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An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates of metabolism, and generally low population density. Oligotrophic environments are those that offer little to sustain life. These environments include deep oceanic sediments, caves, glacial and polar ice, deep subsurface soil, aquifers, ocean waters, and leached soils.
Examples of oligotrophic organisms are the cave-dwelling olm; the bacterium " Candidatus Pelagibacter communis", which is the most abundant organism in the ocean (with an estimated 2 × 1028 individuals in total); and , with their extremely low metabolic rate.
Oligotrophs have acquired survival mechanisms that involve the expression of genes during periods of low nutrient conditions, which has allowed them to find success in various environments. Despite the capability to live in low nutrient concentrations, oligotrophs may find difficulty surviving in nutrient-rich environments. The presence of excess nutrients overwhelm oligotroph's metabolic systems, which cause them to struggle to regulate nutrient uptake. For example, oligotroph's enzymes function well in low nutrient environments, but struggle in high nutrient environments.
Lake Vostok, a freshwater lake which has been isolated from the world beneath 4 km (2.5 mi) of Antarctic ice is frequently held to be a primary example of an oligotrophic environment. Analysis of ice samples showed ecologically separated microenvironments. Isolation of microorganisms from each microenvironment led to the discovery of a wide range of different microorganisms present within the ice sheet. Traces of fungi have also been observed which suggests potential for unique symbiotic interactions. The lake’s extensive oligotrophy has led some to believe parts of the lake are completely sterile. This lake is a helpful tool for simulating studies regarding extraterrestrial life on frozen planets and other celestial bodies.
Krok Lake is an ultra-oligotrophic glacial lake with a thin distribution of and autotrophic microorganisms. The microbial loop plays a big role in cycling nutrients and energy within this lake, despite particularly low bacterial abundance and productivity in these environments. The little ecological diversity can be attributed to the lake's low annual temperatures. Species discovered in this lake include Ochromonas, Chlamydomonas, Scourfieldia, Cryptomonas, Ankistrodesmus, and Daphniopsis studeri (a microcrustacean). It is proposed that low competitive selection against Daphniopsis studeri has allowed the species to survive long enough to reproduce in nutrient limiting environments.
The vegetation in these regions, however, is remarkable for its biodiversity, which in places is as great as that of a tropical rainforest and produces some of the most spectacular wildflowers in the world. It is however, severely threatened by climate change which has moved the winter rain belt south, and also by clearing for agriculture and through use of , which is primarily driven by low land costs which make farming economic even with yields a fraction of those in Europe or North America.
Generally, the nutrient becomes less available along the depth of the soil environment, because on the surface, the organic compounds decomposed from the plant and animal debris are consumed quickly by other microbes, resulting in the lack of nutrient in the deeper level of soil. In addition, the metabolic waste produced by the microorganisms on the surface also causes the accumulation of toxic chemicals in the deeper area. Furthermore, oxygen and water are important for some metabolic pathways, but it is difficult for water and oxygen to diffuse as the depth increases. Some factors such as: soil aggregates, pores and extracellular enzymes, may help water, oxygen and other nutrients diffuse into the soil. Moreover, the presence of mineral under the soil provides the alternative sources for the species living in the oligotrophic soil. In terms of the agricultural lands, the application of fertilizer has a complicated impact on the source of carbon, either increasing or decreasing the organic carbon in the soil.
Collimonas is one of the genera that are capable of living in the oligotrophic soil. One common feature of the environments where Collimonas lives is the presence of fungi, because Collimonas have the ability of not only hydrolyzing the chitin produced by fungi for nutrients, but also producing materials (e.g., P. fluorescens 2-79) to protect themselves from fungal infection. The mutual relationship is common in the oligotrophic environments. Additionally, Collimonas can also obtain electron sources from rocks and minerals by weathering.
In terms of polar areas, such as Antarctic and Arctic region, the soil environment is considered as oligotrophic because the soil is frozen with low biological activities. The most abundant species in the frozen soil are Actinomycetota, Pseudomonadota, Acidobacteriota and Cyanobacteria, together with a small amount of archaea and fungi. Actinomycetota can maintain the activity of their metabolic enzymes and continue their biochemical reactions under a wide range of low temperature. In addition, the DNA repairing machinery in Actinomycetota protects them from lethal DNA mutation at low temperature.
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