Microbial loop
The microbial loop is a major trophic and recycling pathway that regulates nutrient availability and primary production in the marine environment[1]. This article gives an introduction to the microbial loop and its position in the marine food web. The biogeochemical processes involved in the microbial loop are described in the article Nutrient conversion in the marine environment. The organisms involved in the microbial loop are described in the articles Marine Plankton and Marine microorganisms.
Definition of Microbial loop:
The microbial loop is the set of pathways through which dissolved organic matter is taken up by heterotrophic microorganisms, especially bacteria and archaea, and partly transferred through grazing by protists to larger planktonic consumers, while much of the organic matter and associated nutrients is recycled within the microbial community.
This is the common definition for Microbial loop, other definitions can be discussed in the article
|
Notes
The microbial loop both transfers organic matter to larger consumers and retains and recycles carbon and nutrients within the microbial community.
Phytoplankton and other marine organisms release organic compounds as dissolved organic matter (DOM). Its carbon component is referred to as dissolved organic carbon (DOC). DOM is produced through extracellular release, excretion, incomplete feeding, cell damage and death, degradation, dissolution and lysis. Some DOM also enters coastal waters from terrestrial and sedimentary sources. Heterotrophic bacteria and many archaea take up suitable dissolved compounds and use them for growth and respiration. These microorganisms are grazed mainly by small heterotrophic and mixotrophic protists, including nanoflagellates and ciliates. The protists are in turn consumed by larger protists and mesozooplankton, thereby connecting the microbial loop with the wider planktonic food web.
Seawater commonly contains about one million prokaryotic cells per milliliter, although abundance varies widely among marine environments[3]. Their populations are regulated by substrate availability, viral infection and grazing, particularly by heterotrophic nanoflagellates[4].
Microbial metabolism, grazing and cell lysis continually regenerate inorganic nutrients, which can again support phytoplankton and microbial production. Depending on the composition of the organic matter and the availability of nutrients, heterotrophic microorganisms can also temporarily retain nitrogen and phosphorus in their biomass.
Viruses infect bacteria, archaea and microbial eukaryotes. Lysis of infected cells releases organic matter and nutrients back into the surrounding water. This 'viral shunt' redirects part of the microbial biomass away from grazing pathways and back toward dissolved and particulate organic matter, thereby strengthening microbial recycling. Viruses therefore play a major role in the microbial food web, although they are not a trophic link in the same sense as grazing protists[2].
The relative importance and structure of the microbial loop differ among estuarine, coastal and open-ocean waters according to nutrient supply, the quantity and quality of organic matter, plankton size structure, water residence time and physical mixing.
Related articles
References
- ↑ Azam, F., Fenchel, T., Field, J.G., Gray, J.S., Meyer-Reil, L.A. and Thingstad, F. 1983. The ecological role of water-column microbes in the sea. Mar. Ecol. Prog. Ser. 10: 257–263.
- ↑ 2.0 2.1 Shiah, F-K., Lai, C-C., Chen, T-Y., Ko, C-Y., Tai, J-H. and Chang, C-W. 2022. Viral shunt in tropical oligotrophic ocean. Science Advances 8, eabo2829
- ↑ Whitman, W.B., Coleman, D.C. and Wiebe, W.J. 1998. Prokaryotes: the unseen majority. Proceedings of the National Academy of Sciences 95: 6578–6583
- ↑ Sanders, R.W., Caron, D.A. and Berninger, U.-G. 1992. Relationships between bacteria and heterotrophic nanoplankton in marine and fresh waters: an inter-ecosystem comparison. Marine Ecology Progress Series 86: 1–14.
