The o-fish-al site for accessible marine visualizations. All animations are for science outreach and educational purposes. This website serves as an open access point for geoscience students. Want to use one of these animations for your school presentation or project? Please email the artist, Katie O'Malley (komalley@ucsb.edu).

Measuring Microbial Sulfate Reduction with Sulfur Isotope Tracers

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Dissimilatory Sulfate Reduction (DSR)

Dissimilatory Sulfate Reduction (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.

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

Each time an enzyme initiates a chemical reaction it will have a slight preference to break the bonds in a the reactant molecules containing the lighter isotopes. This is because lighter isotopes have longer and easier to break internuclear bonds than their heavier counterparts. Thus it requires less kinetic energy to "use" a lighter isotopes than a heavier isotope. Overtime, we notice an "isotope effect" meaning the overall pool of reactants contains a higher proportion of heavy isotopes to light isotopes.

Organic Matter Degradation

Organic matter are very large carbon containing molecules which are found sinking with depth in the ocean. These molecules are essential to bacterial metabolisms which occur in the water column and seafloor. These microbial populations intake OM as big molecules and use enzymes to break down the molecule into smaller inorganic (no C-H bonds) molecules.

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.

Collaborator: Alexandra Phillips, Post Doctoral Researcher and Founder of Women Doing Science