Saprotrophic bacteria

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Saprotrophic bacteria are bacteria that are typically soil-dwelling and utilize saprotrophic nutrition as their primary energy source. They are often associated with soil fungi that also use saprotrophic nutrition and both are classified as saprotrophs. [1]

Contents

A saprotroph is a type of decomposer that feeds exclusively on dead and decaying plant matter. [2] Saprotrophic organisms include fungi, bacteria, and water molds which are critical to decomposition and nutrient cycling, providing nutrition for consumers at higher trophic levels. They obtain nutrients via absorptive nutrition, in which nutrients are digested by a variety of enzymes and subsequently secreted by the saprotroph. [1]

Community composition and proliferation rates of saprotrophic indicator bacteria are often considered signals of community health in soil, aquatic, [3] and bodily systems. [4]

Structure and life cycle

All saprotrophic bacteria are unicellular prokaryotes, and reproduce asexually through binary fission. [2] Variation in the turnover times (the rate at which a nutrient is depleted and replaced in a particular nutrient pool) of the bacteria may be due in part to variation in environmental factors including temperature, soil moisture, soil pH, substrate type and concentration, plant genotype, and toxins. [5] These factors can, in turn, alter the rates of decomposition and soil organic matter turnover, impacting ecosystem productivity. [6]

When colonizing a new environment, the population of a saprotrophic strain of bacteria initially decreases and then reaches a point of population stabilization. [7] [8] While they are common in soil environments, they can persist anywhere with available food resources, such as in aquatic environments, or in fecal matter. [8] As such, they are a common organism in waste products, where they break down various compounds to obtain nourishment. [2]

Growth rate

Saprotrophic bacterial growth rate is very sensitive to changes in environmental conditions, making it a good variable to detect rapid and subtle changes in microbial communities. Growth rates are also used to measure interactions between bacteria and fungi, with research suggesting bacterial inhibition of fungal growth as it may exert a competitive pressure on fungi. Under normal soil conditions, bacterial biomass production remains relatively steady, as the growth of microorganisms is balanced by predation and other types of cell death. Studies on bacterial growth rates using leucine or thymidine incorporation suggest the turnover times of soil bacterial communities to be in the order of days to weeks at a temperature of around 20 °C. Other studies have estimated a longer turnover time varying between 107–160 days at 25 °C. This large discrepancy could be due to differences in the methods used for these estimations, as well as differences in the incubation temperatures, which are of utmost importance in determining growth rates. Studies have shown that optimal bacterial growth is achieved at temperatures around 25-30 °C in temperate soils, which is usually much higher than the mean annual temperature.[ citation needed ]

Bacterial growth in the rhizosphere presents a special situation, as it supports the rapid proliferation of bacteria compared with the surrounding soil due to the input of root exudates into the soil. Here, bacterial turnover times are estimated to be in the range of just 12–19 hours, with shorter times exhibited on younger roots.[ citation needed ]

Overall, there has not been sufficient research on bacterial growth rates in soil. This contrasts with our comparatively vast knowledge of bacterial growth rate measurements in aquatic environments. We may blame this disparity on the complexity of the soil matrix, which includes both bacterial and fungal decomposers with different feeding strategies. [5]

Environmental factors

Several environmental factors may impact the activity of saprotrophs, including soil moisture, pH, and the presence of substrates. Soil moisture, indicated by carbon mineralization, is positively correlated with bacterial growth, with bacterial growth increasing as soil moisture content increases.[ citation needed ]

In terms of soil pH, there is a well-known pattern of bacterial dominance in neutral or slightly alkaline soils, though clear evidence for the differential growth of bacteria in soils with different pH is scarce.[ citation needed ]

