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Ecology Lecture 5 Ralph Kirby
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What is Decomposition Decomposition is the breakdown of the chemical structure of living organisms that occurs on death of those organisms It involves the release of the energy stored in those compounds as part of autotrophic growth Releases the chemicals that form part of the organic matter ultimately as inorganic chemicals Essential for life on Earth because without it, all carbon would be trapped as dead organic matter and there would be no recycling
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Decomposition is made up of many processes
Ingestion Excretion Chemical decomposition Leaching Physical structure changes All heterotrophs need to be decomposers to come extent in order to obtain nutrients However, more usual to consider decomposers to be those that feed on dead organic matter as their major source of energy and carbon Microbial Decomposers Bacteria, Fungi, etc Detritivores Animals
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Stages of decomposition
Leaching Loss of soluble compounds Occurs in a cycle with fragmentation and is abiotic Fragmentation Reduction of organic matter into smaller particles either physically or chemically Both biotic and abiotic Both biotic microbial and none microbial fragmentation involves some uptake by the decomposer for respiration etc Mineralization This is the release of various end products that are none organic in nature Carbon dioxide Water Nitrate Ammonia Sulphate Immobilization Incorporation of part of the decomposed matter into the decomposers own organic form
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Very large number of different organisms are involved in decomposition
Bacteria Aerobic Most efficient soil Anaerobic Less efficient Termite gut Ungulate gut Aquatic sediment Fungi Cellulose degrading Cellulases Lignin degrading\ Ligninases Microfauna nematodes Microflora Protozoa and algae Mesofauna Mites and springtails Macrofauna Millipedes and termites Megafauna Earthworms and snails
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Note the presence of a food chain
Large insects feed on smaller insects feed on fungi and bacteria Dead organic matter provides both energy and nutrients However, dead organic matter is made up of a wide variety of different chemical These chemicals have different stabilities during decomposition Sugar, amino acids etc are valuable and utilized first Proteins, lipid and less complex polysaccharides like starch are easy to break down and utilized next Next, the complex hard to break down polysaccharides like cellulose Finally, the complex materials like lignin are utilized The order is based on the energy needed to complete the breakdown and uptake of the material
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Complex Organic Molecules in Detritis
Proteins Amino bonds easily broken by proteases Starch, glycogen and similar polysaccharides Storage material and easily broken down by various enzymes Lipids Cell membrane and storage material. Lipases break this down with varying efficiencies Hemicellulose Plant cell wall material, most easily broken down part. Cellulose Structural material. Cyrstaline and can be difficult to breakdown. Variety of cellulases and other enzymes needed Lignin Very complex random structured molecules Requires specialised enzymes for breakdown utilizing free radicals Lignin peroxidases, polyphenol oxidases and laccases
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Note effect of lignin content
Usually stated that fungi are the major lignin degraders Not absolutely true Many bacteria produce lignin degrading enzymes like polyphenol oxidases and lignin peroxidases particularly the Actinomycetes Thermophilic degradation of organic matter rich in cellulose and lignin particularly involves thermophilic Actinomycetes – Composting
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Note anaerobic decomposition of plants in water environment
Anaerobic bacteria involved Also affect large fauna Termites like cellulose and lignin Earthworms do not like lignin
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Decomposition of animal material is much quicker than plant material especially at high temperatures
Bacteria are very important and enzymes needed are less specialised Some insects such as blowflies involved Succession of insects (and also bacteria) allow estimation of time of death in murder cases Also allows season when death occurred to be identified as well as if the body has been moved Small animal body will be reduced to bone and hair (30% of original mass) in 7-10 days during summer At lower temperatures, mummification can occur followed by fragmentation and scattering
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Fecal matter is also important due to the various fauna involved in decomposition
However, it is already high degraded, but provides a rich sources of nutrient to specialised detritivores Examples are dungbeetles Lay their eggs in a made dung pat and develop through larvae and pupae in it
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Decomposition is affected by the physical environment Water
Moisture needed by bacteria and fungi for optimal decomposition But not soaked Temperature Microbial Activity New Hampshire 7.2oC 621mm evaporation West Virginia 12.2oC 720mm evaporation Virginia 14.4oC 806mm evaporation
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Carbon dioxide release versus day temperature
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Decomposers need to get something for their efforts
Carbon Nitrogen Other minerals Balance between immobilization and mineralization Note initial leaching and loss
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Litterbag studies Not Ideal however, because they do not involve soil
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Litterbag studies are not ideal
See net accumulation of immobilized nitrogen from outside the bag when there is a high initial nitrogen level to excourage initial microbial decomposition Leaching Increase Good carbon sources Decrease Poor carbon sources
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Note importance of all macro and micronutrients in decomposition, not just carbon and nitrogen
Sulphur Calcium Manganese Phosphorus etc All are cycled
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More stable physical environment for fauna
Overall pattern for aquatic environment is the same as terrestrial environment More stable physical environment for fauna Shredders Collectors Grazers Scrapers Particulate organic matter to bottom Anaerobic bacteria Note presence of algae etc increasing organic matter too Ciliates and zooplankton eat bacteria
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