Amino sugars indicate microbial extracellular polymeric substances (EPS) in soil

Biofilms are a matrix of extracellular polymeric substances (EPS) produced by microorganisms in several environments. When they form in soils, biofilms provide fitness advantages for the microbial community, but also several other benefits for the soil ecosystem, such as aggregation and improving the water retention of soils. Amino sugars (AS) are thoroughly used as indicators for soil microbial necromass as they are not produced by plants. Further, the origin of galactosamine (GalN), the second most common AS present in soils, has not yet been identified, because unlike other AS such as muramic acid or glucosamine, GalN has rarely been found as cell a wall component. In this study, we hypothesized that GalN would therefore be an indicator for microbial EPS. In order to assess this, we have carried out two incubation experiments with bacterial and fungal species commonly found in soils. In the first experiment, we induced EPS production in two fungal and two bacterial species and determined AS in the cell culture and in the extracted EPS. As a result, we found that not only GalN, but also mannosamine were fully removed from the medium by EPS extraction, showing that they are integral parts of the EPS matrix. In the second experiment, ten bacterial and ten fungal species were incubated either with or without a quartz surface for attachment and also either with a glycerol- or starch-based medium. Following microbial incubation, we extracted the EPS and measured four components: total proteins, carbohydrates, DNA, and amino sugars. We observed EPS composition shifted according to the microbial growth conditions, suggesting that each biofilm component may have distinct functions within the matrix. The presence of a surface, for instance, increased the production of proteins, carbohydrates, and DNA in the EPS fraction, while total AS amounts decreased. Additionally, only the production of EPS-carbohydrates augmented with increasing substrate complexity, while the production of EPS proteins, DNA, and amino sugars decreased under the same conditions. Overall, our results reinforce the significance amino sugars have in the microbial necromass fraction, not only as cell wall components, but also as integral parts of biofilms; calling attention to the importance of biofilms for soil organic carbon stocks in soil.