Measuring Microbial Sulfate Reduction with Sulfur Isotope Tracers
Dissimilatory Sulfate Reduction (DSR)
Envisioning the Sulfur Cycle in Marine Sediments
The sulfur cycle in marine sediments is an important site for microbial sulfate reduction a process which is important for the energy creation of half of all cells in the global ocean. Pyrite, a sedimentary product, of the sulfur cycle is used as a paleoclimate proxy which tells marine scientists about the chemical processes and presence of life in Earth's oceans and other planetary bodies.
Left Figure: "Bird-eye view" Sulfur Cycle in Marine Sediments, Adapted from Jørgensen 1979
Right Figure: Simplified, Species Specific Cycling (Click Image to Expand)
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.