Composition of extracellular polymeric substances (EPS) produced by a range of soil bacteria and fungi

Abstract

Microorganisms are known to form a matrix of extracellular polymeric substances (EPS) on several solid surfaces such as soils. The formation of this matrix provides various benefits not only to the microbial community, but also to the surface it is attached. In soils, for instance, it promotes protection and adherence of microorganisms to soil aggregates and benefits the soil increasing aggregate stability. Even in spite of this knowledge, the contribution of EPS to soil organic matter (SOM) and to SOM turnover is yet unclear, mainly due to methodological limitations. Furthermore, it has also not been determined how the EPS composition affects soil structure, fertility and organic matter dynamics. It is therefore of uttermost importance to study the composition of the EPS matrix and how different microbial functional groups produce EPS in face of varying environmental conditions. To this end, 10 bacterial and 10 fungal species commonly found in soils were grown under different treatments in order to stimulate EPS production. Microorganisms were grown in either glycerol or starch medium with or without the presence of sterile quartz. EPS was extracted using a cation-exchange resin (CER) and its composition was subsequently determined with the quantification of proteins, carbohydrates, amino sugars and DNA. We hypothesized firstly, that EPS production would be higher in cultures with quartz. Secondly, we also expected bacterial EPS production to be higher in cultures with glycerol whereas fungal cultures would produce more EPS in starch medium, reflecting contrasting substrate effects on bacterial and fungal EPS formation. Lastly, we hypothesized that considerable amounts of galactosamine would be found in all extracted EPS, similarly to mucins excreted in the intestines of vertebrates, protecting enzymes. Mucins are a family of proteins with high GalN contents, consequently, GalN might be an indicator to EPS production in soils.