Decomposition and microbial decay practical
Ecology • Organisation of an ecosystem
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Key concepts
What you'll likely be quizzed about
Definition and ecological role of decomposition
Decomposition is the chemical and physical breakdown of dead organisms and organic waste into smaller molecules and inorganic ions. Decomposition returns key elements such as carbon and nitrogen to the abiotic environment, making them available for reuse by producers and other organisms . Microorganisms perform the main work of decomposition. Bacteria digest dissolved substances and fungi release extracellular enzymes to break down complex molecules (for example lignin or cellulose) outside their hyphae and then absorb the products by diffusion. These processes form a critical part of nutrient cycling in ecosystems .
Decomposers and mechanisms
Bacteria act as decomposers by metabolising soluble compounds and converting organic molecules into simpler inorganic forms through respiration. Fungi extend hyphae into dead tissue and secrete enzymes that digest large molecules externally; the resulting small molecules diffuse back into fungal cells for growth and respiration . Anaerobic decomposition produces gases such as methane when oxygen is absent. Aerobic decomposition produces carbon dioxide and water as microorganisms respire. The balance of these pathways affects nutrient release, gas production and preservation of remains under special conditions (for example low temperature, low oxygen and acidic water preserve organic matter in peat bogs) .
Limiting factors affecting rate of decay
Temperature changes affect microbial metabolic rate: higher temperatures increase enzyme activity and microbial growth up to an optimum, so decay rate increases; low temperatures slow microbial metabolism and decrease decay rate . Water availability enables microbial activity and enzyme diffusion; increased moisture usually increases decay rate. Oxygen availability determines whether aerobic respiration (faster in many systems) or anaerobic pathways dominate; more oxygen generally increases the rate of aerobic decomposition. pH affects enzyme function and many decomposers prefer near-neutral conditions; extreme acidity or alkalinity reduces decay rate .
Required Practical: effect of temperature on decay of fresh milk (method and rationale)
The practical compares milk samples held at different controlled temperatures and measures pH change as an indirect indicator of microbial decay. Equal volumes of fresh milk are placed in labelled beakers at four temperatures for several days; initial pH is recorded and the pH change after incubation indicates the extent of microbial acid production (for example lactic acid from lactose fermentation) . Microbial and chemical breakdown of milk fat and sugars alters pH. Warmer samples show larger pH changes because bacterial growth and metabolic rates increase, producing more acidic products. Frozen or refrigerated samples show smaller or slower pH change due to reduced microbial activity. Autoclaving and correct disposal follow practical safety requirements .
Quantitative calculation of decay rates
Rate of decay can be expressed as change in a measurable variable (mass, pH, concentration) per unit time. The basic calculation uses: rate = (change in value) ÷ (time interval). Percentage change uses: percentage change = ((initial − final) ÷ initial) × 100 ÷ time if rate per unit time is required. Example calculation using leaf-litter data: a sample of pine leaf litter decreases from 500 g to 430 g in one month, so mass lost = 70 g in 1 month, and rate = 70 g month−1. Percentage loss in that month = (70 ÷ 500) × 100 = 14% month−1. Example data and practice questions on mass loss appear in worked examples for decay calculations .
Key notes
Important points to keep in mind