Microbial loop

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The microbial loop (from the English: microbial loop ) describes a material cycle in the food chain in the marine plankton was added to the dissolved organic carbon compounds by bacteria and along the classical food chain phytoplankton zooplankton Nekton be passed. The term microbial loop was developed by Farooq Azam et al. (1983) to describe the importance of bacteria in the material cycles of marine ecosystems .

Dissolved organic carbon compounds usually come from the microbial breakdown of organic particles and detritus , or are specifically specified as waste products from plant and animal cells. Particularly small organic molecules can also accidentally escape from cells. Heterotrophic bacteria break down organic particles and break down macromolecules such as polysaccharides , fats and proteins into simple monomers in order to breathe them for energyor to create your own cell substance from it. This makes the organically bound carbon and the energy stored in it usable for the rest of the ecosystem.

history

Before the discovery of the microbial loop, the marine food web was viewed as a link between linear food chains, in which carbon and energy were fixed by primary producers (phytoplankton), this primary production being used by herbivores , which in turn are eaten by predators . All of these organisms are ultimately broken down by heterotrophic bacteria, which release the elements they contain as inorganic compounds. Heterotrophic bacteria in water were not considered to be significant consumers of organic matter. Traditional methods of counting bacteria (e.g. by culturing on agar plates) only resulted in very low cell numbers, so that the influence of bacteria was considered to be very small in early work on marine ecology. This changed when new methods such as epifluorescence microscopy led to significantly higher estimates of bacterial density. These observations led to a reassessment of the importance of bacteria for the marine ecosystem. The term "microbial loop" was introduced by Azam et al. (1983) who pointed out that bacterivorous protists belong to the same size class as phytoplankton and therefore probably represent an important part of the diet of planktonic crustaceans .

Influencing factors

The efficiency of the microbial loop is determined by the population density of marine bacteria. The bacterial density, in turn, is controlled by eating by protozoa and various classes of bacterivorous flagellates . In addition, infections of the bacteria with viruses also cause the destruction of bacterial cells, with organic compounds being released into the water. The combined effect of protozoa and viruses compensates for most of the bacterial population growth, thereby suppressing bacterial blooms.

Importance for the marine ecosystem

The microbial loop is of particular importance because it uses dissolved organic carbon compounds that most other organisms cannot utilize, thereby increasing the overall efficiency of the ecosystem. The dissolved organic substances are recycled and not broken down into mineral compounds. At the same time, other nutrients (e.g. phosphorus, nitrogen) are recycled that would otherwise sink to unreachable depths, so that the microbial loop is of particular importance in nutrient-poor waters far from the coast. There, the biomass of the microbial loop corresponds to five to ten times the biomass of all multicellular organisms (e.g. fish). Heterotrophic bacteria then form the basis of the food web of many oceanic ecosystems. Heterotrophic bacteria convert up to 50% of the net primary production .

The work by Kerner et al. (2003) supplemented the microbial loop with the process of abiotic aggregation of dissolved organic substances into particles ( flocculation ). These particles can be ingested directly by otherwise bacterivorous protists. Heterotrophic bacteria breathe in about 30% of the absorbed organic carbon and thus only make about two thirds available as biomass. The abiotic aggregation bypasses the loss through bacterial respiration and thus increases the efficiency of recycling.

