## 4 Discussion
### Correlations between the Composition of Clay Materials and Microbial Processes  
The correlation between the clay elemental composition and the rates of microbial processes was assessed using the Spearman rank correlation coefficient (Figure 2). A high direct correlation was observed between microbial respiratory activity and contents of phosphorus, nitrogen, and carbon in clay samples, while the correlation with iron content was relatively weak. Suffide production also correlated strongly with the levels of C, N, and P, while its correlation with iron content was inverse. This was probably the result of competition between the processes of sulfur and iron reduction, as was the case with sample B-1 containing over 10 mass % iron. Dissolution of the aluminum silicate structure determined by aluminum and silicon leaching into the solution also exhibited a strong direct correlation with the content of C, N, and P in the samples and the strongest direct correlation with iron. Potassium release inversely correlated with the levels of iron and sulfur, while iron release showed a weak positive correlation with iron and sulfur content and a weak negative one with the levels of C, N, and P. The absence of clear correlations may be explained by decreased release of these elements into the solution due to their accumulation in microbial biofilms on clay.  
Figure 1: Sulfide content in clay samples after 1-month cultivation with acetate or hydrogen as electron donors.  
Assessment of the strength of correlational relationships between the clay minerals composition and the rates of microbial processes is presented in Figure 3. Strong direct correlations (\(p<0.05\)) between the respiratory activity and kaolinite and potassium feldspar levels should be noted. The PFS is the most weathering-susceptible mineral, which acts as a potassium source for the cells. Similar to other clay minerals, microbial growth on kaolinite may be initiated by iron. The data on microbial conversion of kaolinite to illite due to Fe, Si, and Mg absorption and subsequent iron reduction with the absorption of potassium and ammonium ions are presented in the review by [PERSON] [15]. This work also describes the microbial conversion of illite to smectite. Microbial growth in the presence of illite in various polymineral samples was due to the high content of potassium and iron. A relatively strong correlation was also found between microbial respiration and illite content. Interestingly, a strong inverse correlation was revealed between microbial respiration and montmorillonite content. This may be explained by pronounced swelling of montmorillonite and morphological limitations for biofilm formation. At the same time, a strong positive correlation between microbial respiration and the content of quartz, which contains no biophilic elements, may be explained by the same factor, i.e., the possibility of biofilm formation on its large grains.  
Figure 2: Relations between elemental composition and microbial processes in clay materials. “L” after the symbol of an element indicates bioleaching.  
Sulfide production exhibited a strong positive correlation with the levels of illite, plagioclase, calcite, and kaolinite. A negative correlation was also found for montmorillonite content, although, in contrast the case of microbial respiration, its reliability was low.  
The degradation of the aluminum silicate lattice of clay materials correlated directly with the contents of kaolinite, illite, calcite, anatase, and PFS in the samples. Similar to other microbial processes, a strong inverse correlation with montmorillonite content was observed.