Agriculture Reference
In-Depth Information
microbial activity leading to the production of both root mucigels and microbial exudates as
“cementing agents”, physical compaction by root growth, faunal activity and mineral
interactions (Six et al., 2002). The prokaryotic basis of soil structure formation or aggregation
is likely similar to the process described by Mueller (1996) with regards to flocculation in
more aqueous conditions with aggregation occurring in steps: through microbiological
degradation or modification of organic material, attachment of cells to soil particles, the
production of extracellular polymeric substances (EPS) (through decomposition and
utilization of organic matter or based on utilization of root exudates) and the concerted
construction of biofilms by microbial communities, which can cause aggregation of soil
particles. The production of EPS by bacteria are regarded as a means of stabilization against
fluctuations in water potential, protection against antibiotics and regulation of the diffusion of
nutrients and waste products and are thus necessary for survival. The formation of soil
aggregates thus would not necessarily be an imperative for survival of the bacteria but a
consequence of EPS and biofilm production that additionally may indirectly benefit the
bacteria by providing the aeration and porosity for soil moisture, both required for growth of
the microbes (Chenu and Strotzky, 2002). While there is a relative abundance of studies
descriptive of soil aggregation, there is currently a paucity of research concerned with the
species and proportional composition of bacteria involved in soil aggregation.
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Knowledge of the microbiology of soil aggregation can perhaps be informed by some
major findings and studies in microbial adhesion and biofilm formation. Lectins are a
prominent mode of bacterial adhesion and two similar lectins have been described from a
soil-inhabiting basidiomycete species and a soil-inhabiting plant pathogenic bacterial species
(Sudakevitz et al., 2002). Both lectins had affinities for plant hemicelluloses, essentially plant
residues. Ofek et al. (2007) found that some bacterial lectins recognize glycolipids, terminal
sugars and internal sugar sequences as well. Whether certain soil microbial species have
lectins with higher affinities for binding to plant residues is in need of investigation. This may
lead to a better understanding of the role individual species may play in soil aggregation.
Perhaps species with such lectins might predominate in the densest part of the bacterial
density and substrate gradients that mark the processes of residue degradation (Nunan et al.,
2003) leading to EPS formation, and thus may be the most important contributors to soil
aggregation.
Studies investigating structured networks or matrices such as “nanowires” and
“honeycombs” (Schaudinn et al., 2007) in bacterial biofilms may provide the missing link as
to the bacterial role in soil aggregation. Methodologies such as staining and thin sectioning of
undisturbed soil (Nunan et al., 2003) to analyze spatial distribution of bacteria in soil
matrices, and nano-scale secondary secondary ion mass spectrometry of microbes (Herrmann
et al., 2007) that have assimilated isotopically labeled substrate in soil could lead to
determination of whether biofilm networks occur in undisturbed soil. Such networks could
provide the means for quorum sensing (cell-cell signaling) to occur.
Quorum sensing, the production of signaling molecules to sense bacterial population
densities in the surrounding environment among members of the same and other species, has
