Lecture 13 - Carbon

Carbon Overview

• Carbon is a critical element and the main energy source of all ecosystems.

• Understanding the cycling of carbon is essential for grasping the ecology of aquatic ecosystems.

• There are two primary forms of carbon:

  • Inorganic Carbon

  • Organic Carbon

Inorganic Carbon

Characteristics

• Main reservoir of inorganic carbon is in the atmosphere as Carbon Dioxide (CO2).

• CO2 can dissolve in water, resulting in several forms:

  • Carbon dioxide (CO2)

  • Carbonic acid (H2CO3)

  • Bicarbonate (HCO3-)

  • Carbonate (CO3 2-)

Bicarbonate Equilibrium

• The state of carbon forms in water depends on the pH (proton concentration, H+ ions).

• The pH influences the acid neutralizing or buffering capacity of water.

• Greater amounts of dissolved bicarbonate increase the buffering capacity.

• An increase in atmospheric CO2 contributes to heightened acidity in aquatic ecosystems.

Organic Carbon

Characteristics

• Organic carbon consists of carbon chains bonded to other elements, primarily hydrogen and oxygen.

• Organic carbon can be found in two states:

  • Particulate Organic Carbon (POC)

    • Divided into Fine Particulate Organic Matter (FPOM) and Coarse Particulate Organic Matter (CPOM).

    • Can consist of living or dead organic material, serving as a key food source for heterotrophic organisms.

  • Dissolved Organic Carbon (DOC)

    • Two main categories:

      • Humic compounds (by-products from the breakdown of cellulose, tannins, and lignins).

      • Non-humic compounds (e.g., sugars, carbohydrates, amino acids) which are broken down by heterotrophs to yield humic compounds.

Transformation of Carbon

Carbon Cycling

• The cycling of carbon is primarily dominated by photosynthesis and respiration processes.

• Photosynthesis: Converts carbon dioxide and water into organic carbon and oxygen using light energy:

  • Equation: CO2 + H2O + light energy → CH2O + O2

• Respiration: Breaks down organic carbon into carbon dioxide and water while releasing chemical energy:

  • Equation: CH2O + O2 → CO2 + H2O + chemical energy

  • This process involves the oxidation of large organic molecules and is considered the most efficient.

Other Carbon Processes

• Carbon cycling can also involve:

  • Anaerobic processes (occurring without oxygen)

  • Oxidation with other molecules (e.g., nitrate and sulfate)

  • Fermentation

  • Methanogenesis

The Carbon Cycle

Key Processes

• Carbon cycling varies based on the presence of oxygen:

  • Oxygen Present (oxic):

    • Heterotrophy

    • Oxygenic photosynthesis and chemoautotrophy

    • Respiration

    • Methanotrophy

  • Oxygen Absent (anoxic):

    • Fermentation and anoxic respiration

    • Anoxygenic photosynthesis

Oxidation States

• Carbon can have various oxidation states in organic and inorganic compounds:

  • Reduced ( -4) and oxidized (+4).

Summary of Key Points

1. Carbon is the main energy source in ecosystems.

2. There are two forms of carbon: inorganic and organic.

   • Inorganic carbon has three forms in water:

     • Carbonic acid, bicarbonate, carbonate; which are determined by the pH levels in water.

3. Organic carbon is composed of chains of carbon and can exist in dissolved or particulate forms:

   • Includes humic and non-humic varieties.

4. Carbon cycling mainly occurs through respiration and photosynthesis.

   • The cycling process differs in anaerobic conditions (absence of oxygen).