Transcript Overview
Lecture 2 ROLES OF FUNGI & MICROBES IN NATURAL FOREST ECOSYSTEM Overview • In tropical forests all organisms are dependent to some extent on bacteria and fungi. • Some animals such as wood and leaf-eating insects depend on symbiotic gut microbes to digest cellulose in their food supply, while other insects utilize fungi directly as a food source. What are microbes? • Microbes are organisms that we need a microscope to see. • The lower limit of our eye's resolution is about 0.1 to 0.2 mm or 100 - 200 um. • Most microbes range in size from about 0.2 um to the 200 um upper limit, although some fruiting bodies of fungi can become much larger. • Microbes include the bacteria, algae, fungi, and protozoa. Cont’ • Throughout the history of time, bacteria have caused more human deaths on Earth than any other known cause, directly through the diseases of cholera, dysentery, meningitis, measles, pneumonia, scarlet fever, tuberculosis, and others. • At the same time, the Good is that microbes provide many essential services to Earth, including allowing plant productivity (the dominant base of Earth's food web) to be sustainable, and allowing humans to live - basically, without microbes, humans wouldn't be alive. Cont’ • Microbial organisms are powerful at the global scale – 50% of the total oxygen produced over the history of the Earth is from bacteria; 75% of additions of nitrogen to the atmosphere, and 92% of removal from the atmosphere are due to bacteria. • For nitrogen, bacteria produce 88% of the nitrous oxide released to the atmosphere, N2O, which is 300 times more potent than CO2 as a greenhouse gas. Microbes are also responsible for ~70% of the methane production on Earth (25x more potent than CO2), and ~50% of the CO2 put into the atmosphere comes from bacteria. Bacteria • Bacteria are found everywhere in water, soil, and even air. • They are small prokaryotic cells, typically from 0.2 to 1 um in length, are capable of living in boiling water, frozen ground, acid volcanoes, and at the bottom of the ocean. • They can reproduce by doubling with a generation time of 20 minutes, or survive for centuries in a resting stage. In natural waters (lakes, streams, oceans) their generation time is around 1 day. In soils they live in a film of water around plant roots or other particles, and their activity is dependent on the temperature and the amount of available moisture. Cont’ Some bacteria are capable of locomotion, and they possess the only rotary motor known in all of biology. This motor, similar to a wheel and axle, is capable of spinning a flagellum at speeds of 100 revolutions per second, or 6,000 rpm. Bacteria can propel themselves at a rate of 10 times their body length each second. Bacteria, like all cells, are composed mostly of carbon, oxygen, nitrogen, hydrogen, phosphorus, and sulfur in the following percentages: Fungi • Fungi grow in the form of a finely-branched network of strands called hyphae which are 5-10 um in diameter. These hyphae can release digestive enzymes and take up nutrients over their entire length. • Fungi can absorb only small molecules such as sugars or peptides less than size amino acids. • The reproductive organs of the fungi are called fruiting bodies or sporangia, which are sacs or other tissues that contain the fungi spores. Cont’ • Fungi are uncommon in aquatic environments. • On land, the amount of hyphae in the soil is measured in hundreds or thousands of meters of length per gram of soil. For example, the total length of hyphae in a gram of soil (about the amount that would fit on the fingernail of your little finger) can reach up to 1,600 meters. • Fungi secrete enzymes that can break down cellulose into glucose, one of the few kinds of organisms able to do this. • Fungi are the only known organisms that degrade lignin completely. Cellulose and lignin are structural materials in plants that are difficult to degrade. The fungi do not use the breakdown products of lignin, but instead they use hydrogen peroxide to oxidize lignin in place. The breakdown products diffuse away, exposing the cellulose to enzymatic attack. Fungi Protozoans • Protozoans are single-celled eukaryotes, not photosynthetic, that move by flagella or cilia. • In oceans and lakes, the small 2-10 um long flagellates are the most important predators on bacteria. The larger ciliates (e.g., Paramecium) prey mostly upon photosynthetic cyanobacteria and small eukaryotic algae. • In some termites, anaerobic protozoans in the gut degrade cellulose. Types of Micro-Organisms • Microorganisms are classified as autotrophs or heterotrophs based on whether or not they require pre-formed organic matter. • Autotrophs derive energy from either light absorption (photoautotrophs) or oxidation of inorganic molecules (chemoautotrophs). • In most of the light reactions the bacteria are fixing carbon dioxide into organic carbon, just as green plants do. Some photosynthetic bacteria (photoheterotrophs) require pre-formed organic matter as reducing agents, but generate ATP from the absorption of light energy. Cont’ • Some bacteria and fungi (heterotrophs) used pre-formed organic matter as both a source of energy to generate ATP and as a source of carbon for the cell, just as animals do. • The following table summarizes the classification of the ways in which microbes process energy. Classification Energy source for generating ATP Source of carbon for Example of organisms the cell Photoautotroph Light CO2 Bacteria, plants Chemoautotroph Inorganic compounds CO2 Bacteria Photoheterotroph Light CO2, organic matter Bacteria Heterotroph Organic matter Organic matter Bacteria, fungi, animals Nutrient Cycling • Decomposition: Without microbes, organic matter on the forest floor and in the soil would never decompose. • The rate at which these microorganisms decompose dead material is directly responsible for the availability of nutrients for plants. • As the humidity and temperatures in rainforests are high, conditions are ideal for rapid microbial decomposition. • However, the rates of decomposition will differ according to which microorganisms