Dissimilatory sulfate reduction is a form of anaerobic respiration where certain microorganisms generate energy by using sulfate as the terminal electron acceptor for the breakdown of organic compounds. This process differs from assimilatory sulfate reduction, where organisms absorb sulfate to synthesize sulfur-containing molecules like amino acids, not for energy production. The primary byproduct of the dissimilatory process is hydrogen sulfide, which is released into the environment.
The Microorganisms and Their Habitats
The key players in dissimilatory sulfate reduction are a diverse group of microbes known as sulfate-reducing microorganisms (SRMs). This group is not defined by a single lineage but includes various species of both bacteria and archaea. Examples include bacteria from phyla such as Proteobacteria and Firmicutes, as well as archaea like Archaeoglobus fulgidus. These organisms are obligate anaerobes, meaning they cannot survive and multiply in the presence of oxygen.
They are common in aquatic sediments, both in marine and freshwater systems, where organic matter settles and oxygen is quickly depleted. Salt marshes and mudflats are classic examples, where the activity of SRMs is often noticeable. Beyond these natural settings, SRMs also colonize man-made anaerobic environments. These include wastewater treatment plants, oil fields, and even within the complex microbial communities of the human gut.
The common thread linking these disparate locations is the availability of sulfate and the lack of oxygen. Seawater, for instance, is rich in sulfate, making marine sediments a prime location for this process. In these environments, SRMs play a part in the decomposition of organic material that sinks to the seafloor.
The Biochemical Respiration Process
The process of dissimilatory sulfate reduction is a multi-step biochemical pathway. It begins with the uptake of sulfate (SO₄²⁻), which is chemically stable and must be “activated” by the cell. The enzyme ATP sulfurylase accomplishes this by attaching the sulfate to an adenosine monophosphate (AMP) molecule, forming adenosine 5′-phosphosulfate (APS). This activation makes the sulfur atom accessible for the subsequent reduction steps.
Once activated, the APS molecule is acted upon by another enzyme, adenylyl-sulfate reductase (Apr), which reduces the sulfur atom. This step transfers two electrons to the APS, converting it into sulfite (SO₃²⁻) and releasing the AMP molecule. The sulfite is then the target of the final and most significant energy-releasing step in the pathway.
The final stage is catalyzed by the enzyme dissimilatory sulfite reductase (Dsr). This complex enzyme facilitates the transfer of six electrons to the sulfite molecule, reducing it to sulfide (S²⁻). When this sulfide combines with protons (H⁺) in the cell, it forms hydrogen sulfide (H₂S). Throughout this pathway, electrons are shuttled from an electron donor, which can be simple organic molecules or hydrogen gas (H₂), to the sulfur compounds, powering the cell’s metabolic machinery.
Function in Global Biogeochemical Cycles
Dissimilatory sulfate reduction is a significant driver of global biogeochemical cycles, particularly those of sulfur and carbon. In the sulfur cycle, SRMs perform a transformation by taking sulfate, the most oxidized and abundant form of sulfur in the oceans, and reducing it to sulfide. This conversion is a major flux in the global sulfur budget, moving sulfur from the vast oceanic reservoir into a more reactive form that can precipitate with metals or be re-oxidized by other microbes.
This microbial process also has a substantial impact on the carbon cycle, especially in marine sediments. In anaerobic environments, SRMs are major contributors to the decomposition of organic matter. It is estimated that in many coastal marine sediments, dissimilatory sulfate reduction is responsible for up to 50% of the total mineralization of organic carbon. This process breaks down complex organic molecules, releasing carbon back into the environment.
By linking the sulfur and carbon cycles, SRMs influence the chemistry of the oceans and sediments. Their activity ensures that elements locked away in organic material in oxygen-free zones are recycled.
Industrial and Ecological Consequences
The primary byproduct of dissimilatory sulfate reduction, hydrogen sulfide (H₂S), has significant consequences for both industrial operations and natural ecosystems. One of the most costly impacts is microbially influenced corrosion. In industries such as oil and gas, SRMs can colonize the surfaces of metal pipelines. Their production of H₂S creates a highly corrosive environment, leading to the rapid degradation of iron pipes and infrastructure, a process known as biocorrosion. This can result in equipment failure, leaks, and substantial economic losses.
In natural settings, the presence of H₂S can have noticeable and sometimes harmful effects. The gas is known for its strong “rotten egg” smell, which is a common indicator of SRM activity in stagnant ponds, salt marshes, and mudflats. Beyond its unpleasant odor, hydrogen sulfide is toxic to many organisms, including fish and other aquatic life, by inhibiting cellular respiration. The black color of anaerobic sediments is also a direct result of DSR, as the H₂S produced reacts with iron and other metals in the sediment to form dark, insoluble metal sulfides like ferrous sulfide (FeS).
Despite these negative effects, the activity of SRMs can be harnessed for environmental benefits. In the field of bioremediation, this microbial process is used to treat industrial wastewater contaminated with heavy metals and sulfate. By stimulating SRM activity, dissolved metals like zinc, copper, and cadmium can be precipitated out of the water as insoluble metal sulfides.