Sulfur and Nitrogen Cycles: Microbial Roles and Pathways

Sulfur Cycle: Sulfate Reducers and Sulfur Oxidizers

Core idea: Plants cannot use elemental sulfur directly; they need sulfur in oxidized forms that they can uptake, mainly as sulfate. The cycle in soils and water involves microbial transformations that shuttle sulfur between sulfate, sulfide, and elemental sulfur.
Two main bacterial groups drive sulfur transformations:
- Sulfate reducers: reduce sulfate to sulfide.
- Sulfur oxidizers: oxidize sulfide back up to elemental sulfur and to sulfate.
Key redox flow (conceptual):
- Reduction step: \mathrm{SO_4^{2-} \rightarrow S^{2-}}
- Oxidation steps: \mathrm{S^{2-} \rightarrow S^0} and \mathrm{S^0 \rightarrow SO_4^{2-}}
Pyrite formation and breakdown:
- Pyrite is formed when sulfur combines with iron, giving the mineral FeS₂, which has a distinctive gold-like appearance.
- Over time, pyrite oxidizes/breaks down to release sulfate back into the environment: \mathrm{FeS2 + O2 + H2O \rightarrow Fe^{2+} + 2\,SO4^{2-} + 2\,H^+} (general oxidation form).
Concept of nutrient availability for plants:
- Sulfate (SO₄²⁻) is the form plants typically take up; elemental sulfur and sulfide are not directly usable without microbial oxidation/reactions.

Nitrogenase enzyme in legume root nodules enables biological nitrogen fixation:
- Process: atmospheric N₂ is reduced to ammonia, which is rapidly converted/kept as ammonium in soil.
- Representative overall reaction (simplified):
  \mathrm{N2 + 8\, H^+ + 8\, e^- + 16\, ATP \rightarrow 2\, NH3 + H2 + 16\, ADP + 16\, Pi}
Ammonia in soil and its protonation:
- Ammonia (NH₃) equilibrates to ammonium (NH₄⁺) in soil: \mathrm{NH3 + H^+ \rightarrow NH4^+}
Nitrification by nitrifiers (soil microbial community):
- Ammonium is oxidized stepwise to nitrate:
  \mathrm{NH4^+ \rightarrow NO2^- \rightarrow NO_3^-}
- The transcript mentions that this arrow represents a sequence of biochemical steps; in reality, this involves multiple reactions (often cited as 13 to 40 distinct steps in detailed pathways).
- The course often uses a simplified flow to capture the general traffic pattern rather than every intermediate step.
Biological context and terminology:
- The bacteria involved in this portion of the cycle are referred to as nitrifiers.
- The nitrogen fixed by legume bacteria contributes to soil N, reducing the need for synthetic fertilizer and benefiting subsequent crops (e.g., corn).
Educational note on memorization:
- The instructor emphasizes that the full nitrification pathway is complex, with many steps, but a simplified schematic (NH₄⁺ → NO₂⁻ → NO₃⁻) is useful for understanding the flow of nitrogen through the system.

Mortality and detritus:
- When organisms die, they generate detritus which is broken down by decomposers.
- This applies to both soil and water ecosystems; the same general processes operate with different microbial players.
Water-specific microbial names:
- In water, similar decomposition and nutrient cycling occur, but the microbial players are often referred to by different names; the transcript mentions cyanobacteria as an example of bacteria relevant to aquatic contexts.
Conceptual takeaway:
- The soil and water sulfur and nitrogen cycles share a common structure: inputs, transformations by microbial groups, and uptake or release of nutrients by plants and the environment.

Plants require oxidized nutrients for uptake (sulfur as sulfate; nitrogen as ammonium/nitrate).
Microbes mediate redox transformations that convert between different chemical forms:
- Sulfate reducers: SO₄²⁻ → S²⁻ (reduction)
- Sulfur oxidizers: S²⁻ → S⁰ → SO₄²⁻ (oxidation)
Pyrite formation (FeS₂) creates a reservoir of sulfur that can be released as sulfate via oxidative weathering.
Legumes foster soil nitrogen through symbiotic nitrogen fixation via nitrogenase, producing NH₄⁺ after fixation and assimilation.
Nitrification is a multi-step process converting NH₄⁺ to NO₂⁻ to NO₃⁻; while often depicted as a simple arrow, it comprises many biochemical steps (often cited as 13–40 depending on the pathway model).
Detritus and decomposition link the death of organisms to nutrient recycling; water ecosystems use similar cycles with different microbial players (e.g., cyanobacteria).
Practical implications:
- Crop rotations with legumes can enhance soil nitrogen availability for subsequent crops like corn.
- Understanding sulfur cycling helps explain why adding elemental sulfur to soils requires microbial processing before plants can utilize it.

Sulfate reduction vs oxidation flows:
- \mathrm{SO_4^{2-} \rightarrow S^{2-}} (sulfate reduction by sulfate-reducing bacteria)
- \mathrm{S^{2-} \rightarrow S^0} and \mathrm{S^0 \rightarrow SO_4^{2-}} (oxidation by sulfur-oxidizing bacteria)
Pyrite formation and breakdown:
- \mathrm{Fe^{2+} + 2 S^{2-} \rightarrow FeS_2}
- \mathrm{FeS2 + O2 + H2O \rightarrow Fe^{2+} + 2 SO4^{2-} + 2 H^+}
Nitrogen fixation (legumes):
- \mathrm{N2 + 8 H^+ + 8 e^- + 16 ATP \rightarrow 2 NH3 + H2 + 16 ADP + 16 Pi}
- \mathrm{NH3 + H^+ \rightarrow NH4^+}
Nitrification (simplified):
- \mathrm{NH4^+ \rightarrow NO2^- \rightarrow NO_3^-}
Pathway nuance:
- The specific sequence from NH₄⁺ to NO₃⁻ involves many steps (often 13–40 in detailed diagrams).
Microbial players mentioned:
- Sulfate-reducing bacteria
- Sulfur-oxidizing bacteria
- Legume-associated nitrogen-fixing bacteria (nitrogenase-containing)
- Nitrifiers
- Decomposers
- Cyanobacteria ( aquatic context)

The transcript ends with a classroom cue: "Let's do our attendance" indicating a transition to roll call. This line marks the end of the content focus here and signals a classroom routine rather than a scientific concept.