Each year a large amount of plant material, cellulose, is deposited on the surface of Planet Earth. What are the natural processes this cellulose undergoes before yielding carbon dioxide, water and other end products ?

Stages of Cellulose Decomposition

The decomposition of cellulose is a multi-stage process involving a complex interplay of biological, chemical, and geological factors. It can be broadly categorized into three main phases:

1. Microbial Decomposition

This is the initial and most significant stage. Cellulose is not easily broken down due to its strong β-1,4-glycosidic linkages. However, a variety of microorganisms, primarily bacteria and fungi, possess the enzyme cellulase, which hydrolyzes these linkages.

• Bacteria: Cellulomonas, Clostridium, and Ruminococcus are key bacterial genera involved in cellulose degradation, particularly in anaerobic environments like wetlands and the digestive tracts of herbivores.
• Fungi: White-rot fungi (e.g., Phanerochaete chrysosporium) and brown-rot fungi (e.g., Fomitopsis pinicola) are crucial decomposers in terrestrial ecosystems. They secrete cellulases and other enzymes to break down cellulose and lignin.
• Process: Cellulase enzymes break down cellulose into cellobiose and glucose. These sugars are then metabolized by the microorganisms through aerobic or anaerobic respiration, releasing carbon dioxide (CO₂), water (H₂O), and energy.

2. Chemical Oxidation

While microbial decomposition is dominant, chemical oxidation also contributes to cellulose breakdown, especially in environments with limited microbial activity. This process is slower but significant over geological timescales.

• Photodegradation: Exposure to ultraviolet (UV) radiation from sunlight can cause cellulose chains to break down, although this is a relatively minor contributor.
• Oxidation by Reactive Oxygen Species (ROS): ROS, such as hydroxyl radicals (OH^•), generated through photochemical reactions or microbial metabolism, can attack and degrade cellulose.
• Hydrolysis: Water itself can slowly hydrolyze cellulose, particularly under acidic conditions.

3. Geological Sequestration & Further Transformation

The end products of microbial and chemical decomposition – CO₂ and organic matter – undergo further transformations and can be sequestered in geological formations.

• Carbon Dioxide Release: CO₂ released during respiration enters the atmosphere, contributing to the global carbon cycle.
• Humification: Partially decomposed organic matter, including cellulose fragments, is transformed into humus, a stable, dark-colored substance that enriches soil.
• Peat Formation: In waterlogged environments, incomplete decomposition leads to the accumulation of peat, a precursor to coal.
• Fossil Fuel Formation: Over millions of years, buried organic matter undergoes further transformation under high pressure and temperature, eventually forming fossil fuels like coal, oil, and natural gas. These represent long-term carbon sequestration.

Factors Influencing Decomposition Rate

Several factors influence the rate of cellulose decomposition:

• Temperature: Decomposition rates generally increase with temperature up to an optimal point, beyond which enzyme activity declines.
• Moisture: Adequate moisture is essential for microbial activity, but excessive moisture can lead to anaerobic conditions and slower decomposition.
• Oxygen Availability: Aerobic decomposition is generally faster than anaerobic decomposition.
• Nutrient Availability: The availability of nutrients like nitrogen and phosphorus can limit microbial growth and decomposition rates.
• Lignin Content: Lignin, another component of plant cell walls, is more resistant to decomposition than cellulose and can hinder its breakdown.