Microbial Roles in Carbon, Nitrogen, and Sulfur Cycles | Chapter 21 Summary from Brock Biology of Microorganisms

Microorganisms are essential drivers of Earth's nutrient cycles. Chapter 21 of Brock Biology of Microorganisms provides a comprehensive look at how microbes facilitate the cycling of carbon, nitrogen, sulfur, iron, and other key elements. These microscopic chemists mediate redox transformations, form syntrophic partnerships, and shape global climate feedbacks. This chapter also addresses how human activities are altering these natural microbial processes.

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Carbon Cycle

• CO₂ is cycled through the biosphere, atmosphere, ocean, and lithosphere.
• Photosynthesis fixes CO₂; respiration and decomposition release it.
• Microbial decomposition is the major natural source of atmospheric CO₂.
• Methanogenesis produces CH₄ in anoxic environments — a potent greenhouse gas.
• Methane hydrates in sediments and permafrost are major carbon reservoirs.

Syntrophy and Methanogenesis

Syntrophic bacteria partner with methanogens to degrade compounds that are otherwise energetically unfavorable. This cooperation is essential in environments like:

• Termite hindguts
• Anoxic sediments

Nitrogen Cycle

Key microbial processes in nitrogen cycling include:

• Nitrogen fixation: N₂ → NH₃ (e.g., by Rhizobia or Cyanobacteria)
• Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (aerobic)
• Denitrification: NO₃⁻ → N₂ (anaerobic); produces N₂O (greenhouse gas)
• Anammox: NH₄⁺ + NO₂⁻ → N₂ (anaerobic oxidation)
• DNRA: NO₃⁻ → NH₄⁺ (alternative to denitrification)

These processes regulate nitrogen availability in soils, oceans, and wastewater systems.

Sulfur Cycle

• Sulfate reducers convert SO₄²⁻ to H₂S in anoxic zones.
• Sulfur oxidizers convert H₂S back to sulfate in oxic or anoxic environments.
• DMS (dimethyl sulfide) production by marine microbes influences cloud formation and climate.
• Sulfur often binds with iron, forming black sediments (e.g., FeS).

Iron and Manganese Cycles

Microbes help cycle metal ions between soluble and insoluble forms:

• Geobacter and Shewanella reduce Fe³⁺ and Mn⁴⁺ to Fe²⁺/Mn²⁺ using nanowires or electron shuttles.
• Iron-oxidizing bacteria restore Fe³⁺ in microoxic zones.

Phosphorus, Calcium, and Silicon Cycles

• Phosphorus: Cycled as PO₄³⁻; microbes degrade organic phosphates and phosphonates.
• Calcium: Marine organisms like coccolithophores use it to form CaCO₃, impacting carbon sequestration.
• Silicon: Diatoms require it for their frustules, tying it to the biological pump.

Mercury Transformations

• Microbes convert inorganic Hg²⁺ to neurotoxic methylmercury (CH₃Hg⁺) in anoxic habitats.
• Mer operon genes help detoxify mercury via reduction to volatile Hg⁰.

Human Impacts on Microbial Nutrient Cycles

• Fossil fuel emissions increase atmospheric CO₂ → global warming
• Ocean acidification disrupts marine microbes and carbonate producers
• Methane release from permafrost and wetlands triggers feedback loops
• Haber-Bosch process adds massive synthetic nitrogen → eutrophication, N₂O emissions
• Nitrogen cycle disruption alters microbial community function globally

Glossary Highlights

• Syntrophy: Cooperative microbial metabolism of complex compounds
• Anammox: Anaerobic ammonium oxidation → N₂ gas
• DNRA: Dissimilative nitrate reduction to ammonium
• DMS: Sulfur compound affecting cloud albedo
• Radiative Forcing: Degree to which a gas affects Earth's energy balance
• Nanowires: Bacterial filaments for long-distance electron transfer
• Biological Pump: Sinking of carbon via marine biomass
• Mer Operon: Gene cluster that detoxifies mercury

Conclusion

Microbial processes shape Earth's biogeochemical cycles, balancing greenhouse gases, nutrient availability, and ecosystem stability. However, human-driven changes are disrupting these microbial systems, with consequences for global climate and biodiversity. Chapter 21 emphasizes the need to understand microbial metabolism not just in the lab, but in the planetary context.

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