Decomposers bacteria and fungi

If scavengers do not take a dead resource immediately it dies (such as hyenas consuming a dead zebra), the process of decomposition usually starts with colonization by bacteria and fungi. Other changes may occur at the same time: enzymes in the dead tissue may start to autolyze it and break down the carbohydrates and proteins into simpler, soluble forms. The dead material may also become leached by rainfall or, in an aquatic environment, may lose minerals and soluble organic compounds as they are washed out in solution.

Bacteria and fungal spores are omnipresent in the air and the water, and are usually present on (and often in) dead material before it is dead. They usually have first access to a resource. These early colonists tend to use soluble materials, mainly amino acids and sugars that are freely diffusible. They lack the array of enzymes necessary for digesting structural materials such as cellulose, lignin, chitin and keratin. Many species of Penicillium, Mucor and Rhizopus, the so-called 'sugar fungi' in soil, grow fast in the early phases of decomposition. Together with bacteria having similar opportunistic physiologies, they tend to undergo population explosions on newly dead substrates. As the freely available resources are consumed, these populations collapse, leaving very high densities of resting stages from which new population explosions may develop when another freshly dead resource becomes available. They may be thought of as the opportunist 'r-selected species' among the decomposers (see Section 4.12). Another example is provided by the early colonizers of nectar in flowers, predominantly yeasts (simple sugar fungi); these may spread to the ripe fruit where they act on sugar in the juice to produce alcohol (as happens in the industrial production of wine and beer).

In nature, as in industrial processes such as the making of wine or sauerkraut, the activity of the early colonizers is dominated by the metabolism of sugars and is strongly influenced by aeration. When oxygen is in free supply, sugars are metabolized to carbon dioxide by growing microbes. Under anaerobic conditions, fermentations produce a less complete breakdown of sugars to by-products such as alcohol and organic acids that change the nature of the environment for subsequent colonizers. In particular, the lowering of the pH by the production of acids has the effect of favoring fungal as opposed to bacterial activity.

Anoxic habitats are characteristic of waterlogged soils and, more particularly, of sediments of oceans and lakes. Aquatic sediments receive a continuous supply of dead organic matter from the water column above but aerobic decomposition (mainly by bacteria) quickly exhausts the available oxygen because this can only be supplied from the surface of the sediment by diffusion. Thus, at some depth, from zero to a few centimeters below the surface, depending mainly on the load of organic material, sediments are completely anoxic. Below this level are found a variety of bacterial types that employ different forms of anaerobic respiration decomposition ... ... of dead bodies,...

bacteria and fungi are early colonists of newly dead material domestic and industrial decomposition aerobic and anaerobic decomposition in nature

Figure 11.1 (a) Spores (conidia) of aquatic hyphomycete fungi from river foam. (b) Rhizomycelium of the aquatic fungus Cladochytrium replicatum within the epidermis of an aquatic plant. The circular bodies are zoosporangia. (After Webster, 1970.)

Hyphomyc Tes

Figure 11.1 (a) Spores (conidia) of aquatic hyphomycete fungi from river foam. (b) Rhizomycelium of the aquatic fungus Cladochytrium replicatum within the epidermis of an aquatic plant. The circular bodies are zoosporangia. (After Webster, 1970.)

- that is, they use terminal inorganic electron acceptors other than oxygen in their respiratory process. The bacterial types occur in a predictable pattern with denitrifying bacteria at the top, sulfate-reducing bacteria next and methanogenic bacteria in the deepest zone. Sulfate is comparatively abundant in sea water and so the zone of sulfate-reducing bacteria is particularly wide (Fenchel, 1987b). In contrast, the concentration of sulfate in lakes is low, and methanogenesis plays a correspondingly larger role (Holmer & Storkholm, 2001).

A strong element of chance determines which species are the first to colonize newly dead material, but in some environments there are specialists with properties that enhance their chances of arriving early. Litter that falls into streams or ponds is often colonized by aquatic fungi (e.g. Hyphomycetes), which bear spores with sticky tips (Figure 11.1a) and are often of a curious form that seems to maximize their chance of being carried to and sticking to leaf litter. They may spread by growing from cell to cell within the tissues (Figure 11.1b).