Compared to fungi, bacteria are considered more competitive in degrading easily available substrates. In addition to quality and type, the concentration of substrate is also important to bacterial growth in soil. For example, a study utilizing the addition of different concentrations of glucose found that bacterial growth increased significantly at low concentrations, and was inhibited at very high concentrations. On the other hand, increased substrate flow in the rhizosphere due to root exudation has been shown to significantly increase bacterial growth rates. Here, there is a plant species and genotype effect on growth, presumably due to different exudation rates. [5]

Parasitism

Some saprotrophic bacteria are common pathogens in medicine and agriculture, as they move readily between individuals via consumption or other modes of exposure, such as contact with excrement. [8] For example, certain bacteria may be vectors for food borne illnesses such as Escherichia coli. [9] Others have the ability to decompose cellulose, and are often found in the rumen of cows, aiding in their digestion by fermenting the cellulose in grass. [9]

Nutrient cycling and MEEs

Through saprotrophic nutrition, saprotrophic bacteria release microbial extracellular enzymes (MEEs) into the environment to break down soil organic matter (SOM). MEEs are released when an organism's energy and nutrient needs are not being met. This allows for the monitoring of MEEs as an indicator of nutrient availability in soil. [10] Some significant MEEs are:

Role in forest ecosystems

In forest soils, bacteria are important in the decomposition of fungal mycelia and in nitrogen cycle processes, including nitrogen fixation. Additionally, bacteria, alongside fungi, mediate the bulk of biogeochemical processes, determine the availability of mineral nutrients, and determine the fate of carbon in these soils. However, bacteria’s higher demand for nitrogen and inability to translocate nutrients makes them less efficient decomposers than fungi.

Ecosystem disturbances such as fires, insect invasions, and timber harvesting can lead to a slight decrease in bacterial abundance. Furthermore, the bacterial community composition may change in response to changes in nutrient availability and overall chemistry. [14]

Related Research Articles

<span class="mw-page-title-main">Heterotroph</span> Organism that ingests organic carbon for nutrition

A heterotroph is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but not producers. Living organisms that are heterotrophic include all animals and fungi, some bacteria and protists, and many parasitic plants. The term heterotroph arose in microbiology in 1946 as part of a classification of microorganisms based on their type of nutrition. The term is now used in many fields, such as ecology, in describing the food chain.

<span class="mw-page-title-main">Decomposition</span> Process in which organic substances are broken down into simpler organic matter

Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is essential for recycling the finite matter that occupies physical space in the biosphere. Bodies of living organisms begin to decompose shortly after death. Animals, such as earthworms, also help decompose the organic materials. Organisms that do this are known as decomposers or detritivores. Although no two organisms decompose in the same way, they all undergo the same sequential stages of decomposition. The science which studies decomposition is generally referred to as taphonomy from the Greek word taphos, meaning tomb. Decomposition can also be a gradual process for organisms that have extended periods of dormancy.

<span class="mw-page-title-main">Decomposer</span> Organism that breaks down dead or decaying organisms

Decomposers are organisms that break down dead or decaying organisms; they carry out decomposition, a process possible by only certain kingdoms, such as fungi. Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and development. While the terms decomposer and detritivore are often interchangeably used, detritivores ingest and digest dead matter internally, while decomposers directly absorb nutrients through external chemical and biological processes. Thus, invertebrates such as earthworms, woodlice, and sea cucumbers are technically detritivores, not decomposers, since they are unable to absorb nutrients without ingesting them.

<span class="mw-page-title-main">Detritivore</span> Animal that feeds on decomposing plant and animal parts as well as faeces

Detritivores are heterotrophs that obtain nutrients by consuming detritus. There are many kinds of invertebrates, vertebrates, and plants that carry out coprophagy. By doing so, all these detritivores contribute to decomposition and the nutrient cycles. Detritivores should be distinguished from other decomposers, such as many species of bacteria, fungi and protists, which are unable to ingest discrete lumps of matter. Instead, these other decomposers live by absorbing and metabolizing on a molecular scale. The terms detritivore and decomposer are often used interchangeably, but they describe different organisms. Detritivores are usually arthropods and help in the process of remineralization. Detritivores perform the first stage of remineralization, by fragmenting the dead plant matter, allowing decomposers to perform the second stage of remineralization.

