Glucose-stimulation of natural microbial activity causes transient aggregation and alteration of clay mineralogy in sandy and loamy sediments

Subsurface sediments usually show limited microbial activity, however, inputs of nutrients due to anthropogenic spills or infiltration from the surface may quickly activate the native microbial communities, thereby changing composition, structure and properties of sediments. We studied the effect of glucose addition, an easily available carbon source, on mineralogy, microstructure and properties of several (0.5-35 m) loamy and sandy sediments over 30 days in laboratory experiments. We followed the time changes in biomass by direct cell count; respiratory activity by CO₂ emission; clay mineralogy by X-ray diffraction (XRD); microaggregate size distribution by pipette analysis; and observed microbial binding via scanning electron microscopy (SEM).

Glucose addition caused transient buildup of respiratory activity and biomass with maximal values 3-10 times more than in control (water-treated) samples appearing around the 7^th day after the treatment. The biomass of bacteria, archaea, actinomycetes and fungi increased. After that the biomass and the CO₂ emission declined sharply and reached stable values about twice as much as in control samples.

On day 7, we noted an increase in the proportion of smectite layers in the disordered mixed layer illite-smectite minerals (MLM), yet no changes in content and composition of other clay and non-clay minerals. After 30 days of observation, XRD showed further transformation of MLM composition, as well as partial destruction of other clay minerals. We hypothesize that with abundant external nutrition, microbes mined the lacking K from illite layers of the MLM. After the consumption of glucose, all clay minerals were a source of essential elements.

The content of microaggregates of 0.1-0.05 mm in size increased in loams on the 7^th day after the treatment, presumably due to microbial binding and gluing of aggregates by cells and EPS. With the decline of the biomass, the previously-formed microaggregates partially disintegrated. We assume that after the consumption of glucose, the microorganisms lived on biomass and EPS, thereby removing previously formed glue and meshes from the aggregates.

SEM performed on air dried sands collected during maximal microbial activity revealed biofilms consisting of microbial cells and EPS, attaching to the fine clay coatings around sand grains. SEM on lyophilized loams showed filamentous structures, which we interpret to be actinomycete mycelium that enmeshes particles into microaggregates.

Irreversible changes in clay mineralogy and transient aggregation caused temporary alteration of stress-strain properties: increased cohesion, and decreased friction and compressive strength.

Our data show that ongoing/continued microbial activity is crucial for the formation of aggregates as well as for the clay mineral paragenesis in sediments. Both processes affect sediment quality, e.g. in terms of soil organic matter stabilization or with respect to the overall mechanical properties.