Two important symbioses involve fungi: the mycorrhizae that occur on the roots of almost all vascular plants and the lichens that have evolved entirely different body forms from those of their symbionts.
Two general types of mycorrhizae occur, differentiated by whether the hyphae live within the cortical cells of the roots or remain outside the cells: endomycorrhizae (endo = within; myco = fungus; rhizae = roots) and ectomycorrhizae (ecto = outside). Zygomycete taxa are components of most endomycorrhizae while basidiomycetes and a few ascomycetes form ectomycorrhizae.
Life is becoming precarious for lichens in many urban environments today. Many lichens are intolerant of air pollutants. They have no means of getting rid of the elements, toxic or useful, which they absorb. Sulfur is particularly toxic to many, and sulfur dioxide released from burning coal has eliminated many susceptible species from cities. Lichens can be used as biomonitors—and warnings—of the quality of the air we breathe.
An interesting proposal postulates that a symbiosis between early fungi and early plants permitted the plants to establish themselves on land before they had evolved roots with which to absorb vital water and minerals from the soil. The fungi could do this for them and already were associated with some plants, hence the start of the mycorrhizal association.
http://www.sciencedaily.com/releases/2008/03/080305144228.htm
...................................................................................................
The cycles through the lithosphere are said to be sedimentary cycles (from the sedimentary rocks in which the elements reside) and are of such long duration that the elements are essentially removed from further cycling until tectonic (mountain building) or volcanic eruptions expose the rock layers to new weathering. Elements have shorter residence times in the air in the atmospheric cycles and generally the least of all in the biota. A surprisingly small amount of the world's matter is held in living organisms at any time; the reservoirs for the elements of concern to life are almost entirely abiotic ones.
Plants are more than merely users of the chemicals of the Earth; through their metabolic processes they exert a considerable influence on the cycling of the major chemicals. Plants have been indispensable through deep (geologic) time in maintaining the steady-state condition of most of the biogeochemical cycles. All of the ecologically significant chemical elements have both an abiotic and a biotic component. Carbon, hydrogen, and oxygen enter plants from the air and from the decomposition of organic matter, but the other 14 essential nutrients are taken from the soil, as are the miscellaneous other elements used in small amounts by a variety of organisms. Nutrients released from weathered rocks enter the soil solution and move by diffusion and mass flow to the sites of biological activity. Rock weathering is a long-term process that adds small quantities of minerals slowly, over time to the ecosystem. Plants and other organisms, therefore, obtain most of the minerals they need by recycling existing organic matter.
http://www.cliffsnotes.com/study_guide/Materials-Used-by-Organisms-Are-Recycled.topicArticleId-23791,articleId-23782.html
A dynamic movement of a colonist and its host move along a continuum between antagonism and cooperation. A pathogen may live with us but not cause disease most of the time, like herpes simplex viruses (that cause fever blisters.) The pathogen occasionally causes serious illness. The movement along the continuum is dynamic and changing.
The bacteria that live in our large intestines can also move along the continuum. Most of the time they are mutualists, benefitting us and being benefitted by living inside us, but can you guess when they might instantly shift to the pathogenic end of the continuum? Answer: If your appendix ruptures, or if you get a gunshot wound to the abdomen, those very same bacteria move into other compartments of your body and become serious pathogens.
Natural selection is the pervasive explanatory paradigm throughout all of these odysseys into the coexistence of hosts and colonists. Both evolve together, or co-evolve. We have two new terms here: coevolution and symbiosis. Coevolution is nothing more than two or more organisms evolving in response to each other, with natural selection working independently on both. Symbiosis is just what the word is constructed to mean: living together. Symbiosis includes the entire continuum between antagonism and cooperation, with commensalism in the middle (in which one organism benefits and the other is more or less unaffected.)
Mycorrhizae
Fungi and the roots of almost all vascular plants form mutualistic associations called mycorrhizae (singular, mycorrhiza). The fungus gets its energy from the plant, and the plant acquires an efficient nutrient absorbing mechanism—the actively growing hyphae that penetrate regions of the soil untapped by root hairs. Phosphate uptake especially is increased when mycorrhizae are present.Two general types of mycorrhizae occur, differentiated by whether the hyphae live within the cortical cells of the roots or remain outside the cells: endomycorrhizae (endo = within; myco = fungus; rhizae = roots) and ectomycorrhizae (ecto = outside). Zygomycete taxa are components of most endomycorrhizae while basidiomycetes and a few ascomycetes form ectomycorrhizae.
Lichens
The symbiotic relationship of fungi with either algae or cyanobacteria produces a body—a lichen—so distinctly different from either of its symbionts that it is treated as a separate organism. The fungal hyphae give the lichen thallus its characteristic shape, and the cells of its photosynthetic partner are dispersed among them. While the algal or cyanobacterial member can live independently, the fungus cannot, so the fungus in essence is a parasite on the photosynthesizer in the lichen thallus. The fungus, however, provides a “home” for the photosynthetic cells as well as absorbing water and nutrients that the photobiont uses. This makes the symbiosis mutualistic as much as parasitic in the view of some biologists.Life is becoming precarious for lichens in many urban environments today. Many lichens are intolerant of air pollutants. They have no means of getting rid of the elements, toxic or useful, which they absorb. Sulfur is particularly toxic to many, and sulfur dioxide released from burning coal has eliminated many susceptible species from cities. Lichens can be used as biomonitors—and warnings—of the quality of the air we breathe.