Individual evidence

  1. ^ A b Farooq Azam, Tom Fenchel, John G. Field, John S. Gray, Lutz A. Meyer-Reil, Frede Thingstad: The ecological role of water-column microbes in the sea. In: Marine Ecology Progress Series. Vol. 10, 1983, pp. 257-263, ( digital version (PDF; 3.64 MB) ).
  2. ↑ Karel Van den Meersche, Jack J. Middelburg, Karline Soetaert, Pieter van Rijswijk, Henricus TS Boschker, Carlo HR Heip: Carbon-nitrogen oupling and algal-bacterial interactions during an experimental bloom: Modeling a 13 C tracer experiment. In: Limnology and Oceanography. Vol. 49, No. 3, 2004, pp. 862-878, doi : 10.4319 / lo.2004.49.3.0862 .
  3. ↑ Donald E. Francisco, Robert A. Mah, Albert C. Rabin: Acridine Orange-Epifluorescence Technique for Counting Bacteria in Natural Waters. In: Transactions of the American Microscopical Society. Vol. 92, No. 3, 1973, pp. 416-421, doi : 10.2307 / 3225245 .
  4. ↑ John E. Hobbie, Ralph J. Daley, Steve Jasper: Use of nuclepore filters for counting bacteria by fluorescence microscopy. In: Applied and Environmental Microbiology. Vol. 33, No. 5, 1977, ISSN  0099-2240 , pp. 1225-1228, ( online ).
  5. ↑ Lawrence R. Pomeroy: The Ocean's Food Web, A Changing Paradigm. In: Bioscience. Vol. 24, No. 9, 1974, pp. 499-504, doi : 10.2307 / 1296885 .
  6. ^ Arnold H. Taylor, Ian Joint: Steady-state analysis of the 'microbial loop' in stratified systems. In: Marine Ecology Progress Series. Vol. 59, 1990, pp. 1-17, ( digital version (PDF; 1.48 MB) ).
  7. ↑ Martin Kerner, Heinz Hohenberg, Siegmund Ertl, Marcus Reckermann, Alejandro Spitzy: Self-organization of dissolved organic matter to micelle-like microparticles in river water. In: Nature . Vol. 422, No. 6928, 2003, pp. 150-154, doi : 10.1038 / nature01469 .

supporting documents

  • Tom Fennel: Marine Plankton Food Chains. In: Annual Review of Ecology and Systematics. Vol. 19, 1988, pp. 19-38, doi : 10.1146 / annurev.es.19.110188.000315 .
  • Tom Fennel: The microbial loop - 25 years later. In: Richard Warwick (Ed.): Marine Ecology. A Tribute to the Life and Work of John S. Gray (= Journal of Experimental Marine Biology and Ecology. Vol. 366, No. 1/2). Elsevier, New York NY et al. 2008, pp. 99-103, doi : 10.1016 / j.jembe.2008.07.013 .
  • Jed A. Fuhrman, Farooq Azam: Thymidine incorporation as a measure of heterotrophic bacterioplankton production in marine surface waters. In: Marine Biology. Vol. 66, No. 2, 1982, pp. 109-120, doi : 10.1007 / BF00397184 .
  • David Kirchman, John Sigda, Richard Kapuscinski, Ralph Mitchell: Statistical analysis of the direct count method for enumerating bacteria. In: Applied and Environmental Microbiology. Vol. 44, 2, 1982, pp. 376-382, ( online ).
  • Simon Meinhard, Farooq Azam: Protein content and protein synthesis rates of planktonic marine bacteria. In: Marine Ecology Progress Series. Vol. 51, 1989, pp. 201-213, ( digital version (PDF; 1.1 MB) ).
  • Uwe Münster: Investigations about structure, distribution and dynamics of different organic substrates in the DOM of Lake Plußsee. In: Archives for Hydrobiology. Supplement. Vol. 70, No. 4, 1989, ISSN  0341-2881 , pp. 429-480.
  • Lawrence R. Pomeroy, Peter J. le B. Williams, Farooq Azam, John E. Hobbie: The Microbial loop. In: Oceanography. Vol. 20, No. 2, 2007, pp. 28-33, doi : 10.5670 / oceanog.2007.45 .
  • Karen Stoderegger, Gerhard J. Herndl: Production and Release of Bacterial Capsular Material and its Subsequent Utilization by Marine Bacterioplankton. In: Limnology and Oceanography. Vol. 43, No. 5, 1998, pp. 877-884, doi : 10.4319 / lo.1998.43.5.0877 .