Dissimilatory Sulfate Reduction (DSR)
DSR is an important metabolic process for sulfate reducers in anoxic (without oxygen) marine sediments. This is the enzyme specific pathway which starts by intaking sulfate from seawater into the microbial cell and outputting bisulfide (HS-) as a product.
SO42- (extracellular) ⇄ SO42- (intracellular) ⇄ APS ⇄ SO32- → H2S
The Marine Sulfur Cycle
The sulfur cycle in marine sediments mirrors the carbon cycle. In areas of the ocean where there is no oxygen in along the seafloor and in the watery surface sediments, microbes conduct DSR (defined above) to create energy to survive the cold and desolate environment. This metabolic process accounts for half of all microbial cells in the ocean. That's a lot of cells!
After DSR, the sulfur becomes pyrite (sometimes called fool's gold!) which scientists use as a paleoclimate proxy or "historical evidence" of the presence of these microbial populations. The isotopes of pyrite found in rock samples on earth, the moon, and other planets can tell us about the presence of life and the conditions they once lived.
Sulfur Isotopes (Graduate Level)
How do isotope effects evolve in an open system vs. a closed systems?
In microbial sulfate reduction, sulfate (red line) is taken is as the reactant, and sulfide (HS-) (blue line) is produced as a product. Microbes will consume the lightest sulfur isotopes first as they are energetically easier to consume (see potential energy diagram gif below!). The middle line depicts the base enrichment of 34-S in seawater which is ~21‰.
In a closed system (left) because there is less isotopes to choose from over time the system becomes depleted faster creating a more dramatic isotope effect than in the open system.
In an open system (right) a fresh supply of sulfate is flowing into the system while sulfides are diffusing out of the system creating a parallel relationship.
Isotope Fractionations: Heavier vs. Lighter
Organic Matter Degradation
How does pH change which species of dissolved inorganic sulfur is present in seawater?
At different pH values different types of sulfide species will be present due to equilibrium effects. Moving across different pH levels will dictate which form your sulfide species is in. Seawater is ~pH 8, meaning most of the sulfide present will be dissolved HS− ions, however there may be small portion of H₂S as a dissolved gas.
This concept is central for the marine carbon cycle too. We observe this equilibrium "offset" during ocean acidification due to climate change.
For the sake of sulfur research, we purposely acidify our sediments which may contain a mixture of sulfide species. Adding acid to the sediments pushes the pH to low values which produces H₂S, a gas, which can be trapped in a separate container to be measured later. Species separated during this process are called acid volatile sulfides (AVS).
X-Ray Absorption Spectroscopy (XAS)
Shown on the left is an atomic view of X-Ray Absorbance spectroscopy (XAS) in action. The XAS beam can be tuned to specific photon energies in order to excite core electrons in a molecular sample.
In marine science, this tool is used to learn about local geometric and/or electronic structure of a sample in order to identify the types and amounts of organic compounds present in a sample of organic matter.