation; SEEP-SRB2 (Dissulfuribacterales), syntrophic partners of ANME archaea in methane oxidation; and SEEP-SRB3 (Desulfosarcinaceae), specialized in propane and butane degradation. SEEP-SRB3 exhibited the highest substrate uptake rates per cell, comparable to those of ANME archaea, confirming their pivotal role in short-chain alkane oxidation. Estimated biomass turnover times of 20–40 days revealed elevated metabolic activity within geochemical hotspots, challenging prevailing assumptions of universally slow metabolism in deep-sea microbial communities. This study provided direct evidence that uncultivated SEEP-SRB lineages are not passive constituents of the benthic microbial assemblage but rather active agents in biogeochemical cycling, substantiating their substantial contribution to sulfate reduction in hydrocarbon seep ecosystems.

3. Features of Anaerobic Degradation of Polycyclic Aromatic Hydrocarbons

3.1. Mechanisms of PAH Activation in SRB

In addition to alkanes, SRB are capable of degrading polycyclic aromatic hydrocarbons (PAH). Studies using a sulfidogenic consortium [50] and an enriched sulfate-reducing culture affiliated with Deltaproteobacteria [51] demonstrated that carboxylation is the initial step in the anaerobic degradation of phenanthrene. These studies reported the formation of phenanthroic acid, supporting the hypothesis that phenanthrene degradation proceeds via primary carboxylation, analogous to pathways described for naphthalene [52] and benzene [53]. A more recent study isolated strain PheS1, a pure culture of an SRB phylogenetically affiliated with Desulfotomaculum, which degrades phenanthrene via carboxylation to yield phenanthrene-2-carboxylic acid [8]. Himmelberg et al. [54] successfully enriched a sulfate-reducing culture (TRIP1) capable of growth on phenanthrene as the sole carbon and electron source with sulfate as the electron acceptor. The degradation of PAH is considerably slower than that of alkanes due to the high stability of condensed aromatic rings, rendering these compounds persistent organic pollutants that can remain in the environment for extended periods [24].

3.2. Pathways of Naphthalene and Methylnaphthalene Degradation

The anaerobic degradation of naphthalene and methylnaphthalene has been characterized primarily using two model strains: NaphS2 and N47 [7,55]. In methylnaphthalene degradation, the methyl group is activated through fumarate addition catalyzed by naphthylmethylsuccinate synthase, a glycyl radical enzyme [24]. This enzyme belongs to the same family as benzylsuccinate synthase (Bss), which is responsible for the anaerobic activation of toluene and other alkylaromatic hydrocarbons via fumarate addition [32,56].

Subsequently, the CoA derivatives are further degraded via reactions analogous to β-oxidation, leading to ring cleavage and formation of intermediates that enter central metabolic pathways, ultimately yielding acetyl-CoA and CO₂ [57,58,59].

3.3. Key Enzymes in Alkylaromatic Hydrocarbon Degradation

The degradation of alkylaromatic hydrocarbons requires the presence of another enzyme, benzylsuccinate synthase, which is known to be present in bacteria of the family Clostridiales [32,53]. Strain EbS7, isolated from Guaymas Basin sediments (Gulf of California), degrades ethylbenzene under sulfate-reducing conditions [45], while strains NaphS2, NaphS3, and NaphS6, obtained from North Sea and Mediterranean Sea sediments, utilize naphthalene and 2-methylnaphthalene [7,26]. Desulfobacula toluolica Tol2, a marine strain, was the first described sulfate reducer capable of the complete oxidation of toluene to CO₂ under strictly anoxic conditions [60].

3.4. Phylogenetic Diversity and Metabolic Specialization of Aromatic-Degrading SRB

SRB exhibit pronounced specialization with respect to hydrocarbon class, with certain lineages adapted to metabolize aromatic compounds such as benzene, toluene, and naphthalene. Representative examples of sulfate-reducing bacteria capable of degrading aromatic hydrocarbons, along with their degradation pathways and supporting references, are provided in the Table S1.

