MCB 202: Exhaustive Study Notes on Biogeochemical Cycles

Definition and Scope of Biogeochemical Cycles

• The biogeochemical cycle refers to the natural processes that recycle elements and compounds essential for life through Earth’s major systems:

  • Biosphere

  • Lithosphere

  • Atmosphere

  • Hydrosphere

• These cycles ensure a continuous supply and movement of vital nutrients including carbon, nitrogen, oxygen, phosphorus, and water.

• The term "biogeochemical" is an amalgamation of three types of interacting processes:

  • Biological (bio)

  • Geological (geo)

  • Chemical

• These cycles are interdependent and critical for maintaining the overall ecological balance of the planet.

The Carbon Cycle: The Backbone of Life

• Carbon is considered the backbone of all organic life and is an essential element in biological molecules:

  • Carbohydrates

  • Proteins

  • Lipids

  • Nucleic acids

• Forms and Reservoirs of Carbon:

  • atmosphere: Carbon dioxide ($CO_2$).

  • Oceans: Dissolved carbon ($CO_2$) and bicarbonates.

  • Biosphere: Organic carbon in living organisms.

  • Lithosphere: Stored in fossil fuels and sedimentary rocks like limestone.

• Key Processes of the Carbon Cycle:

  • Photosynthesis: This is the primary method for carbon to enter the biosphere. Plants, algae, and cyanobacteria use sunlight to absorb atmospheric $CO_2$.

  • Equation: $6CO_2 + 6H_2O + \text{sunlight} \rightarrow C_6H_{12}O_6 + 6O_2$

  • Respiration: Carried out by all living organisms (plants, animals, and microbes). Glucose is broken down with oxygen to release energy.

  • Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

  • Decomposition: Decomposers (bacteria and fungi) break down dead organisms.

  • Aerobic conditions: Releases carbon as $CO_2$.

  • Anaerobic conditions: Releases carbon as methane ($CH_4$).

  • Combustion: The burning of wood, coal, oil, and natural gas. This happens naturally (wildfires) or through human activities (fossil fuel burning for energy).

  • Carbon Storage (Sequestration): Long-term reservoirs include marine organisms and sinks like forests and soils.

• Oceans as Sinks: Oceans absorb large quantities of atmospheric $CO_2$.

The Nitrogen Cycle: Essential for Proteins and Nucleic Acids

• Nitrogen is a major component of amino acids (building blocks of proteins) and nucleic acids (DNA and RNA).

• Although Nitrogen gas ($N_2$) makes up $78\%$ of the atmosphere, it is unusable in gaseous form by most organisms.

• The Five Key Processes:

  • 1. Nitrogen Fixation: Conversion of atmospheric $N_2$ into ammonia ($NH_3$) or ammonium ($NH_4$).

  • Biological fixation: Performed by nitrogen-fixing bacteria like Rhizobium in the root nodules of legumes (beans, peas).

  • Abiotic fixation: Lightning breaks nitrogen bonds; nitrogen combines with oxygen to form nitrates ($NO_3$) which reach the earth via rain.

  • Industrial fixation: The Haber-Bosch process combines nitrogen and hydrogen under high pressure to produce ammonia for fertilizer.

  • 2. Nitrification: A two-step process conducted by soil bacteria.

  • Step One: $NH_3$ is converted to nitrite ($NO_2$) by bacteria such as Nitrosomonas.

  • Step Two: $NO_2$ is converted to nitrate ($NO_3$) by Nitrobacter. Nitrates are the form most easily absorbed by plants.

  • 3. Assimilation: Plants absorb nitrates or ammonium through roots. Nitrogen is then incorporated into organic molecules like proteins. Animals obtain nitrogen by eating plants or other animals.

  • 4. Ammonification (Mineralization): When organisms die or excrete waste, decomposers (bacteria and fungi) return nitrogen to the soil by converting organic nitrogen compounds back into $NH_3$ or $NH_4$.

  • 5. Denitrification: Denitrifying bacteria like Pseudomonas and Clostridium convert soil nitrates back into $N_2$ gas, releasing it back into the atmosphere.

The Oxygen Cycle and Its Linkages

• The oxygen cycle is closely intertwined with the carbon cycle.

• Primary Sources/Sinks:

  • Photosynthesis: Plants release $O_2$ into the atmosphere.

  • Respiration: Animals and other organisms use $O_2$ and produce $CO_2$.

• Other Processes:

  • Decomposition: Oxygen is utilized in the breakdown of organic materials.

  • Geological processes: Weathering of rocks involves oxygen.

