Material Cycles

A unit of energy moves through an ecosystem only once, indicating a linear path. This energy traverses through various trophic levels of a food web, moving from producers to consumers and ultimately dissipates as heat.
In contrast, certain substances, such as chemical elements like carbon or nitrogen, follow a cyclical path between the physical environment and living organisms, and back to the environment.
These substances are continuously recycled through biogeochemical cycles.

Four major Biogeochemical Cycles:
1. Global cycles: Hydrologic, Carbon, & Nitrogen
2. Local cycles: Phosphorous
Biogeochemical cycles involve:
- Biological mechanisms: Involves the absorption of chemicals by organisms and their subsequent release back into the environment.
- Geological mechanisms: Involve the weathering and erosion of rocks, alongside the transport of elements through surface and subsurface drainage.
- Chemical transport mechanisms: These are observed in the movement of dissolved materials via rain, snow, atmospheric gases, and airborne dust.

The Hydrologic Cycle refers to the path of water as it moves through the environment. This cycle is driven by solar energy and consists of the following processes:
- Evaporation
- Condensation
- Precipitation
- Surface runoff
Solar energy is the primary driving force behind the evaporation of water. Water vapor generated can be transported globally.
Water in its condensed form can be stored in:
- Oceans
- Land bodies (lakes, rivers)
- Organisms
- Ice
Water condenses over land surfaces which supports all terrestrial systems.
Approximately 2% of the world’s total volume of water is stored in:
- Organisms
- Frozen as ice or snow
- Groundwater (which serves as drinking water)
The remaining 98% cycles through the hydrologic cycle.
Transpiration: The process whereby plants release water vapor into the atmosphere.

The hydrologic cycle is crucial for:
- Cellular metabolism
- Nutrient flow in ecosystems
- Global heat and energy distribution

The Carbon Cycle describes the movement of carbon from the atmosphere into organisms and back:
- Components that remove carbon dioxide from the atmosphere are termed carbon sinks (e.g., phototrophs).
- Some carbon is sequestered with a low turnover rate (e.g., fossil fuels, peat, or limestone).
The cycle begins with the intake of CO₂ during photosynthesis:
- Carbon atoms are incorporated into sugars.
- These sugars release carbon back into the environment via cellular respiration.

Components that release carbon dioxide are known as carbon sources.
Key processes include:
- Decomposition in heterotrophic organisms
- Respiration in autotrophs and heterotrophs
Human impact:
- Burning fossil fuels generates excessive carbon dioxide.
- The accumulation cannot be absorbed by carbon sinks quickly enough, contributing to global warming.

The carbon cycle is predominantly driven by two key processes:
1. Photosynthesis by photoautotrophs that absorb CO₂ from the atmosphere.
2. Respiration by heterotrophs that release CO₂ back into the atmosphere.
The cycle is essential for recycling carbon organic molecules, which are vital for life and play a role in regulating Earth’s temperature and climate.

Sources of Nitrogen:
- Weathering of bedrock contributes to 25% of nitrogen in ecosystems.
- Approximately 75% of nitrogen derives from the atmosphere.
- N₂, composes nearly 78% of Earth's atmosphere but is inaccessible to most organisms without transformation.
- The triple bond in N₂ necessitates specific bacteria to break it down through nitrogen fixation.

Nitrogen-fixing bacteria extract nitrogen from atmospheric N₂ to meet their metabolic needs, releasing NH₃ or NH₄⁺ as a byproduct,
Certain plants, notably legumes, harbor nitrogen-fixing bacteria (e.g., rhizobia) within root nodules.
Other plants form associations with cyanobacteria that also fix nitrogen.

Nitrifying bacteria convert NH₃ or NH₄⁺ into nitrate (NO₃⁻) or nitrite (NO₂⁻) through nitrification.
Specific bacteria can convert NO₂⁻ to NO₃⁻:
- Nitrates are commonly utilized by plants.
- Nitrites can be taken up by plants but are considered less preferable and are potentially toxic in high concentrations.

Assimilation is the process where inorganic substances (like NH₃, NH₄⁺, and NO₃⁻) are incorporated into organic molecules (e.g., amino acids).
Ammonification involves converting organic nitrogen into NH₃ or NH₄⁺ through the decomposition of dead plants and animals or animal waste.
- Decomposers (bacteria and fungi) convert organic nitrogen to NH₃, which quickly converts to NH₄⁺ in slightly acidic soils.

Denitrification refers to the conversion of NO₃⁻ to N₂ by anaerobic denitrifying bacteria that utilize NO₃⁻ instead of O₂ in respiration.
- This process effectively returns nitrogen to the atmosphere and occurs in low oxygen environments.
- The overall reaction sequence:
  NO₃^{-}
  ightarrow NO₂^{-}
  ightarrow NO
  ightarrow N₂O
  ightarrow N₂
The nitrogen cycle actively recycles nitrogen crucial for the formation of essential biomolecules such as amino acids, nucleic acids (DNA), and chlorophyll.

The Phosphorous Cycle differs from other cycles due to the absence of a gaseous phase.
Lacking an atmospheric component, phosphorous cannot be transported via wind but can move through aqueous forms in water.
The primary storehouse for phosphorous is the Earth’s crust.

Phosphorous is released into the soil through the weathering and erosion of rocks.
Producers (e.g., photoautotrophs) absorb and incorporate dissolved forms such as phosphates.
Consumers acquire the phosphorous needed for survival by consuming plants or other organisms.

Phosphorous is reintroduced into the environment through:
- Excretion of waste by plants and animals,
- Decomposition of deceased organisms.
Leaching of phosphorous leads to runoff, introducing phosphates into aquatic systems where they may be utilized by algae.

Increased phosphorous in aquatic ecosystems leads to rapid algal growth, a process known as eutrophication.
In lakes, the relationship between primary production and total phosphorous exhibits a linear correlation:
- When algae and plants decay, bacteria decompose the material, depleting oxygen levels, potentially leading to fish kills.

Phosphorous is essential for the synthesis of vital biological molecules, including:
- DNA (deoxyribonucleic acid)
- ATP (adenosine triphosphate), which acts as the energy currency of the cell.
Many fertilizers are derived from mined phosphate, emphasizing the economic importance of this cycle.