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.

<span class="mw-page-title-main">Soil biology</span> Study of living things in soil

Soil biology is the study of microbial and faunal activity and ecology in soil. Soil life, soil biota, soil fauna, or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a soil profile, or at the soil-litter interface. These organisms include earthworms, nematodes, protozoa, fungi, bacteria, different arthropods, as well as some reptiles, and species of burrowing mammals like gophers, moles and prairie dogs. Soil biology plays a vital role in determining many soil characteristics. The decomposition of organic matter by soil organisms has an immense influence on soil fertility, plant growth, soil structure, and carbon storage. As a relatively new science, much remains unknown about soil biology and its effect on soil ecosystems.

<span class="mw-page-title-main">Saprotrophic nutrition</span> Type of heterotrophic nutrition based on decayed organic matter

Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed organic matter. It occurs in saprotrophs, and is most often associated with fungi and soil bacteria. Saprotrophic microscopic fungi are sometimes called saprobes. Saprotrophic plants or bacterial flora are called saprophytes, although it is now believed that all plants previously thought to be saprotrophic are in fact parasites of microscopic fungi or other plants. In fungi, the process is most often facilitated through the active transport of such materials through endocytosis within the internal mycelium and its constituent hyphae.

<span class="mw-page-title-main">Microbial loop</span> Trophic pathway in marine microbial ecosystems

The microbial loop describes a trophic pathway where, in aquatic systems, dissolved organic carbon (DOC) is returned to higher trophic levels via its incorporation into bacterial biomass, and then coupled with the classic food chain formed by phytoplankton-zooplankton-nekton. In soil systems, the microbial loop refers to soil carbon. The term microbial loop was coined by Farooq Azam, Tom Fenchel et al. in 1983 to include the role played by bacteria in the carbon and nutrient cycles of the marine environment.

A copiotroph is an organism found in environments rich in nutrients, particularly carbon. They are the opposite to oligotrophs, which survive in much lower carbon concentrations.

<span class="mw-page-title-main">Wood-decay fungus</span> Any species of fungus that digests moist wood, causing it to rot

A wood-decay or xylophagous fungus is any species of fungus that digests moist wood, causing it to rot. Some species of wood-decay fungi attack dead wood, such as brown rot, and some, such as Armillaria, are parasitic and colonize living trees. Excessive moisture above the fibre saturation point in wood is required for fungal colonization and proliferation. In nature, this process causes the breakdown of complex molecules and leads to the return of nutrients to the soil. Wood-decay fungi consume wood in various ways; for example, some attack the carbohydrates in wood, and some others decay lignin. The rate of decay of wooden materials in various climates can be estimated by empirical models.

<span class="mw-page-title-main">Soil respiration</span> Chemical process produced by soil and the organisms within it

Soil respiration refers to the production of carbon dioxide when soil organisms respire. This includes respiration of plant roots, the rhizosphere, microbes and fauna.

<span class="mw-page-title-main">Microbiology of decomposition</span>

Microbiology of decomposition is the study of all microorganisms involved in decomposition, the chemical and physical processes during which organic matter is broken down and reduced to its original elements.

Soil microbiology is the study of microorganisms in soil, their functions, and how they affect soil properties. It is believed that between two and four billion years ago, the first ancient bacteria and microorganisms came about on Earth's oceans. These bacteria could fix nitrogen, in time multiplied, and as a result released oxygen into the atmosphere. This led to more advanced microorganisms, which are important because they affect soil structure and fertility. Soil microorganisms can be classified as bacteria, actinomycetes, fungi, algae and protozoa. Each of these groups has characteristics that define them and their functions in soil.

