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Laboratory experiments
Fuelled by carbohydrate supplied by the host, many ectomycorrhizal fungi have the capacity to acidify the surrounding substrate and exude organic acids, both when growing in axenic cultures and in symbiosis with plants under laboratory conditions (rewiew by Rosling et al., Hoffland et al. ref, Schmalenberger et al). Using flow through systems, Calvaros et al 2013 estimated weathering rates to be 10 times higher when ectomycorrhizal pine seedlings were present compared to unplanted systems, and attributed this to exudation of organic acids and acidifying effects by the EMF fungus. There is however, a large step to transfer these results to natural systems according to the authors.
Similar activities can be performed by other microorganisms in the soil. For instance, many soil bacteria have a strong capacity to acidify the surrounding when sufficient carbon resources is available. Trees foster specific communities of bacteria in the rhizosphere, or mycorrhizasphere (Collingnon et al 2011, Calvarusu 2008, 2013), and these organisms may be the active partner of weathering reactions in the soil (review by Uroz et al. 2009). In addition, brown rotting fungi produce large amounts of oxalic acids when degrading wood (ref), which may have secondary effects on phosphorous release from the soil (Fransson et al ) ,
It is not clear if organic acid release and acidification is a primary mechanism to release nutrients from soil minerals, or a secondary effect related to the way microorganisms are growing in the soil. Proton are exuded in response to uptake of positively charged ions regardless of if minerals are present or not, and exudation of organic acids can occur in response to many different factors (ref). Furthermore, concentrations of organic acids in soil solution (Hees et al) seldom reach levels that have significant effects on mineral dissolution (ref).
On the other hand, fungal hypha attach to surfaces to be able to grow and proliferate through the soil, which may affect the surface of the minerals both physically and chemically (McMaster 2014). Attachment is enhanced by organic compounds produced by the fungus (Denny and Wilkins 1987, Gadd and Sayer 2000 in adeke) and Balogh-Brunstad suggested that organic acids may accumulate under such biolayers to sufficient concentrations to affect mineral dissolution. Although these authors call for more experiments to confirm this possible effect.
Using atomic force microscopy Gazze et al (2013) demonstrated a biolayer of Extracellular Polymeric Substances EPS (40-80 nm thick) to form around hyphal tips of the EMF Paxillus involutus. This material fused to form a biofilm that covered most of the mineral surface where the fungus was growing. Saccone et al (2011) found similar biolayes formed by P. involutus growing on hornblende, chlorit and biotite surfaces. Furthermore, the hornblende surface was less resistant to mechanical forcing under the biolayer compared to freshly cleaved surfaces suggesting that enhanced weathering had occurred. In addition, several studies have demonstrated dissolution channels on minerals where EMF hyphae have been growing. Sometimed these tracks can be 50 nm deep (Gazze et al 2012)
In some laboratory experiment with axenic cultures of microorganism it has been found that attachment to biotite surfaces yielded stronger dissolution compared to when they separated by a membrane. This demonstrate that not only the chemicals produced but also the physical attachment is important for mineral dissolution (Ahmed and Holmström). However, in other experiments, no additional effect on weathering was found when EMF hyphae were attached to minerals in axenic growth in solution cultures (Balogh-Brunstrad 2008b).
One problem with axenic solution experiments is that no sink (the plant) is available for the released elements. In one study with axenic pine seedlings ectomycorrhizal with P. involutus, Bonneville (2011) found large removal of K, Mg and Fe under hyphae attached to biotite, which suggests that hyphal attachment and a sink for element removal (the plant) is important for mineral dissolution. Furthermore, it could be demonstrated that the biotite surface was strongly acidified under the hyphae, which suggests that specific chemical conditions occur under biolayers formed by the fungal mycelium. Schmalenberger et al (2015) demonstrated mineral specific exudation of oxalate by P. involutus using labelled 14CO2 given to the host plant. Oxalate was exuded in response to minerals in the following sequence Gabbro> limestone, olivine and basalt > granite and quartz.
Active weathering by EMF presume that elements provided by the minerals are in short supply, and that fungal weathering activity will increase if these nutrients become growth limiting. Unfortunately, very few studies have measured weathering capacity by EMF under different nutrient conditions, but results by Rosling et al (2007) does not suggest that oxalic acid exudation by soil fungi (no EMF in this study) is enhanced by P limitation. Work by Schöll et al 2006 on the other hand demonstrated that limitation of nutrients (P, Mg, K) affected the composition of organic acids exuded by EMF (more oxalate) but not the total amounts. Interestingly they found different results when EMF were growing in pure cultures and when growing in symbiosis with plant seedlings, which again highlights the importance of having in a nutrient sink (the plant) for accurate interpretations of EMF effects on weathering. Smits et al (2013) demonstrated significant weathering of apatite by P. involuts when P was in limiting supply while this effect diminished when sufficient P was supplied. Also Schöll et al. (2006b) demonstrated significant weathering of muscovite by P. involutus when K was in low supply while no effect on hornblende was found under Mg deficiency. Other tested species had no effect on weathering of muscovite under K limitation.
Carefully designed experiment under controlled conditions are necessary in order to increase or understanding of fungal weathering under varying nutrient conditions. Such experiments are necessary in order to predict how forest will respond to future changes in climate and nitrogen deposition rates.
