Microbial loop

into the food chain. 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. In general, dissolved organic carbon (DOC) is introduced into the ocean environment from bacterial lysis, the leakage or exudation of fixed carbon from phytoplankton (e.g., mucilaginous exopolymer from diatoms), sudden cell senescence, sloppy feeding by zooplankton, the excretion of waste products by aquatic animals, or the breakdown or dissolution of organic particles from terrestrial plants and soils. Bacteria in the microbial loop decompose this particulate detritus to utilize this energy-rich matter for growth. Since more than 95% of organic matter in marine ecosystems consists of polymeric, high-molecular- weight (HMW) compounds (e.g., proteins, polysaccharides, lipids), only a small portion of total dissolved organic matter (DOM) is readily utilizable to most marine organisms at higher trophic levels.…

into the food chain.]] 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. In general, dissolved organic carbon (DOC) is introduced into the ocean environment from bacterial lysis, the leakage or exudation of fixed carbon from phytoplankton (e.g., mucilaginous exopolymer from diatoms), sudden cell senescence, sloppy feeding by zooplankton, the excretion of waste products by aquatic animals, or the breakdown or dissolution of organic particles from terrestrial plants and soils. Bacteria in the microbial loop decompose this particulate detritus to utilize this energy-rich matter for growth. Since more than 95% of organic matter in marine ecosystems consists of polymeric, high-molecular- weight (HMW) compounds (e.g., proteins, polysaccharides, lipids), only a small portion of total dissolved organic matter (DOM) is readily utilizable to most marine organisms at higher trophic levels. This means that dissolved organic carbon is not available directly to most marine organisms; marine bacteria introduce this organic carbon into the food web, resulting in additional energy becoming available to higher trophic levels. Recently the term "microbial food web" has been substituted for the term "microbial loop". ==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 ultimate fate of soil organic carbon is a function of the combined activities of plants and below ground organisms, including soil microbes. Although soil microorganisms are known to support a plethora of biogeochemical functions related to carbon cycling, the vast majority of the soil microbiome remains uncultivated and has largely cryptic functions. Only a mere fraction of soil microbial life has been catalogued to date, although new soil microbes and viruses are increasingly being discovered. This lack of knowledge results in uncertainty of the contribution of soil microorganisms to soil organic carbon cycling and hinders construction of accurate predictive models for global carbon flux under climate change. 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 organic carbon at rates higher than model expectations, resulting in error within predictive models of carbon fluxes. 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. 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). 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 ( ), 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. 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. 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 or between deciduous and coniferous forest litter (14)]. In turn, these biochemical differences influence soil organic carbon levels through changing decomposition dynamics. Also, increased diversity of plant communities increases rates of rhizodeposition, stimulating microbial activity and soil organic carbon storage, although soils eventually reach a saturation point beyond which they cannot store additional carbon. ==See also== Biological pump f-ratio Plankton Marine snow Phycosphere ==References== ==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

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