Impact of bacterial biomass on the surface wettability of soil particles under varying moisture conditions
Final Report Abstract
Surface properties of soil bacteria may significantly alter upon drought stress, but the reactions of different strains were highly variable. Degree and type of changes depend on the type of the drought stress, on growth conditions as well as on the cell wall architecture (i.e., Gram-positive or -negative) and on adaptation strategies of each strain; we thus found a large variety of reactions. More detailed analysis of two strains, B. subtilis and P. fluorescens, which become less wettable due to drought stress, indicated that different mechanisms were active. These included contraction of teichoic acids or lipopolysaccharides in high ionic strength environment, resulting in higher N/C ratios at the cell surfaces due to a higher amenability of cell wall proteins to XPS analysis, and different changes in cell size and stiffness in case of matric stress. In case of P. fluorescens, mainly hydrophobic interactions of the cells with surfaces were affected by drought stress. Interactions of the cells with mineral surfaces again were strain-specific, but also dependent on cell surface properties and the mineral. For example, P. fluorescens, which has a higher CA than B. subtilis had more attractive or at least less repulsive interactions with quartz, kaolinite, montmorillonite or goethite. Stress decreased the repulsive hydrophilic and increased the attractive hydrophobic interactions, with the strength of stress effects differing between the two strains. The wettability of the CMA was determined by both the quantity and the quality of the microbial compounds on their surface. The bacterial impact on soil wettability has the potential to persist. In particular in experiments with no addition of soil microorganisms, the wettability of the CMA remained constant over time. The reduced wettability of stressed cells also persisted in this experiment. However, in the presence of soil microorganisms, fast changes in CMA wettability seem possible, and the lower wettability of stressed cells did not persist. These results may be related to (1) stress induced by the incubation experiment itself, in particular to the less adapted unstressed cells, (2) shifts in the microbial community as some members may grow on the residues of the added cells, and (3) to loss of C from the system due to respiration during degradation of cell residues and thus a decreasing surface coverage of the CMA with organic material. These results, however, were obtained in a rather artificial system and may not directly be transferred to real soil. Soil incubation experiments showed that drought stress induces low wettability, in particular in the unadapted soil from the moister site. In parallel, we detected characteristic shifts in the microbial community during incubation at different water potentials. Although these shifts were small compared to the differences in community composition between the two soils or the effects of increasing the microbial population by substrate amendment, we were able to identify taxa affected by the moisture conditions. Stress thus affects the bacterial abundance and community composition, and the characteristics of CMA found in soil are imprinted by the properties of the attached cells. The initially postulated positive feedback thus seems highly probable although further studies are necessary to finally prove this.
Publications
- (2020) A new approach for repeated tip-sample relocation for AFM imaging of nano and micro sized particles and cells in liquid environment. Ultramicroscopy 211:112945
Abu Quba AA, Schaumann GE, Karagulyan M, Diehl D
(See online at https://doi.org/10.1016/j.ultramic.2020.112945) - (2021) Quality control of direct cell–mineral adhesion measurements in air and liquid using inverse AFM imaging. RSC Advances 5384–5392
Abu Quba AA, Schaumann GE, Karagulyan M, Diehl D
(See online at https://doi.org/10.1039/d1ra00110h)