A Kingdom Separate from Plants
The fungi (singular,fungus) once were considered to be plants because they grow out of the soil and have rigid cell walls. Now they are placed independently in their own kingdom of equal rank with the animals and plants and, in fact, are more closely related to animals than to plants. Like the animals, they have chitin in their cell walls and store reserve food as glycogen. (Chitin is the polysaccharide that gives hardness to the external skeletons of lobsters and insects.) They lack chlorophyll and are heterotrophic. Familiar representatives include the edible mushrooms, molds, mildews, yeasts, and the plant pathogens, smuts and rusts.
Fossil record
Evidence of fungi growing within the cells of 400-million-year-old Silurian-Age vascular plants suggests an early origin for the fungi. The first fungi developing from very early eukaryotes undoubtedly were unicellular; coenocytic filamentous forms were a later development.An interesting proposal postulates that a symbiosis between early fungi and early plants permitted the plants to establish themselves on land before they had evolved roots with which to absorb vital water and minerals from the soil. The fungi could do this for them and already were associated with some plants, hence the start of the mycorrhizal association.
Secrets Of Cooperation Between Trees And Fungi Revealed
ScienceDaily (Mar. 5, 2008) — Plants gained their ancestral toehold on dry land with considerable help from their fungal friends. Now, millennia later, that partnership is being exploited as a strategy to bolster biomass production for next generation biofuels. The genetic mechanism of this kind of symbiosis, which contributes to the delicate ecological balance in healthy forests, also provides insights into plant health that may enable more efficient carbon sequestration and enhanced phytoremediation, using plants to clean up environmental contaminants.
http://www.sciencedaily.com/releases/2008/03/080305144228.htm
...................................................................................................
Materials Used by Organisms Are Recycled
The Earth is essentially a closed chemical system through which the elements necessary for life are reused and move from abiotic reservoirs to the biota and back in global biogeochemical cycles. Some elements are held as gases in the atmosphere, others are components of the lithosphere (rocks and soil of the Earth's crust), many move through the hydrosphere (marine and freshwaters) before or after their sojourn in the biosphere (the living components).
The cycles through the lithosphere are said to be sedimentary cycles (from the sedimentary rocks in which the elements reside) and are of such long duration that the elements are essentially removed from further cycling until tectonic (mountain building) or volcanic eruptions expose the rock layers to new weathering. Elements have shorter residence times in the air in the atmospheric cycles and generally the least of all in the biota. A surprisingly small amount of the world's matter is held in living organisms at any time; the reservoirs for the elements of concern to life are almost entirely abiotic ones.
Plants are more than merely users of the chemicals of the Earth; through their metabolic processes they exert a considerable influence on the cycling of the major chemicals. Plants have been indispensable through deep (geologic) time in maintaining the steady-state condition of most of the biogeochemical cycles. All of the ecologically significant chemical elements have both an abiotic and a biotic component. Carbon, hydrogen, and oxygen enter plants from the air and from the decomposition of organic matter, but the other 14 essential nutrients are taken from the soil, as are the miscellaneous other elements used in small amounts by a variety of organisms. Nutrients released from weathered rocks enter the soil solution and move by diffusion and mass flow to the sites of biological activity. Rock weathering is a long-term process that adds small quantities of minerals slowly, over time to the ecosystem. Plants and other organisms, therefore, obtain most of the minerals they need by recycling existing organic matter.
http://www.cliffsnotes.com/study_guide/Materials-Used-by-Organisms-Are-Recycled.topicArticleId-23791,articleId-23782.html
A dynamic movement of a colonist and its host move along a continuum between antagonism and cooperation. A pathogen may live with us but not cause disease most of the time, like herpes simplex viruses (that cause fever blisters.) The pathogen occasionally causes serious illness. The movement along the continuum is dynamic and changing.
The bacteria that live in our large intestines can also move along the continuum. Most of the time they are mutualists, benefitting us and being benefitted by living inside us, but can you guess when they might instantly shift to the pathogenic end of the continuum? Answer: If your appendix ruptures, or if you get a gunshot wound to the abdomen, those very same bacteria move into other compartments of your body and become serious pathogens.
Natural selection is the pervasive explanatory paradigm throughout all of these odysseys into the coexistence of hosts and colonists. Both evolve together, or co-evolve. We have two new terms here: coevolution and symbiosis. Coevolution is nothing more than two or more organisms evolving in response to each other, with natural selection working independently on both. Symbiosis is just what the word is constructed to mean: living together. Symbiosis includes the entire continuum between antagonism and cooperation, with commensalism in the middle (in which one organism benefits and the other is more or less unaffected.)
http://www.barbican.org.uk/radical_nature
No comments:
Post a Comment