Introduction
The epilithic and endolithic microflora of all rocks consists of algae, cyanobacteria, mineral respiring bacteria and symbiotic lichen communities with for example fungi and cyanobacteria in symbiosis (Adamo 1993, Gadd 2007).
The use of the rocks and minerals vary from species to species. Some only use the free space and ecological niche that a surface rock provides, but more often the microorganisms take actual advantage of the chemical properties of the rocks and minerals. There are thousands of species of bacteria, fungi and lichens that make use of hundreds of different types of minerals. Carbonate rocks like marble and limestone and their carbonate minerals are more commonly prefered sources, but there are several species also living on both intermediate and mafic rocks and minerals (Burford 2003, Gadd 2007).
This paper will focus on the role of fungi in the process and how these organisms make use of, alter and erode minerals and rocks via mycorrhiza. A field of science sometimes called “geomycology”.
Geomycology is far from only a microbiological topic despite being a field dominated by microbiology researchers. Microbiologists are generally interested in analyzing and document how these organisms function and their role in the ecology but the geological questions are just as important. It gives us important clues and new explanations to economically interesting topics within sedimentary processes such as biomineralization, ore formation, bioleaching but also valuable knowledge on how bioerosion and bioweathering works (Gadd 2007, Hoffland et al 2004).
Mycorrhiza
Mycorrhiza (Greek for “fungal roots”) is the name for the symbiotic union (or association) of plant roots and fungi and is most likely the most important bioweathering force on dry land that not only erodes solid bedrock but also dissolves mineral grains in the sediments, aiding in the formation of clays and other sediments necessary for land plants to grow in (Bonneville 2009).
Over 90% of all land plant families have some form of mycorrhiza association with fungi and over 6000 species of fungi are known to live in mycorrhiza association with roots (Landeweert 2001).
There are several forms of mycorrhiza but three ecologically significant variations of mycorrhiza dominate. Endomycorrhiza, Ectomycorrhiza and Ericoid mycorrhiza and subvariations of these.
They are basically slightly different methods for the same goal: the exchange of nutrients between plant and funghi via a physical connection between the root and the fungi mycellium, in most cases beneficial for both sides, but in some rarer cases the fungi is a parasite on the root. Often many different types of mycorrhiza can be found associated with plants at the same time.
Purpose and function of symbiosis
The fungi basically gains access to important carbohydrates from the plant such as sucrose or glucose which are essential for the fungi (Gadd 2007).
The fungi in return provides the plant with minerals from the surrounding soils such as calcium, magnesium, potassium and phosphorous – all essential for plant life. The fungi have also been observed providing the roots with protection from toxic elements, immobilizing them in the hyphae. But also giving protection for the roots from diseases and parasites. And as already said it also helps in the formation of the sediments in which the plants needs to be able to create stable roots for growth. The beneficial effects for the plant to have fungi mycorrhiza are simply manifold.
The finely netted hyphae provides a much larger surface to surface ratio between it and the surrounding soil than the much thicker roots alone can, which greatly increases the potential amount of input of nutrients for the plant. The hyphae also acts like an improved dissolver of nutrients from the surrounding soils and mineral grains than the roots alone can since it produces efficient low molecular weight acids. It dissolves essential elements out of siliciclastic mineralgrains like apatite (for access to phospherous), hornblende, mica and feldspars (for access to calcium, magnesium and potassium).
In the case of apatite it has been observed that the fungi hyphae actually tunnels it self through other mineral grains with acid to gain access to smaller apatite inclusions inside. These types of weathering separates clearly from standard chemical weathering which simply follows the crystal structure of the mineral. These tunnels form independently from the crystal structure as shown on figure 2 below. There has also been found both traces of acids and the hyphae it self inside the tunnels (Blum et al 2002, Landeweert 2007).
Bacterial associations
Also associated with the mycorrhiza are several species of bacteria that forms a third part in the process by living in increased numbers near the hyphae. While not being able to erode the minerals in an efficient manner such as the fungi can with their acids, they make efficient use of different types of minerals that are eroded out of larger grains like oxides, sulphates and sulfides and concentrate free ions of these as nanoparticles/ores around the hyphae (so called “biomineralization”). The bacteria use these mineral elements for bacterial respiration. These increased concentrations of elements are then taken advantage of by the fungi. Its not completely understood what the fungi contributes to the bacteria other than a good source of freshly weathered rocks (Gadd 2007, Landeweert 2001).
Variations of mycorrhiza
Endomycorrhiza
The endomycorrhiza (or more commonly known as arbuscular mycorrhizal fungi or AM-fungi) exists with some 85% of all known plant families today. AM is formed solely by fungi of the phyla Glomeromycota. AM-fungi is thought to have existed as long as land plants have and also be one of the major reasons for the evolutionary success of land plants.
With the rise of vascular land plants some 450 MA ago the first signs of AM-fungi are also shown in the paleontological record as for example in the Rhynie chert in plants of Aglaophyton major and this long evolutionary existence is also calculated and verified by DNA-mutations (Simon et al 1993). AM-fungi works by fungi hyphae invading the root cells. The hyphae enters the cell membranes and by this greatly expands the possible contact surface between the hyphae and the cell cytoplasm for nutrient transfer.
Ericoid mycorrhiza
The ericoid mycorrhiza fungi species also invades the outer layers of cells of the root like AM-Fungi does, but is more limited to the number of plant species it formes symbiosis with. It differs from AM-fungi by having more limited hyphae growth. It is also better adapted to extract nutrients from fresh dead organic matter than soil (so called “saprotrophic capabilities”). It is locally very important for organic decomposition but its more limited influence on soils and erosion of siliciclastic minerals makes it a less important group from a geological perspective (Read 1996).
Ectomycorrhiza
The ectomycorrhiza (or EcM/EMF) is an extra cellular symbiosis where the hyphae penetrates the structure between roots cells instead of invading the actual cell membrane. This symbiosis is common among more forest type species of plants like birch, oak and pine and exists in around 10% of all plant families. The fungi hyphae creates a net around the roots called a so called “hartig net” (se fig 1) that more or less completely covers the root. Large amounts of hyphae then expands out in the surrounding soils in an extent larger than AM-fungi or Ericoid mycorrhiza hyphae does (Wang 2006). EcM is just like AM major contributors to mineral weathering on rocks and smaller mineral grains in the soil (Gadd 2006.
Environmental interest
Bioremediation
Understanding mycorrhiza provides both greater understanding to weathering and erosion of minerals, but also how to make use of this by targeting soil acidification and pollutions more efficiently. The solubilization, mobilization and also immobilization of toxic elements (such as cadmium, mercury, copper and uranium) by the aid of variations of mycorrhiza makes the active use of the symbiosis a tool for cleaning up industrial waste. Thus providing an instrument for mineral recycling. As already mentioned the mycorrhiza mobilizes various elements for nutrients, but in a toxic environment they also have to control toxic elements so that the fungi doesnt die. The fungi do so by immobilizing these elements around the hyphae, and sometimes also transforming the elements to more inert chemical variations (Gadd 2006).
Understanding CO2 cycles
The weathering properties of the fungi hyphae can help us understand and analyze the history of CO2-levels in the atmosphere. Taylor et al (2009) and Hoffland et al (2004) proposes that the rise of ectomycorrhiza in the Cretaceous explains some part of the declining trend in CO2-levels since then. The EcM aids in the CO2-fixation in the soils. The hyphae simply helps binding the soil and thus slows the carbon cycle (se fig 3 below). Before EcM arose there seem to have been more soil movement in general.
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