Unlike many aerobic hydrocarbon-degrading microorganisms, individual SRB strains rarely possess the capacity to metabolize multiple aromatic compounds with equal efficiency [60]. This metabolic specialization reflects the evolutionary optimization of enzyme complexes for specific substrate classes—a trade-off that restricts metabolic breadth but enhances catalytic performance within defined ecological niches [15].

4. The Role of Sulfate-Reducing Bacteria in Bioremediation

SRB play a crucial role in natural bioremediation processes in anaerobic environments. They can use sulfate ions as a terminal electron acceptor, enabling them to oxidize a wide range of pollutants, including petroleum hydrocarbons and toxic metals, transforming them into less harmful or inert substances [1,24]. In addition, SRB help immobilize toxic heavy metals and metalloids through their precipitation as virtually insoluble sulfides [61].

A promising application of SRB is the remediation of oil-contaminated marine and river sediments. In these environments, heavy oil fractions create anoxic zones where aerobic degradation is inhibited. The high concentration of sulfate in seawater establishes SRB as the dominant decomposers. Biostimulation strategies, involving the addition of limiting nutrients such as nitrogen and phosphorus, can significantly enhance the activity of indigenous SRB communities and accelerate the degradation of alkanes, aromatic, and PAH [25,46]. For recalcitrant compounds, bioaugmentation—the introduction of specific pre-adapted microbial consortia, often immobilized on carriers—is employed to increase treatment efficiency [62]. Optimization of conditions such as pH, redox potential, and the C:N:P:S ratio is crucial for successful biostimulation of SRB and prevention of toxic hydrogen sulfide accumulation [63].

A distinctive aspect of SRB activity is their ability to form complex syntrophic consortia based on metabolic interdependence. A canonical example is their association with anaerobic archaea-oxidizing methane. In these systems, SRB act as electron-accepting partners, consuming electrons (in the form of hydrogen or via direct interspecies electron transfer) to make the archaeal oxidation of short-chain alkanes thermodynamically feasible. This process underlies natural hydrocarbon filtration at cold seeps and can potentially be harnessed for bioremediation [13,27]. Similar syntrophic consortia involving various bacteria are responsible for anaerobic PAH degradation, where SRB support the process thermodynamics by scavenging molecular hydrogen at terminal stages [15].

The practical application of SRB in bioremediation is realized through several technological approaches. The most common and cost-effective method is in situ biostimulation, which involves enhancing the activity of indigenous SRB populations directly within contaminated soil or groundwater by amending the environment with sulfate-containing compounds, such as gypsum, together with nitrogen and phosphorus sources [64]. A more recent direction is the development of bioelectrochemical systems, in which SRB can either transfer electrons to an anode while oxidizing pollutants or accept electrons from a cathode to drive sulfate reduction. This approach uniquely combines contaminant removal with bioelectricity generation or valuable product synthesis, while enabling precise process [65]. For controlled treatment applications ex situ technologies, utilizing anaerobic bioreactors are effective for treating industrial wastewater containing sulfates, phenols, and aromatic compounds. An additional benefit is the removal of heavy metals, which are precipitated as insoluble metal sulfides. Together, these approaches demonstrate the versatility of SRB-based technologies for addressing a wide range of environmental pollution challenges [66].

The scheme below (Figure 1) illustrates the pathways of anaerobic degradation of petroleum hydrocarbons by sulfate-reducing bacteria in marine sediments, in which sulfate reduction drives the mineralization of the contaminant.

5. Discussion

Summarizing the presented material, knowledge about the wide range of metabolic capabilities of SRB is of great practical importance in the field of bioremediation. The widespread natural occurrence of aliphatic hydrocarbons and PAH identifies SRB, which are capable of the anaerobic degradation of these persistent pollutants, as crucial agents for the restoration of oil-contaminated ecosystems [67,68,69].

Previously, aerobic bacteria were considered the preferred participants in degradation processes due to their thermodynamic advantages [70]. However, the lack of oxygen limits their activity in natural hydrocarbon reservoirs [71]. While the efficiency of bioremediation can be improved through engineering solutions such as aerators, oxidizers, and dispersants [72], these methods often conflict with the core advantages of natural attenuation and environmental sustainability. Consequently, anaerobic hydrocarbon degradation mediated by SRB has been recognized as a viable and environmentally compatible alternative [72].