<span class="mw-page-title-main">Mycorrhizal fungi and soil carbon storage</span> Terrestrial ecosystem

Soil carbon storage is an important function of terrestrial ecosystems. Soil contains more carbon than plants and the atmosphere combined. Understanding what maintains the soil carbon pool is important to understand the current distribution of carbon on Earth, and how it will respond to environmental change. While much research has been done on how plants, free-living microbial decomposers, and soil minerals affect this pool of carbon, it is recently coming to light that mycorrhizal fungi—symbiotic fungi that associate with roots of almost all living plants—may play an important role in maintaining this pool as well. Measurements of plant carbon allocation to mycorrhizal fungi have been estimated to be 5 to 20% of total plant carbon uptake, and in some ecosystems the biomass of mycorrhizal fungi can be comparable to the biomass of fine roots. Recent research has shown that mycorrhizal fungi hold 50 to 70 percent of the total carbon stored in leaf litter and soil on forested islands in Sweden. Turnover of mycorrhizal biomass into the soil carbon pool is thought to be rapid and has been shown in some ecosystems to be the dominant pathway by which living carbon enters the soil carbon pool.

<span class="mw-page-title-main">Fungal extracellular enzyme activity</span> Enzymes produced by fungi and secreted outside their cells

Extracellular enzymes or exoenzymes are synthesized inside the cell and then secreted outside the cell, where their function is to break down complex macromolecules into smaller units to be taken up by the cell for growth and assimilation. These enzymes degrade complex organic matter such as cellulose and hemicellulose into simple sugars that enzyme-producing organisms use as a source of carbon, energy, and nutrients. Grouped as hydrolases, lyases, oxidoreductases and transferases, these extracellular enzymes control soil enzyme activity through efficient degradation of biopolymers.

<span class="mw-page-title-main">Root microbiome</span> Microbe community of plant roots

The root microbiome is the dynamic community of microorganisms associated with plant roots. Because they are rich in a variety of carbon compounds, plant roots provide unique environments for a diverse assemblage of soil microorganisms, including bacteria, fungi, and archaea. The microbial communities inside the root and in the rhizosphere are distinct from each other, and from the microbial communities of bulk soil, although there is some overlap in species composition.

Priming or a "priming effect" is said to occur when something that is added to soil or compost affects the rate of decomposition occurring on the soil organic matter (SOM), either positively or negatively. Organic matter is made up mostly of carbon and nitrogen, so adding a substrate containing certain ratios of these nutrients to soil may affect the microbes that are mineralizing SOM. Fertilizers, plant litter, detritus, and carbohydrate exudates from living roots, can potentially positively or negatively prime SOM decomposition.

<span class="mw-page-title-main">Mycoplankton</span> Fungal members of the plankton communities of aquatic ecosystems

Mycoplankton are saprotrophic members of the plankton communities of marine and freshwater ecosystems. They are composed of filamentous free-living fungi and yeasts that are associated with planktonic particles or phytoplankton. Similar to bacterioplankton, these aquatic fungi play a significant role in heterotrophicmineralization and nutrient cycling. Mycoplankton can be up to 20 mm in diameter and over 50 mm in length.

<span class="mw-page-title-main">Viral shunt</span>

The viral shunt is a mechanism that prevents marine microbial particulate organic matter (POM) from migrating up trophic levels by recycling them into dissolved organic matter (DOM), which can be readily taken up by microorganisms. The DOM recycled by the viral shunt pathway is comparable to the amount generated by the other main sources of marine DOM.

Hydrocarbonoclastic bacteria are a heterogeneous group of prokaryotes which can degrade and utilize hydrocarbon compounds as source of carbon and energy. Despite being present in most of environments around the world, several of these specialized bacteria live in the sea and have been isolated from polluted seawater.