After the colonization of terrestrial litter by the 'sugar' fungi and bacteria, and perhaps also after leaching by rain or in the water, the residual resources are not diffusible and are more resistant to attack. In broad terms, the major components of dead terrestrial organic matter are, in a sequence of increasing resistance to decomposition: sugars < (less resistant than) starch < hemicellu-loses, pectins and proteins < cellulose < lignins < suberins < cutins. Hence, after an initial rapid breakdown of sugar, decomposition proceeds more slowly, and involves microbial specialists that can use celluloses and lignins and break down the more complex proteins, suberin (cork) and cuticles. These are structural compounds, and their breakdown and metabolism depend on very intimate contact with the decomposers (most cellulases are surface enzymes requiring actual physical contact between the decomposer organism and its resource). The processes of decomposition may now depend on the rate at which fungal hyphae can penetrate from cell to cell through lignified cell walls. In the decomposition of wood by fungi (mainly homobasidiomycetes), two major categories of specialist decomposers can be recognized: the brown rots that can decompose cellulose but leave a predominantly lignin-based brown residue, and the white rots that decompose mainly the lignin and leave a white cellulosic residue (Worrall et al., 1997). The tough silicon-rich frustules of dead diatoms in the phytoplankton communities of lakes and oceans are somewhat analogous to the wood of terrestrial communities. The regeneration of this silicon is critical for new diatom growth, and decomposition of the frustules is brought about by specialized bacteria (Bidle & Azam, 2001).

The organisms capable of dealing with progressively more refractory compounds in terrestrial litter represent a natural succession starting with simple sugar fungi (mainly Phy-comycetes and Fungi Imperfecti), usually followed by septate fungi (Basidiomycetes and Actinomycetes) and Ascomycetes, which are slower growing, spore less freely, make intimate contact with their substrate and have more specialized metabolism. The diversity of the microflora that decomposes a fallen leaf tends to decrease as fewer but more highly specialized species are concerned with the last and most resistant remains.

The changing nature of a resource during its decomposition is illustrated in Figure 11.2a for beech leaf litter on the floor of a cool temperate deciduous forest in Japan. Polyphenols and soluble carbohydrates quickly disappeared, but the resistant structural holocellulose and lignin decomposed much more slowly. The fungi responsible for leaf decomposition follow a succession that is associated with the changing nature of the resource. The frequency of occurrence of early species, such as Arthrinium sp. (Figure 11.2b), was correlated with declines in holocellulose and soluble carbohydrate concentrations; Osono and Takeda (2001) suggest that they decomposition of more resistant tissues proceeds more slowly succession of decomposing microorganisms

Takeda Bag

Figure 11.2 (a) Changes in the composition of beech (Fagus crenata) leaf litter (in mesh bags) during decomposition on a woodland floor in Japan over a 3-year period. Amounts are expressed as percentages of the starting quantities. (b, c) Changes in the frequency of occurrence of fungal species representative of: (b) early species (Arthrinium sp.) and (c) late species (Mortierella ramanniana). (After Osono & Takeda, 2001.)

Figure 11.2 (a) Changes in the composition of beech (Fagus crenata) leaf litter (in mesh bags) during decomposition on a woodland floor in Japan over a 3-year period. Amounts are expressed as percentages of the starting quantities. (b, c) Changes in the frequency of occurrence of fungal species representative of: (b) early species (Arthrinium sp.) and (c) late species (Mortierella ramanniana). (After Osono & Takeda, 2001.)

depend on these components for their growth. Many late species, such as Mortierella ramanniana, seem to rely on sugars released by other fungi capable of decomposing lignin.

Individual species of microbial decomposer are not biochemically very versatile; most of them can cope with only a limited number of substrates. It is the diversity of species involved that allows the structurally and chemically complex tissues of a plant or animal corpse to be decomposed. Between them, a varied microbiota of bacteria and fungi can accomplish the complete degradation of dead material of both plants and animals. However, in practice they seldom act alone, and the process would be much slower and, moreover, incomplete, if they did so. The major factor that delays the decomposition of organic residues is the resistance to decomposition of plant cell walls - an invading decomposer meets far fewer barriers in an animal body. The process of plant decomposition is enormously speeded up by any activity that grinds up and fragments the tissues, such as the chewing action of detritivores. This breaks open cells and exposes the contents and the surfaces of cell walls to attack.

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Responses

  • james
    What was the predominant type of cellulose decomposer that developed?
    7 years ago
  • armando
    Which species is the decomposer?
    6 years ago
  • joesph
    Is mucor a decomposer?
    3 years ago
  • laila
    What is rhizomycelium?
    3 years ago
  • ALANNAH
    Is fungi a decomposer?
    2 years ago
  • belinda
    Does escharishia coli act as a decomposer?
    2 years ago
  • vappu
    Is aspergillus act as a decomposer?
    2 years ago
  • bernd
    How do bacteria and fungi act as decomposer?
    2 years ago
  • PRIMA
    What is species richness for fungi?
    7 months ago
  • AILI
    What are organisms such as bacteria and fungi that are decomposers called?
    3 months ago
  • Olga
    How do decomposers such as fungi store food as sugar and what does this do for them?
    8 days ago

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