Microbial loop
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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.[1] in 1983 to include the role played by bacteria in the carbon and nutrient cycles of the marine environment.
In general,
History
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Carbon cycle |
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Prior to the discovery of the microbial loop, the classic view of marine
Early work in marine ecology that investigated the role of bacteria in oceanic environments concluded their role to be very minimal. Traditional methods of counting bacteria (e.g., culturing on
In the 1970s, the alternative technique of direct
In 1974, Larry Pomeroy published a paper in BioScience entitled "The Ocean's Food Web: A Changing Paradigm", where the key role of microbes in ocean productivity was highlighted.[3] In the early 1980s, Azam and a panel of top ocean scientists published the synthesis of their discussion in the journal Marine Ecology Progress Series entitled "The Ecological Role of Water Column Microbes in the Sea". The term 'microbial loop' was introduced in this paper, which noted that the bacteria-consuming protists were in the same size class as phytoplankton and likely an important component of the diet of planktonic crustaceans.[1]
Evidence accumulated since this time has indicated that some of these bacterivorous protists (such as ciliates) are actually selectively preyed upon by these copepods. In 1986, Prochlorococcus, which is found in high abundance in oligotrophic areas of the ocean, was discovered by Sallie W. Chisholm, Robert J. Olson, and other collaborators (although there had been several earlier records of very small cyanobacteria containing chlorophyll b in the ocean[4][5] Prochlorococcus was discovered in 1986[6]).[7] Stemming from this discovery, researchers observed the changing role of marine bacteria along a nutrient gradient from eutrophic to oligotrophic areas in the ocean.
Factors controlling the microbial loop
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Biogeochemical cycles |
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The efficiency of the microbial loop is determined by the density of
In marine ecosystems
The microbial loop is of particular importance in increasing the efficiency of the marine food web via the utilization of dissolved organic matter (DOM), which is typically unavailable to most marine organisms. In this sense, the process aids in recycling of organic matter and nutrients and mediates the transfer of energy above the
Many planktonic bacteria are motile, using a flagellum to propagate, and
More currently, the microbial loop is considered to be more extended.
In land ecosystems
Soil ecosystems are highly complex and subject to different landscape-scale perturbations that govern whether soil carbon is retained or released to the atmosphere.
The lack of information concerning the soil microbiome metabolic potential makes it particularly challenging to accurately account for the shifts in microbial activities that occur in response to environmental change. For example, plant-derived carbon inputs can prime microbial activity to decompose existing soil
To account for this, a conceptual model known as the microbial carbon pump, illustrated in the diagram on the right, has been developed to define how soil microorganisms transform and stabilise soil organic matter.[17] As shown in the diagram, carbon dioxide in the atmosphere is fixed by plants (or autotrophic microorganisms) and added to soil through processes such as (1) root exudation of low-molecular weight simple carbon compounds, or deposition of leaf and root litter leading to accumulation of complex plant polysaccharides. (2) Through these processes, carbon is made bioavailable to the microbial metabolic "factory" and subsequently is either (3) respired to the atmosphere or (4) enters the stable carbon pool as microbial necromass. The exact balance of carbon efflux versus persistence is a function of several factors, including aboveground plant community composition and root exudate profiles, environmental variables, and collective microbial phenotypes (i.e., the metaphenome).[18][10]
In this model, microbial metabolic activities for carbon turnover are segregated into two categories: ex vivo modification, referring to transformation of plant-derived carbon by extracellular enzymes, and in vivo turnover, for intracellular carbon used in microbial biomass turnover or deposited as dead microbial biomass, referred to as necromass. The contrasting impacts of catabolic activities that release soil organic carbon as carbon dioxide (CO2), versus anabolic pathways that produce stable carbon compounds, control net carbon retention rates. In particular, microbial carbon sequestration represents an underrepresented aspect of soil carbon flux that the microbial carbon pump model attempts to address.[17] A related area of uncertainty is how the type of plant-derived carbon enhances microbial soil organic carbon storage or alternatively accelerates soil organic carbon decomposition.[19] For example, leaf litter and needle litter serve as sources of carbon for microbial growth in forest soils, but litter chemistry and pH varies by vegetation type [e.g., between root and foliar litter [20] or between deciduous and coniferous forest litter (14)]. In turn, these biochemical differences influence soil organic carbon levels through changing decomposition dynamics.[21] Also, increased diversity of plant communities increases rates of rhizodeposition, stimulating microbial activity and soil organic carbon storage,[22] although soils eventually reach a saturation point beyond which they cannot store additional carbon.[23][10]
See also
References
- ^ .
- ISSN 0024-3590.
- ^ JSTOR 1296885.
- .
- .
- S2CID 4373102.
- S2CID 32682912.
- ISSN 0171-8630.
- ^ S2CID 4380194.
- ^ doi:10.1146/annurev-environ-012320-082720. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- S2CID 203658781.
- PMID 29088705.
- ^ ISBN 978-981-10-8401-0.
- PMID 30451857.
- hdl:10036/68733.
- .
- ^ S2CID 9992380.
- PMID 33873756.
- .
- hdl:2027.42/74496.
- doi:10.3390/f7100231.
- PMID 25848862.
- S2CID 97153551.
Bibliography
- Fenchel, T. (1988) Marine Planktonic Food Chains. Annual Review of Ecology and Systematics
- Fenchel, T. (2008) The microbial loop – 25 years later. Journal of Experimental Marine Biology and Ecology
- Fuhrman, J.A., Azam, F. (1982) Thymidine incorporation as a measure of heterotrophic bacterioplankton production in marine surface waters. Marine Biology
- Kerner, M, Hohenberg, H., Ertl, S., Reckermannk, M., Spitzy, A. (2003) Self-organization of dissolved organic matter to micelle-like microparticles in river water. Nature
- Kirchman, D., Sigda, J., Kapuscinski, R., Mitchell, R. (1982) Statistical analysis of the direct count method for enumerating bacteria. Applied and Environmental Microbiology
- Meinhard, S., Azam F. (1989) Protein content and protein synthesis rates of planktonic marine bacteria. Marine Ecology Progress Series
- Muenster, V.U. (1985) Investigations about structure, distribution and dynamics of different organic substrates in the DOM of Lake Plusssee. Hydrobiologie
- Pomeroy, L.R., Williams, P.J.leB., Azam, F. and Hobbie, J.E. (2007) "The microbial loop". Oceanography, 20(2): 28–33. .
- Stoderegger, K., Herndl, G.J. (1998) Production and Release of Bacterial Capsular Material and its Subsequent Utilization by Marine Bacterioplankton. Limnology & Oceanography