References

  1. 1 2 "Saprotroph | Definition, Description, & Major Groups | Britannica". www.britannica.com. Retrieved 2022-12-22.
  2. 1 2 3 "What are Saprophytes? – Examples and Characteristics". MicroscopeMaster. Retrieved 2022-12-22.
  3. ↑ Donsel, Dale J. Van; Geldreich, Edwin E.; Clarke, Norman A. (1967-11-01). "Seasonal Variations in Survival of Indicator Bacteria in Soil and Their Contribution to Storm-water Pollution". Applied Microbiology. 15 (6): 1362–1370. doi:10.1128/am.15.6.1362-1370.1967. ISSN   0003-6919. PMC   547201 . PMID   16349746. S2CID   237233381.
  4. ↑ Shin, Na-Ri; Whon, Tae Woong; Bae, Jin-Woo (September 2015). "Proteobacteria: microbial signature of dysbiosis in gut microbiota". Trends in Biotechnology. 33 (9): 496–503. doi:10.1016/j.tibtech.2015.06.011. ISSN   0167-7799. PMID   26210164.
  5. 1 2 3 Rousk, Johannes; Bååth, Erland (October 2011). "Growth of saprotrophic fungi and bacteria in soil: Growth of saprotrophic fungi and bacteria in soil". FEMS Microbiology Ecology. 78 (1): 17–30. doi: 10.1111/j.1574-6941.2011.01106.x . PMID   21470255.
  6. ↑ Ed.D, Eddie Funderburg. "What Does Organic Matter Do In Soil?". Noble Research Institute. Retrieved 2022-12-08.
  7. ↑ Guzev, V. S.; Zvyagintsev, D. G. (2003-03-01). "The Biometric Analysis of Bacterial Cells in Soil". Microbiology. 72 (2): 187–192. doi:10.1023/A:1023220115021. ISSN   1608-3237. S2CID   41432044.
  8. 1 2 3 Kupriyanov, A. A.; Semenov, A. M.; Van Bruggen, A. H. C. (2010-06-01). "Transition of entheropathogenic and saprotrophic bacteria in the niche cycle: Animals-excrement-soil-plants-animals". Biology Bulletin. 37 (3): 263–267. doi:10.1134/S1062359010030076. ISSN   1608-3059. S2CID   31464133.
  9. 1 2 "Saprotroph | Definition, Description, & Major Groups | Britannica". www.britannica.com. Retrieved 2022-12-08.
  10. 1 2 Luo, Ling; Meng, Han; Gu, Ji-Dong (2017-07-15). "Microbial extracellular enzymes in biogeochemical cycling of ecosystems". Journal of Environmental Management. 197: 539–549. doi:10.1016/j.jenvman.2017.04.023. ISSN   0301-4797. PMID   28419976.
  11. ↑ Freeman, Chris; Ostle, Nick; Kang, Hojeong (January 2001). "An enzymic 'latch' on a global carbon store". Nature. 409 (6817): 149. doi: 10.1038/35051650 . ISSN   1476-4687. PMID   11196627. S2CID   3152551.
  12. ↑ Luo, Ling; Meng, Han; Gu, Ji-Dong (2017-07-15). "Microbial extracellular enzymes in biogeochemical cycling of ecosystems". Journal of Environmental Management. 197: 539–549. doi:10.1016/j.jenvman.2017.04.023. ISSN   0301-4797. PMID   28419976.
  13. ↑ Arnosti, C. (2003), "Microbial Extracellular Enzymes and their Role in Dissolved Organic Matter Cycling", Aquatic Ecosystems, Elsevier, pp. 315–342, doi:10.1016/b978-012256371-3/50014-7, ISBN   9780122563713 , retrieved 2022-12-08
  14. ↑ Baldrian, Petr (2017-06-01). "Microbial activity and the dynamics of ecosystem processes in forest soils". Current Opinion in Microbiology. Environmental microbiology * CRISPRcas9. 37: 128–134. doi:10.1016/j.mib.2017.06.008. ISSN   1369-5274. PMID   28689057.
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