Bacterial cell-mineral associations and their stability under varying moisture conditions

Soil particle wettability controls the water dynamics of dry and unsaturated soil and has an impact on many processes where water is involved. Pure soil minerals are usually wettable, but under environmental conditions they are easily covered by organic compounds, changing their surface properties and potentially making them water repellent. Besides organic compounds such as alkanes, fatty acids, free lipids and waxes, research also indicates a direct influence of bacterial cells on the development of soil water repellency. In a series of stress experiments with different Gram-negative and Gram-positive strains of bacteria we could show that cell surface wettability measured in terms of contact angle is affected by cell stress response caused by hypertonic or drought environmental conditions. The changes in wettability were found to be accompanied by changes in physicochemical surface properties and surface elemental composition of the cells, as indicated by X-ray photoelectron spectroscopy. Furthermore, coverage of minerals by cells caused significant changes in particle wettability, rendering originally wettable minerals water repellent, with the effect being more pronounced for cell-mineral associations (CMA) formed with stressed cells. To investigate the physical stability of these CMA, we conducted an incubation experiment with CMA formed by quartz particles and Bacillus subtilis cells either grown under physiological or hypertonic conditions. The CMA were incubated at different water potentials (pF 2.5 and 4.2) and part of them subjected to wetting-drying cycles. The results showed that the quartz–B. subtilis CMA formed with stressed cells remained significantly more water repellent than those formed with unstressed cells during the whole incubation time of 80 days and independent of the incubation conditions. Furthermore, we observed a slight tendency of increasing contact angle with increasing incubation time. Besides the generally lower wettability of the stressed cells, the lower wettability of the CMA formed with stressed cells can be related to a higher degree of microbial coverage, as indicated by higher surface C content and lower surface O/C and Si/C ratios compared to the CMA formed with unstressed cells. The higher microbial coverage can probably be explained by attachment conditions being more favorable in case of the stressed cells, as suggested by interaction free energies calculated using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. In summary, the results indicate that the hydrophobizing effect of hypertonic stress on B. subtilis was stable over time and support the assumption that stress-related changes in cell surface properties remained also in necromass and their effect on surface properties of CMA can persist.