The Sulphur Cycle: Essential for Protein Structure

• Sulphur is essential for amino acids (specifically methionine and cysteine), vitamins, and enzymes.

• Forms and Reservoirs:

  • Rocks and minerals.

  • Sulphate ($SO_4^{2-}$) in water.

  • Hydrogen sulphide ($H_2S$) in volcanic gases.

  • Sulphur dioxide ($SO_2$) in the atmosphere.

• Major Steps in the Sulphur Cycle:

  • 1. Weathering of Rocks: Physical and chemical weathering of rocks like pyrite and gypsum releases soluble sulphate ions ($SO_4^{2-}$) into soil and water.

  • 2. Assimilation: Plants absorb $SO_4^{2-}$ through roots to form sulphur-containing amino acids. Animals acquire sulphur via the food chain.

  • 3. Decomposition: Decomposers break down waste/dead matter into inorganic forms like $H_2S$, especially in anaerobic conditions such as swamps or wetlands.

  • 4. Oxidation and Reduction:

  • Oxidation: Sulphur-oxidizing bacteria convert $H_2S$ into elemental sulphur ($S$) and then into $SO_4^{2-}$.

  • Reduction: In oxygen-poor environments, sulphate-reducing bacteria convert $SO_4^{2-}$ back into $H_2S$ via anaerobic respiration.

  • 5. Volcanic and Atmospheric Activity: Volcanoes and hot springs release $SO_2$ and $H_2S$ gas. In the atmosphere, $SO_2$ reacts with water to form sulphuric acid ($H_2SO_4$), leading to acid rain.

The Phosphorus Cycle: The Geosphere to Biosphere Movement

• Unique Characteristic: Unlike carbon or nitrogen cycles, the phosphorus cycle does not include a gaseous phase under normal conditions. It is slower and more localized.

• Biological Importance: Essential for DNA, RNA, ATP (adenosine triphosphate), and phospholipids in cell membranes.

• Key Processes:

  • 1. Weathering: Phosphorus-containing rocks (e.g., phosphate rock and apatite) release phosphate ions ($PO_4^{3-}$) due to rain and chemical reactions.

  • 2. Uptake: Plants absorb $PO_4^{3-}$ from the soil to build nucleic acids. Consumers obtain it by eating plants or herbivores.

  • 3. Decomposition: Decomposers return phosphorus to the soil as inorganic phosphate.

  • 4. Leaching and Sedimentation: Rainwater washes phosphates into water bodies (leaching).

  • In aquatic systems, phosphorus supports algal growth.

  • Excessive amounts cause eutrophication, resulting in algal blooms and oxygen depletion.

  • Phosphorus eventually settles at the bottom of oceans/lakes to form new sedimentary rock over geological time, which may be uplifted millions of years later.

The Water (Hydrological) Cycle: Driving Ecological Processes

• The total amount of water on Earth remains nearly constant, though it changes states (solid, liquid, gas).

• Stages of the Water Cycle:

  • 1. Evaporation: Driven by solar energy, liquid water from oceans and rivers turns into vapor.

  • 2. Transpiration: Plants release water vapor through small openings in leaves called stomata. Together with evaporation, this is called evapotranspiration.

  • 3. Condensation: Water vapor cools and gathers around dust particles to form clouds or fog.

  • 4. Precipitation: Moisture falls as rain, snow, sleet, or hail.

  • 5. Infiltration and Percolation: Water soaks into the soil (infiltration) and moves deeper into aquifers and the water table (percolation).

  • 6. Surface Runoff: Water that does not soak in flows over land into water bodies, contributing to erosion and nutrient transport.

Human Impact and Ecological Balance

• Human interference has significant negative impacts on these cycles:

  • Carbon Cycle: Deforestation and large-scale fossil fuel combustion increase atmospheric $CO_2$, causing global warming and climate change.

  • Nitrogen Cycle: Synthetic fertilizers and burning fossil fuels cause water pollution (eutrophication), acid rain, and loss of biodiversity.

  • Sulphur Cycle: Industrial processes and fossil fuel burning release excess $SO_2$, intensifying acid rain which damages plants, soil, aquatic systems, and infrastructure.

  • Phosphorus Cycle: Mining for fertilizers and detergents leads to high runoff and eutrophication in water bodies. Recovery from phosphorus imbalances is slow because there is no gaseous phase.

  • Water Cycle: Urbanization, damming rivers, and over-extracting groundwater lead to water scarcity, reduced rainfall, and habitat loss.

• Sustainability: Understanding these cycles is vital for managing soil fertility, wastewater treatment, and carbon emission management (including carbon capture and storage/CCS).