MICR2000 Module 5 L4: Biogeochemical cycles

What are biogeochemical cycles?

They are natural pathways that recycle essential elements (C, N, S, P, Fe, etc.) between biological, geological, and chemical forms, connecting microbial activity to global ecosystem function.

Why are microbes central to biogeochemical cycles?

They catalyze key redox transformations (e.g., N₂ fixation, methanogenesis, sulfur oxidation) that drive element cycling and regulate Earth’s climate and nutrient availability.

How do redox reactions underpin biogeochemical cycles?

Cycling elements involves repeated oxidation and reduction. Microbes exploit these reactions for energy, while transforming elements into bioavailable or gaseous forms.

Why is the carbon cycle closely tied to microbial metabolism?

Microbes fix CO₂ (autotrophy), decompose organic matter (heterotrophy), and produce greenhouse gases (CH₄, CO₂) — controlling the balance of carbon flux.

What is methanogenesis, and where does it occur?

Methanogenesis is the production of methane (CH₄) by archaea in anaerobic environments (e.g., sediments, animal guts). It’s a terminal step in carbon cycling when stronger electron acceptors are absent.

How does methane oxidation balance methanogenesis?

Methanotrophic microbes consume CH₄, oxidizing it to CO₂ under aerobic or anaerobic conditions, reducing methane emissions to the atmosphere.

What is the biological nitrogen cycle’s main challenge?

N₂ gas is abundant but inaccessible to most organisms. Microbes must convert it into reactive forms (NH₃, NO₃⁻, NO₂⁻) that plants and animals can use.

What is nitrogen fixation, and why is it critical?

It’s the microbial reduction of N₂ to NH₃, supplying ecosystems with usable nitrogen. Without it, ecosystems would quickly run out of biologically available nitrogen.

What is anaerobic ammonium oxidation (anammox)?

A microbial process that directly converts NH₄⁺ + NO₂⁻ → N₂ gas, bypassing nitrification and denitrification. It is key in wastewater treatment and marine nitrogen loss.

Why is the sulfur cycle important for microbes and ecosystems?

Sulfur compounds serve as both electron donors and acceptors. Microbes oxidize H₂S → SO₄²⁻ and reduce SO₄²⁻ → H₂S, cycling sulfur through ecosystems and linking to carbon and metal cycles.

What role do sulfur-oxidizing microbes play in the environment?

They detoxify H₂S in sediments and hydrothermal vents, while generating sulfate that supports other microbes and nutrient availability.

How do iron cycles support microbial growth?

Microbes reduce Fe³⁺ → Fe²⁺ (anaerobic respiration) or oxidize Fe²⁺ → Fe³⁺ (energy generation). Iron cycling is tightly coupled with carbon, sulfur, and nitrogen transformations.

Why is microbial iron oxidation often linked to acidic environments?

Fe²⁺ is unstable at neutral pH but persists in low pH conditions, where acidophilic microbes exploit it as an energy source.

What is the phosphorus cycle’s main microbial role?

Microbes mineralize organic phosphorus into phosphate (PO₄³⁻), making it bioavailable, and precipitate phosphate with metals in soils and sediments.

Why is phosphorus cycling different from other cycles?

Phosphorus has no gaseous phase; it cycles mainly between dissolved, organic, and mineral forms, making microbial solubilization crucial for ecosystem fertility.

How do microbes link different cycles together?

Microbes simultaneously transform multiple elements (e.g., sulfur oxidizers that also fix CO₂; denitrifiers that impact carbon cycling), interconnecting cycles at local and global scales.

What is the “microbial loop” in aquatic systems?

Microbes recycle dissolved organic matter back into the food web, ensuring nutrients remain available to higher trophic levels.

How do human activities disrupt biogeochemical cycles?

Agriculture, fossil fuel burning, and pollution increase fixed nitrogen, carbon, and sulfur inputs, altering microbial processes and contributing to eutrophication and climate change.