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Concepts of Microbial Ecology |
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Concepts of Microbial Ecology Energy Flow
Ecosystems are in a dynamic steady state. This is made possible by controlling (by complex feedback mechanism) the numbers or activity of individual organisms, thus maintaining an equilibrium. Alternatively, the population may change in a way that reflects changing levels of matter and energy within the environment.
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Energy, usually light, is partly trapped by organisms within the ecosystem and is eventually exported in the form of heat and chemical energy in reduced compounds (carbohydrates). The energy is first trapped by the primary producers which use light in photosynthesis to reduce CO2, The chemosynthetic bacteria which can use chemicals instead of light energy to produce reduced carbon compounds are also primary producers.
The rate at which energy is stored by primary producers is called the
primary productivity and is expressed in energy units (Kcal/ m2 / day) or as
the amount of carbon or organic matter. Flow of energy through an ecosystem
is closely linked with flow of matter.
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Mechanical energy has two forms:
Kinetic or free energy
Potential energy
Kinetic energy is the energy possessed by a body by virtue of its motion
and is measured by the amount of work done in bringing the body to rest.
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Potential energy is the stored energy (energy at rest) and becomes useful
after conversion into kinetic energy. All organisms require a source of potential
energy which is found in the chemical energy of food.
The oxidation of food releases energy which is used to do work. Thus
chemical energy is converted to mechanicals energy Hence, food is the means of transfer of both matter and energy.
A portion of the gross primary production is converted back to CO2 by the respiration of the primary producers. The remaining organic carbon is
the net primary production available to heterotrophic consumers who convert
the organic carbon back to CO2 during their respiration. A net gain in organic
matter produced by photosynthesis and not converted back to CO2 is called
net community productivity.
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The transfer of energy stored in organic compounds from one organism
to the other establishes a food chain. The transfer occurs in steps and each step constitutes a trophic level. The interrelationship of food chain steps establish the food web.
There are two main ways in which the primary production of an ecosystem may be used by heterotrophic organisms.
Grazing food web Organisms that feed directly on primary producers
(herbivores) constitute the trophic level of grazers. These grazers may be
eaten by secondary consumers (carnivores). This is grazing food chain.
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Detritus food web If the primary producers are not eaten alive, then their
dead bodies (those of primary and secondary consumers) enters the detritus
food chain which is linked copiously to complete the web.
Only 10-15 % of the biomass from each trophic level usually is transferred
to the next level. 85-90% is consumed by respiration or enters the detritus
food chain.
In most terrestrial and shallow water habitats, the predominant primary
producers are higher plants. Microbes do not play important roles as primary
producers nor are they generally considered important as consumers. In
these habitats 80-90% of total energy flow is through detritus chain. Most of
the energy in the grazing food chain is in the bodies of the organism involved.
In the aquatic habitats, the entire food web is based on microbial primary
producers, the phytoplanktons (algae, cyanobacteria). Microbes are
responsible for most of the ocean's net primary production which is about
half of the total photosynthesis of the planet.
The main primary consumers in the aquatic habitat also belong to
microbial groups. Most of the primary production (50-90%) goes to the
consumers.
The decay portion of the food web (detritus) is dominated by microbial
forms in both aquatic and terrestrial environments. The decay portions of the food web involve the degradation of incompletely digested organic matter (faeces, urea, dead plants and animals). Part of the microbial biomass formed during decomposition is recycled into the food web.
Most of the energy in the detritus food chain is stored outside the organism. Hence, this chain is dependent on the primary production in the grazing food chain. The detritus food chain is not solely concerned with decomposition, it contains predators that live on decomposers and this detritus chain is also linked to carnivores of the grazing food chain. Hence, the grazing and detritus food chains are interrelated.
The primary producers and various sorts of heterotrophs in a food chain
occupy different trophic levels. In the grazing food chain, microbes occupy
the lower levels as primary producers and consumers and animals occupy
the higher levels but, microbes decompose the bodies from all levels in the
food chain. The higher the trophic level, the less energy it receives, because of
respiratory-loss and therefore the biomass is usually less than that of lower levels.
In any ecosystem, pyramid of trophic levels based on biomass does not reveal the importance of the organism as much as the pyramid of trophic level based on energy flow through the levels. For example, in a trophic level based on biomass, microbes have no importance since their biomass is exceptionally low but with respect to metabolic activity (energy pyramid) their importance is well ascertained.
Though microorganisms are mostly primary producers or decomposers (saprophytes) in the ecosystem they can also be pathogens since they can obtain nutrients directly from live primary producers, herbivores or carnivores. The trophic levels are not therefore self-contained. The movement between the trophic levels may involve the fundamental changes in metabolism. For example, some algae which are normally photosynthetic primary producers can become heterotrophs when put in the dark with organic matter and then they become part of the decomposer system.
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