Sulfate-reducing bacteria, which have mastered the strategy of anaerobic hydrocarbon oxidation, are among the most intriguing and ecologically significant groups of microorganisms in modern marine ecosystems. Their study, far from complete, has evolved from a narrow taxonomic pursuit into an interdisciplinary frontier, addressing questions that span global geochemistry to biotechnological security. These organisms, predominantly belonging to the class Desulfobacteria, serve as key architects of major biogeochemical interfaces in the marine realm. In anaerobic sediments, at the confluence of sulfur and carbon cycles, they act as primary reducers, catalyzing the terminal mineralization of organic matter [73]. Their presence is essential for maintaining ecological balance and for global biogeochemical cycling.

A defining characteristic of SRB is their capacity to degrade recalcitrant hydrocarbons under anoxic conditions.

This capability is facilitated by specialized biochemical pathways, such as the activation of alkanes via fumarate addition, and is often integrated into obligate syntrophic relationships within complex microbial communities. Associations play a significant role in regulating methane emissions by modulating the flux of methane from subsurface reservoirs and gas hydrates into the hydrosphere and atmosphere. In the context of global climate dynamics, understanding the function and resilience of this natural biological filter has emerged as a topic of significant scientific importance [41].

The current state of research is characterized by a paradigm shift: from studying pure cultures to investigating complex microbial communities using systems biology approaches enabled by omics technologies (metagenomics, metatranscriptomics). This shift has begun to reveal the in situ metabolic activities of the uncultivated majority and to reconstruct metabolic networks in anoxic zones. The discovery of syntrophic partnerships between SRB and archaea, utilizing enzymes such as modified methyl-coenzyme M reductase, has fundamentally altered our understanding of carbon cycling. Concurrently, there is a pressing need to expand the collection of cultivated forms; comprehensive study of their physiology, substrate range, and stress tolerance is a crucial step towards translating fundamental knowledge into reliable bioremediation technologies [41,74].

The body of work presented in this review indicates that our understanding of sulfate-reducing bacteria and their role in anaerobic hydrocarbon degradation has expanded considerably in recent decades. Once considered a relatively specialized metabolic capability restricted to a few cultivated strains, anaerobic alkane oxidation is now recognized as a more widespread and ecologically significant process. This expanded understanding carries implications for multiple fields, including microbial ecology, biogeochemistry, and biotechnology [15,75].

Despite this progress, significant knowledge gaps remain. Due to difficulties in accessing natural samples from deep subsurface environments and the well-known challenges associated with cultivating fastidious anaerobic microorganisms, SRB remain a poorly understood group. Consequently, the majority of SRB diversity has not been characterized in pure culture, and the functional roles of these uncultivated lineages remain largely unknown. Furthermore, the regulatory mechanisms governing syntrophic interactions, including potential intercellular communication within consortia, require further investigation. Until these aspects are better understood, models of carbon and sulfur cycling in anoxic environments will remain provisional.

Methodologically, the field is characterized by the increasing integration of traditional and emerging approaches. The shift towards system-level investigation of microbial communities is complemented by the continued isolation and physiological characterization of SRB strains, which remains essential for understanding substrate ranges, stress responses, and metabolic versatility. Such foundational knowledge is a prerequisite for developing reliable bioremediation applications. More broadly, research on anaerobic hydrocarbon-degrading SRB increasingly engages multiple disciplines, including geochemistry, structural biology, and synthetic biology. Interdisciplinary approaches, together with the development of open-access databases and standardized methodologies, may facilitate progress and enable comparability across studies.

Thus, sulfate-reducing bacteria—organisms once considered to be of limited interest—are now recognized as key players in marine carbon and sulfur cycling. Continued investigation of their diversity, metabolic capabilities, and ecological interactions promises to refine our understanding of biogeochemical processes and may inform the development of biotechnological applications for environmental management.