Abstract
Melanised cell walls and extracellular polymeric matrices protect rock-inhabiting microcolonial fungi from hostile environmental conditions. How extracellular polymeric substances (EPS) perform this protective role was investigated by following development of the model microcolonial black fungus Knufia petricola A95 grown as a sub-aerial biofilm. Extracellular substances were extracted with NaOH/formaldehyde and the structures of two excreted polymers studied by methylation as well as NMR analyses. The main polysaccharide (~ 80%) was pullulan, also known as α-1,4-; α-1,6-glucan, with different degrees of polymerisation. Αlpha-(1,4)-linked-Glcp and α-(1,6)-linked-Glcp were present in the molar ratios of 2:1. A branched galactofuromannan with an α-(1,2)-linked Manp main chain and a β-(1,6)-linked Galf side chain formed a minor fraction (~ 20%). To further understand the roles of EPS in the weathering of minerals and rocks, viscosity along with corrosive properties were studied using atomic force microscopy (AFM). The kinetic viscosity of extracellular K. petricola A95 polysaccharides (≈ 0.97 × 10−6 m2 s−1) ranged from the equivalent of 2% (w/v) to 5% glycerine, and could thus profoundly affect diffusion-dominated processes. The corrosive nature of rock-inhabiting fungal EPS was also demonstrated by its effects on the aluminium coating of the AFM cantilever and the silicon layer below.
Highlights
Microbial biofilms, one of the most successful forms of life, are held together by an extracellular matrix (Flemming et al 2016)
Supporting methods for atomic force microscopy (AFM) cantilever surface layer examination after fungal extracellular polymeric substances (EPS) contact To demonstrate the corrosive nature of K. petricola A95 extracellular polysaccharides, a commercially available silicon cantilever (Pointprobe-Plus-NCHR, Nanosensors, Neuchatel, Switzerland: 125 μm length, 30 μm width, 4 μm thickness) covered with a thin film of aluminium, was immersed in solutions of the polysaccharides
The yields of crude K. petricola A95 EPS grown on malt-extract agar were about 103 mg g−1 cell dry weight
Summary
One of the most successful forms of life, are held together by an extracellular matrix (Flemming et al 2016). Almost all microbial groups, including fungi, that produce extracellular polymeric substances (EPS) tend to form biofilms (Blankenship and Mitchell 2006). Fungal cell walls contain chitin and β-glucans surrounded by a complex network of polysaccharides, glycoproteins and enzymes (Barreto-Bergter and Figueiredo 2014). Extracellular polysaccharides produced by fungi (both filamentous- and yeast-like ones) range from glucans to galactans including. Many predominant cell-wall polysaccharides in fungi are soluble and can actively participate in EPS formation (reviewed in Breitenbach et al 2017). This work is dedicated exclusively to the largely unknown EPS formed by rockinhabiting fungi
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