Microbial loop

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Introduction

Depiction of the microbial loop. Original uploader was Umarina at en.wikipedia Released into the public domain (by the author).

The microbial loop is a depiction of the microbial food web. The pathway starts with dissolved organic matter (DOM) which makes its way through various trophic levels ultimately to the highest trophic levels. This is relevant in aquatic and soil systems. Organisms involved in the marine microbial loop include DOM, bacteria, micro flagellates and ciliates, phytoplankton and various other nutrient cycling bacteria [3]. The microbial loop is ecologically significant because it is directly involved in nutrient cycling and primary production of biomass ultimately effecting large-scale ecological functions.

Biological interaction

The microbial loop can best be explained starting with DOM. DOM, or dissolved organic carbon, are concentrations of free organic carbon below 0.45 micrometers [4]. DOM consists of molecules which were once parts of living organisms such as proteins, carbohydrates lipids and nucleic acids. DOM is introduced into the system in many ways including sloppy feeding and excretion of waste products from aquatic organisms [3]. This DOM is then utilized as a food source for bacteria. Bacteria are the only organisms capable of recycling DOM making them an extremely vital component for the continuation ecosystem function. These bacteria are then consumed by protozoans, microbe consumers of bacteria and algae. Zooplankton, like copepods and other organisms then feed off these protozoans and the link to the traditonal food web is established. Zooplankton are eaten by bigger organisms (shrimp ) which are eaten by bigger organisms (fish) and so on down the line. All this organic matter is transferred from smaller levels to the largest levels of ecosystems [1]. The DOM is recycled back into the system as these various organisms from all trophic levels die off and are broken down smaller and smaller via decomposition and other methods mentioned earlier (sloppy feeding, excretion, etc.)

Environments

The microbial loop is found in both aquatic and terrestrial ecosystems. The biological interaction description above is a marine interpretation of the microbial loop. The microbial loop has a significant importance in regulating DOM in marine systems but the loop also has its place in soil systems.

Soil

Exudates from plant roots increase nutrients in the rhizosphere resulting in a bacteria growth increase. If it were not for the bacteria eating protozoa and nematodes in the soil, plant available nutrients would be locked up in bacterial biomass. (Christensen et al., 1992; Griffiths & Caul, 1993; Griffiths et al., 1993; Bonkowski et al., 2000b). This interaction in the soil has both a “top-down effect” on control of bacteria numbers as well as a “bottom-up effect” on the nutrient-acquiring micro-fauna in terrestrial environments. (See current research for more info.)

Ecological Signifigance

The microbial loop is a general interaction in which DOM is utilized as an energy source. As stated before, microbes hold a significant influence in this interaction because they allow for an energy pathway that may have otherwise been lost. The presence of the microbial loop provides for the existence of numerous organisms that feed off the DOM-eating microbes. This interaction carries on throughout higher trophic levels further supporting the microbial loop’s extensive implications [4].

Microbial processes

What microbial processes are important for this microbial interaction? Does this microbial interaction have some ecosystem-level effects? Does this interaction affect the environment in any way? Describe critical microbial processes or activities that are important in this interaction, adding sections/subsections as needed. Look at other topics in MicrobeWiki. Are some of these processes already described? Create links where relevant.

Key Microorganisms

What specific kinds of microbes are typically involved in this interaction? Or associated with important processes? Describe key groups (genera, species) of microbes that we find in this environment, and any special adaptations they may have evolved to survive in this environment. List examples of specific microbes that represent key groups or are associated with important processes found in this environment. Add sections/subsections as needed. Look at other microbe listings in MicrobeWiki. Are some of the groups of microbes from your environment already described? Create links to other MicrobeWiki pages where possible.

Current Research

Protozoa Stimulate

It is universally understood that protozoa stimulate plant growth. Current research is being done to explain this phenomenon. Studies have shown that it is the protozoan presence that enhances soil nitrogen mineralization and plant nitrogen uptake from organic nitrogen sources and not the presence of complex interactions between plant roots, bacteria and protozoa via the soil microbial loop. What this means is that soil nitrogen availability may be the key factor resulting in plant growth via protozoan interaction and that the microbial loop is not relevant in this process. [5]

Sea Surface Warming

Studies have shown that the warming of sea tempratures results in an increased extracellular release of carbohydrates from phytoplankton. This results in an increased C:N ratios of DOM with the higher tempratures. Possible future implications of this could be bottom up effects on the microbial loop in cold seas resulting in a change of aquatic species compostions.[6]

References

1. Azam F, Fenchel T, Field JG, Gray JS, Meyer-Reil LA, Thingstad F. (1983). “The ecological role of water-column microbes in the sea”. Marine Ecology Progress Series. 10: 257-263.

2. Bonkowski M. (2003). “Protozoa and plant growth: the microbial loop in soil revisited”. Tansley Review. 617-631.

3. Fenchel T. (2008). “The microbial loop – 25 years later”. Journal of Experimental Marine Biology and Ecology. 99-103.

4. Stone, Richard (June 18, 2010). "Marine Biogeochemistry: The Invisible Hand Behind A Vast Carbon Reservoir". Science Magazine. doi:10.1126/science.328.5985.1476.

5. Ekelund, Flemming; Saj, Stephane; Vestergård, Mette; Bertaux, Joanne; Mikola, Juha. (2009). “The “soil microbial loop” is not always needed to explain protozoan stimulation of plants”. Soil Biology & Biochemistry. Vol. 41 Issue 11, p2336-2342, 7p.

Edited by Nathan Bohne, a student of Angela Kent at the University of Illinois at Urbana-Champaign.