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Life Strategies

Introduction

Fungi are found in a wide variety of habitats, and their life cycles differ accordingly. LINK A number of strategies adopted by fungi are similar to those found in other organisms, though the fungi appear to have some different yet specific approaches. In part, these differences relate to the relative size of the organisms, the nature of the foods used by the organisms, and the growth and dispersal of the organisms.

Strategies - General

MacArthur and Wilson (1967) developed the ecological concept of stability of population in relation to strategies for using the habitat. They introduced the term ‘r’ selection to denote organisms that had high rates of reproduction, and ‘K’ selection to denote organisms in a population close to their carrying capacity in the habitat.

“r” strategists have several concurrent characteristics: they tend to be able to rapidly occupy a new habitat and they require readily available nutrients. Explosive rates of reproduction are usually followed by a rapid decline in the size of the population with the onset of competition. The life cycle is typically short. Thus the organisms are common colonisers, with large fluctuations in population size.

“K” strategists use resources efficiently, and reproduce more slowly. They require greater resources to reproduce, protect the resources and themselves, and populations tend to be stable.

In effect, the two strategies are at the edges of a two dimensional continuum.

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Plant Strategies

Plants have been suggested to have three fundamental life strategies. Grime (1979) described ‘C’, ‘S’, and ‘R’ selection. “C” stands for competitive selection. These plants are able to compete effectively. Thus they are found commonly in undisturbed habitats where productivity is relatively high. “S” stands for stress, and these plants have adaptations which enable survival in stressful environments. The stress may be related to resources such as minerals, or conditions such as temperature. Finally, “R” stands for ruderal, and these plants are similar to the ‘r’ strategists of MacArthur and Wilson. The plants have rapid rates of reproduction with short life spans. “r” strategists have evolved in productive, usually disturbed, environments. In effect, the three strategies are at the edges of a multi-dimensional space.

The question to resolve is whether these strategies adequately explain the life cycles we see in the fungi. The fungi differ from animals (MacArthur and Wilson) and plants (Grime) in several fundamental ways. Fungi are pleomorphic. One genet can have cytoplasmic continuity linking several different “habitats” at the same time. The cytoplasm within one organism can be closed off from the remainder, and different parts of the fungus can function in different ways. Fungi can form several types survival structures each with different capacities to respond to the environment, and thus have the potential to utilise a variety of strategies for continuation over time. The fungal colony can expand and contract according to the environmental conditions. Growth is indeterminate. Finally, fungi compete using a variety of strategies, against one another and with other organisms. Animal and plant strategies seem inadequate to describe the potential fungal strategies.

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Fungal Strategies

Fungi have been suggested to have specific strategies. As the strategies can change over time, these are referred to as ‘behaviours’. The classification of Grime has been modified (see Andrews 1992) into the following behaviours:

R Selected

Members of the order Mucorales are widely considered to be ruderals. The genera Mucor and Rhizopus are common in soil and air, and Pilobolus and Pilaira on dung. Their sporangiospores germinate rapidly in response to available energy. The mycelium develops rapidly when nutrients are available. Sporangiospores or other reproductive propagules are formed very soon after initiation of the mycelium. At the first sign of depletion of nutrients, or competition, the mycelium starts to collapse. Each individual is "alive" for a short time.

Ruderals capture the readily available resources quickly. However, they appear unable to utilise complex carbohydrates because they lack the appropriate array or activity of enzymes. Energy is rapidly transformed into reproductive units, not hyphae. Sporangiospores appear to have a relatively short life, thus the fungi do not survive for long or at high densities in stable, low nutrient, highly competitive habitats. LINK

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S Selected

Some fungi may grow in environmental conditions that are too extreme for survival of other fungi or organisms. The stresses include extreme osmotic potential, heat, pH, mineral concentration, or lack or type of of organic nutrients. The adaptation may be so complete, that the fungi are unable to metabolise unless in the stressful environment. Others may function at low metabolic rates in less extreme conditions. 'S' selected fungi can be very important, for they include the spoilage fungi found in preserved food.

WATER AVAILABILITY. Fungi, generally, are able to tolerate much lower water availability than other organisms. Survival in extremely low osmotic potential has been studied in relation to food spoilage. Contaminants found on sugary jam and salty foods tend to be fungi. In fact, of the most osmophilic organisms, Monascus bisporus, can grow at a water activity of 0.62 and Saccharomyces rouxii at 0.61. DNA is denatured at just below this activity. The latter fungus is used ripen soy sauce and miso paste. LINK

The mechanisms enabling functionality under osmotic stress are related to the presence of compatible solutes. Compatible solutes such as glycerol and other polyols are stored in high concentrations in the cell countering the effects of the loss of water from the cell. Glycerol appears to protect enzymes from accumulation of Na and loss of water, both of which may denature the enzymes. Polyols may also protect membranes. Osmophilic fungi utilise compatible solutes to maintain water potential in the cell, though their rates of metabolism and thus growth are extremely slow.

