Biogeochemical Cycles: A Comprehensive Guide

Hi, I’m Kathy with Level Up RN! In this lesson, we’ll explore the biogeochemical cycles, including the carbon cycle, nitrogen cycle, sulfur cycle, and phosphorus cycle. These cycles are crucial for maintaining life on Earth by ensuring the continuous recycling of essential elements.

By the end of this lesson, you’ll have a solid grasp of:
✔ How elements move between living organisms and the environment
✔ Key biological and chemical processes driving these cycles
✔ Why these cycles are essential for life and environmental balance

Stay tuned for a quiz at the end to test your understanding! If you have our Level Up RN Microbiology Flashcards, grab the ones covering biogeochemical cycles so you can follow along.

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What Are Biogeochemical Cycles?

A biogeochemical cycle refers to the movement of chemical elements between the biosphere (living organisms), lithosphere (earth’s crust), hydrosphere (water), and atmosphere (air). These cycles ensure the continuous availability of essential nutrients required for life.

Each cycle consists of:

1. Reservoirs – Where elements are stored (e.g., carbon in fossil fuels).
2. Exchange pathways – How elements move between reservoirs (e.g., photosynthesis, decomposition).
3. Human influences – How human activities alter natural cycles (e.g., burning fossil fuels affecting the carbon cycle).

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The Carbon Cycle: The Foundation of Life

Why Is Carbon Important?

Carbon is the backbone of all organic molecules, including carbohydrates, proteins, lipids, and nucleic acids (DNA and RNA). The carbon cycle regulates the flow of carbon between organisms and the environment.

Key Processes in the Carbon Cycle

1. Carbon Fixation

   • Autotrophs (plants, algae, cyanobacteria) absorb CO₂ from the atmosphere during photosynthesis and convert it into organic molecules like glucose.
   • This process stores energy and creates food for heterotrophs.

2. Consumption & Respiration

   • Heterotrophs (animals, fungi, most bacteria) consume organic matter and release CO₂ through cellular respiration.

3. Decomposition & Soil Carbon Cycling

   • When organisms die, decomposers (bacteria, fungi) break down organic material, releasing CO₂ or CH₄ (methane) into the atmosphere.

4. Carbon Storage & Sequestration

   • Carbon is stored in:
     • Oceans (dissolved CO₂, marine life, sediments).
     • Fossil fuels (coal, oil, natural gas).
     • Rock formations (limestone stores carbon as calcium carbonate).

5. Human Impacts

   • Burning fossil fuels releases excess CO₂, contributing to global warming.
   • Deforestation reduces the planet’s ability to absorb CO₂.

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The Nitrogen Cycle: Essential for Proteins & DNA

Why Is Nitrogen Important?

Nitrogen is a major component of proteins, nucleic acids (DNA, RNA), and ATP. However, most organisms cannot use atmospheric nitrogen (N₂) directly—it must be converted into biologically useful forms.

Key Steps in the Nitrogen Cycle

1. Nitrogen Fixation

   • Nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules) convert N₂ into ammonia (NH₃).
   • This process makes nitrogen available for plants.

2. Nitrification

   • Ammonia (NH₃) is converted into nitrite (NO₂⁻) by Nitrosomonas bacteria.
   • Then, nitrite is converted into nitrate (NO₃⁻) by Nitrobacter bacteria.
   • Nitrate (NO₃⁻) is the most readily absorbed form of nitrogen for plants.

3. Assimilation

   • Plants absorb NO₃⁻ and use it to synthesize proteins and nucleic acids.
   • Animals consume these plants, obtaining nitrogen for their own biological needs.

4. Ammonification

   • Decomposers (bacteria, fungi) break down dead organisms and waste, releasing nitrogen back into the soil as ammonia (NH₃).

5. Denitrification

   • Denitrifying bacteria (e.g., Pseudomonas) convert nitrates (NO₃⁻) back into nitrogen gas (N₂), releasing it into the atmosphere.

Human Impacts on the Nitrogen Cycle

• Overuse of fertilizers introduces excess nitrates into water systems, causing eutrophication (harmful algal blooms that deplete oxygen).
• Burning fossil fuels releases nitrogen oxides (NOₓ), contributing to acid rain and air pollution.

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The Sulfur Cycle: Crucial for Proteins & Climate

Why Is Sulfur Important?

Sulfur is a key component of amino acids (cysteine and methionine), which are essential for protein synthesis.

Key Steps in the Sulfur Cycle

1. Atmospheric Sulfur

   • Sulfur is released as sulfur dioxide (SO₂) from volcanoes, decaying matter, and burning fossil fuels.

2. Deposition & Absorption

   • Sulfur dioxide dissolves in rainwater to form weak sulfuric acid, depositing sulfur into the soil.
   • Plants absorb sulfur as sulfate (SO₄²⁻) and incorporate it into proteins.

3. Decomposition & Bacterial Cycling

   • Decomposers break down dead organisms, releasing hydrogen sulfide (H₂S) into the environment.
   • Anaerobic bacteria convert H₂S into elemental sulfur (S) and sulfate (SO₄²⁻).

Human Impacts

• Burning fossil fuels releases excess SO₂, leading to acid rain that damages ecosystems.

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The Phosphorus Cycle: Key for DNA & ATP

Why Is Phosphorus Important?

Phosphorus is a key component of DNA, RNA, ATP (cellular energy), and phospholipids in cell membranes.

Key Steps in the Phosphorus Cycle

1. Weathering & Release

   • Phosphate ions (PO₄³⁻) are released from rocks into soil and water through weathering.

2. Absorption by Plants & Animals

   • Plants absorb phosphates and incorporate them into biological molecules.
   • Animals obtain phosphorus by consuming plants or other animals.

3. Decomposition & Return to Soil

   • When organisms die, decomposers break down tissues, returning phosphorus to the soil.

4. Sedimentation & Rock Formation

   • Over time, phosphorus settles in water bodies, forming new phosphate-rich rocks.

Human Impacts

• Fertilizer runoff causes eutrophication, leading to algal blooms.
• Mining and deforestation disrupt natural phosphorus cycles.

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Quiz Time!

Let’s test your knowledge with four multiple-choice questions!

1. Which process converts atmospheric nitrogen (N₂) into a usable form for plants?
   A. Nitrification
   B. Ammonification
   C. Nitrogen Fixation
   D. Denitrification
   → Answer: C

2. What is the most common form of nitrogen absorbed by plants?
   A. Ammonia (NH₃)
   B. Nitrite (NO₂⁻)
   C. Nitrate (NO₃⁻)
   D. Nitrogen Gas (N₂)
   → Answer: C