are present, the character of the organic matter, the physical and chemical environment of the soil and so forth. • Apparently microbial and fungal populations are quite sensitive to fluctuations in soil moisture and other disturbances. Oxygen Generation in the Atmosphere • Almost all of the production of oxygen by bacteria on earth today occurs in the oceans by the cyanobacteria or "blue-green algae. Consumption of Poor Quality Food • In the ocean, most of the primary productivity is consumed by herbivores. • In contrast, in terrestrial systems most of the primary productivity is not consumed by the herbivores. • The reasons for this difference are: (1) animals lack digestive enzymes capable of using cellulose and lignin and other structural plant compounds; (2) plants often have anti-grazing toxins, aromatic resins, or thorns; (3) most land plant tissue is poor in mineral nutrients (especially N and P) compared to the tissue in the herbivore. Cont’ • Nitrogen fixation: Nitrogen fixation is an essential function of microbes in forests. Without bacteria which are capable of converting gaseous nitrogen into nitrates and nitrites which plants can utilize, rainforest soils would rapidly become depleted of this essential mineral in usable form. • Many million tons of nitrogen are converted annually and added to the soil by these organisms. In the many tropical soils which are nutrient-poor, only nitrogen-fixing bacteria allow plants to survive. Tree dispersion and other ecological effects • Pathogenic microbes play a role in preventing “clumping” of trees or plants of a particular species and ensuring their wide dispersal throughout the forest. • When plants of one species live close together, they are subject to attacks by pathogenic agents, while if they are more widely dispersed, transmission of disease agents is more difficult. • In this way, the presence of microbial and fungal pathogens play a role in structuring the composition of tropical forests, by ensuring that most individuals of a given (tree) species will be fairly widely dispersed (van der Putten, 2000). Cont’ • This has implications for monocrops, such as oil palm, soy beans, and rubber, which are being raised on converted rainforest land in many tropical areas. For example, the large gaps in the forests made to create oil palm forests in Malaysia and Indonesia have contributed to the spread of the root-rot fungus Ganoderma, and another root-rot fungus, Phytophthora cinnamon, spread widely after logging in Australia. • Similarly, cutting in Eucalyptus forests in Australia has led to a great increase in severity of outbreaks of this pathogen (Lodge, et al., 1996; Gilbert and Hubbell, 1996). • These outbreaks have many ancillary effects, including alterations in forest structure, changes in animal populations (including endangerment of rare species), and decreases in tree density. Food sources • Microbes provide food for many small organisms in forests and also as agents which allow the digestion of otherwise indigestible food sources in the guts of many animals. • Fungi are important food sources for some invertebrates such, as ants and their fungus gardens and beetles. • On the island of Sulawesi (Celebes), 40% of the beetles feed on fungi (Lodge, et al., 1996). Regulators of population size • Pathogenic microorganisms have important effects on the population size of any organisms which they infect. • For instance, defoliation of green plants is restricted by the attacks of pathogens on insect predators of those plants. Mycorrhizae • Many fungi are present in rainforest soils, and some form close associations with tree roots. These presumably symbiotic associations are known as mycorrhizae. • Fungal hyphae penetrate the root and associate with roots. Up to 90% of all tree roots are involved in these associations. • The fungi colonize roots through the spread of the hyphae or the dispersion of spores. • Mycorrhizae are believed to be involved in nutrient capture and that much carbon and other minerals (nitrogen and phosphorus, especially) are transferred from the soil to the roots by mycorrhizal associations. • In turn, the plant passes manufactured carbon compounds to the fungi, and since the mycorrhizae are themselves eaten by soil organisms, carbon is transferred rapidly from the host tree to the soil ecosystem. Cont’ • Mycorrhizae apparently facilitate the uptake of water by roots and increase the resistance of roots to pathogens. • Mycorrhizal associations appear to play key roles in growth, nutrient cycling and primary productivity in tropical rainforests. • • They also appear to have some control over the structure of the plant community and the course of succession. Where forest is disturbed, plants which do not form mycorrhizal associations will predominate; later, as the fungi invade the area, there will be a succession of plants which tolerate and, later, require these associations. Cont’ • Mycorrhizal fungi also act as social agents, as they interconnect trees through their hyphae. This may mean that trees can transfer carbon among themselves via the fungal mat, so that trees in the shade (and thus less able to photosynthesize) are “subsidized” by well-illuminated trees. • It is possible that young trees in shady environments are enabled to survive by this mechanism, at least until they can extend their branches into the canopy. • It has been speculated that forests are less competitive than they appear, particularly if the mycorrhizae act to reduce competition for nutrients by equitable distribution (Read, 1977). • In one experiment, tree seedlings were found to transfer carbon between species bidirectionally (Simard, et al., 1997) Cont’ • Little is known about the ecology of mycorrhizae, but they appear to have a narrow range of tolerance. • some can colonize more than one species of tree; others apparently cannot. • Mycorrhizae don’t seem to reform easily in disturbed or logged environments. • Disturbances of forests by logging may contribute to further forest destruction by disrupting them. 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