TEMPERATURE. Survival and growth at extreme temperatures is found widely among microbes. Cold-tolerant (at 0oC) organisms are called psychrotolerant, and heat-tolerant (above 40oC) called thermotolerant. Thermotolerant and psychrotolerant fungi can grow over a wide range of temperatures. Even so, few fungi can grow above 65oC, and even fewer below –3oC. Those which require high or low temperatures for growth are called thermophiles or psychrophiles, respectively.

The functioning of thermophiles and thermotolerant fungi can be followed in the composting process. LINK A variety of fungi are incorporated with the plant material at the beginning of the process. As the plant material is degraded by a variety of microbes, heat is generated, and as heat cannot escape from the centre of the pile, the temperature of the pile increases. Those fungi able to tolerate high temperatures remain active, but as the temperature increases, only thermophilic organisms remain in the centre of the pile. The rest are killed or become quiescent in the high temperatures. A variety of fungi remain active at the outer surface, where heat escapes. Eventually, the rate of decomposition slows down and thermotolerant and then mesophilic fungi reinvade the pile.

The spores of some fungi also survive exposure to extreme temperatures when they are dry. The capacity is referred to as thermostability. Thermostability is also found widely among the fungi.

The fungi that function in extreme aridity, extreme temperatures, and saline conditions are stress tolerant species.

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C Selected

Combative fungi gain organic nutrients from recalcitrant sources by utilisation of a range of enzymes, gain access to resources used by other fungi and also defend themselves using a variety of processes.

Combative fungi utilise a range of enzymes to break down complex organic molecules. For instance, soil fungi utilising the recalcitrant plant remains require enzymes that result in the degradation of cellulose, lignin, waxes, and polyphenolics. LINK A huge array of organic molecules are amenable to digestion by one or other fungus, acting alone or in unison LINK; some of these enzymes have industrial applications.

Lignin is a complex molecule usually found combined with cellulose in wood LINK, and other plant remains. Lignin is very resistant to degradation because of the variety of chemical bonds between phenyl and other aromatic groups. Lignin in different plants often has different chemical constitution. In addition, lignin is commonly associated with tannins, phenol and other aromatic molecules which may inhibit fungal metabolism. Most of the fungi that degrade (oxidise) lignin are found in the Basidiomycota, with a few in Ascomycotina (Xylariales). A degree of specialisation between potential wood rot fungi and host is common, due to the enzymes needed to degrade the wood, tolerate the antifungal molecules, and the capacity to overcome fungi using the less recalcitrant organic molecules. LINK

Combative fungi compete with other fungi by protecting their resources by the use of antagonism including antibiotics, mycoparasitism and contact inhibition. LINK

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Conclusion

Fungi utilise a variety of mechanisms to cope with the variety of habitats and sources of organic carbon. Generally, different specific strategies are found in each of the range of fungi in each habitat. The strategies have been classified into ruderal, stress selected and combative. However, it is important to realise that these categories are artificial. The characteristics exhibited by any one fungus are likely to differ with the specific challenges, and thus differ over space and time, even within one organism.

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References

Andrews JH 1992 Fungal Life-History Strategies. In The Fungal Community 2nd edition, eds: CG Carroll and DT Wicklow, Marcel Dekker, New York pp 119 – 145.

Grime JP 1979 Plant Strategies and Vegetation Processes. John Wiley, Chichester. UK.

MacArthur RH and Wilson ED 1967 The Theory of Island Biogeography. Princeton Uni Press, Princeton, USA.

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Decomposition of Plant Remains

Fungi interact with a wide variety of organisms. They compete and/or function synergistically with small animals, arthropods, protists, bacteria and other fungi. This is illustrated quite clearly in the degradation of plant remains.

Plant litter remains on the soil surface for varying periods: in rainforest the material may be completely transmuted within 12 months, under sclerophyllous vegetation of arid regions it may take decades. In essence, the concentration of phosphorus is highly correlated with the potential rate of plant decay. Plants grown in soils with high available phosphorus form softer leaves which degrade more rapidly. Sclerophyllous plants have higher concentrations of lignin and phenols in their leaves, and the plant material degrades more slowly.

When litter falls, the material is fragmented by arthropods. Larger animals have an insignificant direct influence on these processes. The arthropods increase fragmentation exposing larger surface area of attack for microbes. Their faeces increase the complexity of the food resource, especially increasing available nitrogen. Further, their action increases the compaction of the plant remains, increasing the humidity and decreasing the fluctuations in humidity and temperature. Under European conditions, arthropods are thought to be associated with 10-20% of litter diminution.

In conditions of lower humidity, fungi are the predominant microbe degrading the litter. LINK Further, increased concentrations of phenols and tannins, and low pH slow bacterial activity. The fungi involved with degradation of litter also transform some of the materials. It is now widely beleived that the remnant materials in soil have both plant and fungal origins. The importance of fungal melanins to humus has been suggested, but the processes of degradation of plant remains, transformation to humus, and use by microbes are still unclear.

The material in soil that remains following degradation is called humus. Humus is the recalcitrant remains of plants and microbes (mostly fungi), and includes complex polyphenolic and aromatic materials. These complex organic materials may remain in the soil for decades provided they are protected. LINK They are crucial stores of recalcitrant carbon in soil, and may form the target for attempts to increase the carbon sequestered in soil.

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