Keeping in Touch: Microbial Life on Soil Particle Surfaces

• Published in: Advances in Agronomy Vol. 78; pp. 1 - 43
• Main Author: Mills, Aaron L
• Format: Book Chapter Journal Article
• Language: English
• Published: Elsevier Science & Technology 2003
• ISBN: 9780120007967; 0120007967
• ISSN: 0065-2113; 2213-6789
• DOI: 10.1016/S0065-2113(02)78001-2

Microorganisms in unsaturated soil live in a world dominated by the presence of extensive surfaces, both solid and gas–liquid interfacial surfaces. Particle attachment in soils is similar to particle attachment in aquatic systems, which, because of the high abundance of suspended populations has been widely studied. Although there seems to be a general advantage to the microbes living at the interfaces in terms of enhanced nutrient concentrations and the potential to use many of the physical substrata themselves as energy or nutrient sources, the thickness of the water films in unsaturated conditions leaves the microbes little option except to adhere to the surfaces. Initial attachment to the surfaces appears to be dominated by electrostatic and hydrophobic effects that are described by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for negatively charged cells and particles. These effects result in reversible attachment and the cells are subject to rapid detachment with slight changes in solution chemistry and removal by hydraulic shear. Coatings play an important role in attachment, with metal oxide coatings conferring a positive charge to the particle surface resulting in a much tighter adhesion of the microbial cells to the surface. Attachment of the organisms to the particles by direct surface contact through appendages such as fimbriae or deposition of polysaccharidic slime results in irreversible attachment that can lead to buildup of colonies and biofilms. In this chapter, considerations of theory are presented as they pertain to soil organisms, and abundant use of examples from aquatic habitats exemplifies principles and ideas not easily studied in unsaturated soil. The importance of attachment to the gas–liquid interface is also highlighted.