6.2 - Decomposition and carbon recycling

Microorganisms as decomposers

Dead organisms and organic waste remain food sources because they contain molecules that other organisms can use.

Bacteria and fungi are important microbial decomposers. Dead leaves contain carbohydrates, including cellulose, and dead animals contain carbohydrates, proteins and lipids. Faeces and other organic wastes also supply material for decomposition. These substances contain carbon originally incorporated into organisms, often through photosynthesis followed by feeding. Death does not destroy that carbon or automatically turn it into carbon dioxide.

Many decomposer bacteria and fungi obtain nutrients by saprotrophic nutrition: they digest dead material outside their cells and absorb the soluble products. Fungal hyphae are fine threads that can grow over and into material; their contact with a substrate provides sites for enzyme release and nutrient uptake. Bacteria can also grow on and within decomposing material. Microorganisms are living organisms carrying out metabolism, rather than chemicals that simply make a body disappear.

Larger detritus-feeding animals, such as earthworms, can fragment dead material and expose more surface to microbial attack. Their feeding can assist decomposition, but fragmentation alone does not explain the microbial conversion of organic carbon into carbon dioxide. Likewise, the changing community on a dead animal can provide forensic evidence, but this lesson follows the feeding and carbon transfers performed by the microorganisms themselves.

The surroundings affect these living processes. Water is needed for normal cellular activity and for dissolved substances to move. Temperature changes enzyme-controlled reaction rates: warming within a suitable range can increase activity, whereas excessive heat can damage cells and enzymes. Oxygen availability affects aerobic respiration. Drying, cooling or low oxygen can therefore slow particular pathways; they do not remove the carbon atoms. Different microorganisms tolerate different conditions, so there is no universal decomposition timetable.

Extracellular digestion and uptake

A microorganism must make many food molecules small enough to cross its cell membrane before it can use them.

Much dead material contains large, insoluble molecules. Cellulose chains in a plant cell wall, for example, cannot simply pass intact through a decomposer's membrane. The microorganism secretes extracellular enzymes onto the material. Extracellular means outside the cell: the enzyme acts on a substrate in the surroundings, rather than requiring the whole substrate to enter first.

Enzymes catalyse the breakdown of larger molecules into smaller products. Hydrolytic enzymes use water to break suitable bonds. Different enzymes act on different substrates: cellulases help break down cellulose to smaller sugars, while proteases break proteins into smaller peptides and amino acids. Do not treat one enzyme as able to digest every kind of organic molecule. A community containing different decomposers can act on different components of the same material.

The small soluble products can then be taken across the microorganism's cell membrane. This uptake gives the cell organic molecules for metabolism and growth. The sequence matters:

1. The microorganism secretes enzymes.
2. The enzymes digest suitable large molecules outside its cells.
3. Small soluble products are taken into the cells.

Digestion and uptake are different steps. Digestion changes molecules; uptake moves products across a membrane. Neither step alone means all the carbon has been released into the atmosphere. For example, breaking a carbohydrate into sugars leaves carbon atoms in those sugars. The next question is what happens to them inside the microorganism.

This also explains a useful prediction. If a decomposer cannot secrete an enzyme needed to break down a particular polymer, that polymer provides little usable food even when it is abundant. Supplying an appropriate small soluble digestion product can bypass that initial breakdown problem, provided the cell can take up and metabolise the product. Availability to a microorganism depends on molecular form as well as total amount.

Assimilation, respiration and carbon recycling

Carbon entering a decomposer can become part of a new organism or return to the environment in an inorganic form.

A decomposer uses some absorbed organic material to make cell components, including proteins, membranes and new cell walls. Carbon is then retained in microbial biomass, the material making up the microorganisms. Growth therefore transfers some carbon from dead material into living organisms. If those microorganisms are eaten, their carbon can pass to consumers; when they die, their remains provide further organic material for decomposition.

Other absorbed organic molecules are used as respiratory substrates. In aerobic respiration, their oxidation uses oxygen and releases carbon dioxide and water. The overall reaction transfers energy for cellular work, including synthesis and active transport. For glucose, the familiar word equation is: