ROLES OF SAPROBIC FUNGI IN THE ECOSYSTEM

Discussion Paper

Sarah Jane Ibasco

Angela Menorca

Andrew Tolentino

PPATH 104 B-1L

March 14, 2011

A. DEFINITION OF SAPROBESThey are ecological group of fungi that obtain their nutrition from non-living

organic materials. They have specialized hyphae called rhizoids that allow them to forcibly penetrate most solid materials and anchor to substrates. They also consist of extracellular enzymes that allow them to digest cellulose and lignin found in the organic matter. They are said to be sensitive to disturbance, pollution and changes that occur in the environment. They are usually found growing on woody substrates, humus, soil, grass, leaf litter, dead or decaying matter and animal exudate.

B. AS PRINCIPAL DECOMPOSERS IN ECOSYSTEMS

They play a pivotal role in the decomposition of plant matter, where most of the carbon in terrestrial ecosystems is sequestered. They also play a major role in carbon and nutrient cycling in ecosystems. They can replenish the earth with nutrients for subsequent generations of living things and break down remains of dead plants and animals. They can also provide associated plants a sense of protection from pathogens. Saprotrophic microbes may offer associated plants some measure of protection from pathogens. Plant roots exudates can include sideropohores, which are low molecular mass, iron-binding agents produced under iron-limiting conditions. They can limit growth of microbes in the vicinity of roots by reducing the availability of iron, but soil-borne root-infecting fungi are often inferior to soil saprotrophic fungi in their ability to compete for resources. This superior competitive ability of soil saprotrophic fungi can benefit the neighboring plant. This is due to the reason that the growth of the pathogenic fungi is limited by a shortage of iron, a situation that is enhanced by the harmless saprotrophs out-competing the pathogens for the limiting nutrient. If growth of organisms continues in a medium until a particular nutrient is exhausted, that nutrient is said to be limiting. Therefore, the plants are exposed to fewer pathogens when saprotrophs are present. Furthermore, saprotrophic soil fungi may also produce antibiotics that inhibit the growth of root-infecting fungi.

C. TYPES OF SAPROBESOne type of saprobes is known as litter decomposers which decay leaves. They

employ different strategies to gain access to their substrates. Some reproduce quickly by producing many spores that can colonize news substrates. Other fungi employ rhizomorphs that allow them to explore the forest floor. This includes many mushrooms that grow on the forest floor such as Gymnopus dryophilus and Marasmius. Another type is classified as wood decay fungi. Among them, mostly basidiomycetes are among the few organisms that are capable of degrading lignin. Example of these species are

Xeromphalina campanella, Cytidia salicina, Fomes pinicola and Climacodon septentrionale. Wood decay fungi have two types namely white rot fungi and brown rot fungi. The white rot fungi are capable of degrading both lignin and cellulose. They leave the wood bleached and with a stringy consistency. They can degrade diverse organic compounds besides lignin and they can be used in bioremediation which is known to be the reduction of environmental pollutants using living organisms and biopulping, the treatment of wood fibers in paper production. As for the brown rot fungi, they have a capability to degrade cellulose but leave the lignin behind. They are brown-rotted wood which has a crumbly consistency, breaking up into cubical fragments, and has a red-brown color. Brown rot residues are highly enriched in lignin and are very resistant to further decay they make up a major component of humic soils in some forest types

D. ROLE OF SAPROPHYTIC FUNGI IN DIFFERENT ECOSYSTEMS

Most marine fungi have been identified from substrata containing lignocellulose, and therefore it is not surprising that several genera have been implicated in wood decay activity within marine and estuarine environments. Although marine borers are recognised as particularly aggressive wood degraders in marine environments, they are unable to tolerate the reduced oxygen tensions found in sediments (Blanchette et al., 1990). Many marine fungi appear to be able to tolerate low oxygen tensions and so may be the dominant agent of lignocellulose turnover in marine sediments, since although lignocellulolytic bacteria exist they are not aggressive degraders of this substratum (Holt and Jones, 1983;Singh et al., 1990). This is of particular importance when considering the vast biomass represented by lignocellulose in the form of mangrove and other plant materials in coastal areas with high sediment loading. Fungi are also extremely important decomposers of wood in the upper intertidal region where marine borers are unable to survive.

In the decomposition of herbaceous substrata, the importance of the mangrove ecosystem in terms of their export of plant detritus and faunal biomass supporting of shore biological production is well documented (Lee, 1995). Microbes are responsible for the transformation of the polymeric compounds into dissolved or particulate organic matter utilizable by other consumers in the food web. These microbes comprise the bacteria, the eumycotic fungi including the ascomycetes, the mitosporic fungi, the chytrids, and the chromistan group which were formerly known as the oomycetes, the labyrinthulids and the hyphochytrids (Hawksworth et al., 1995). There are numerous publications describing the association of these various groups with decaying plant litter in mangrove s (e.g. Fell and Master, 1973; Newell, 1976; Nakagiri et al., 1996) and their presence in sediments (e.g. Lee and Baker, 1973; Ulken, 1984; Ito and Nakagiri, 1997). However, comparatively little information is available on their activity so their role in the

recycling of nutrients is unclear. This was essentially hampered by the slow development of methods for the ecological study of the aquatic mycelial eukaryotic decomposers compared with the well-established (published and ®eld-tested) methods used for the measurement of prokaryotic productivity (Newell, 1994). The recently published or re®ned methods formeasuring mycelial mass and productivity have enabled a better understanding of the role of the mycelial decomposers in diferent marine ecosytems. For example, it is now clear that di€erent dominant fungal groups are responsible for the decomposition of herbaceous material in the mangroves and saltmarsh ecosytsems (Newell, 1996).

In the decomposition of animal remains, little is known of the role of fungi in the decomposition of animal remains in the sea. Tunicin is an animal cellulose which occurs in the test of tunicates and it has been shown that marine fungi may play role of degrading tunicate cellulose in nature (Kohlmeyer & Kohlmeyer, 1979). Marine fungi are also known to invade calcareous substances, such as the shells of molluscs, test of barnacles, or linings of burrows. It is also thought that thraustocytrids and other oomycete fungi play an important role in the recycling of elements tied up in animal remains, although evidence is lacking.

E. ROLE OF SAPROPHYTIC FUNGI IN NUTRIENT CYCLING

In agricultural systems the decomposition of plant residues is carried out by microorganisms. The rate of plant residue decomposition depends on the physico-chemical environment, the nature of the decomposer (soil microbe) community (Couteaux et al., 1995; Saetre, 1998) and the biochemical composition of the organic material (Elliott et al., 1993). Most biochemical decomposition of organic materials is carried out by heterotrophic microorganisms, among which fungi are an important group (Shukla et al., 1990). Fungal hyphae penetrate the decomposing material both chemically and mechanically and decompose the more recalcitrant organic matter fractions such as lignin and cellulose. Fungal succession on a natural substratum reflects sequential release of different organic and inorganic nutrients, interaction between each individual and substratum, competition among individual fungi (Kshattriya et al., 1996). Fungal hyphae physically stabilize compost into small aggregates providing the compost with improved aeration and drainage. Microorganism activity and various physico-chemical agents bring about changes in the structure and chemical composition of the organic matter which in turn regulates the species composition of late colonizing microorganisms (Adedji, 1986).

Terrestrial decomposition is mediated by microbes. Generally, litter decomposition is faster initially due to the utilization of readily available energy sources by the microbes, a loss of water-soluble components and non-structural carbohydrates,

and the removal of residue particles by the soil microflora. The increased microbial populations after placing the residues in the soil may be due to a suitable biophysical environment that increased the surface area for microbial colonization and the organic matter as their energy source (Anderson and Domsch, 1985) and other nutrients. The initial concentration of C, N and K showed significant, positive correlations with microbial population. However, P, Na, lignin, and lignin:N showed significant negative correlations. The gradual increase in microbial population and fungal diversity with decomposition period may also have been due to improved moisture levels, temperature moderation and nutrient concentration.

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