Contrasting Fungal-Assisted Aggregation between Layered and Fibrous Clay Particles

Interactions between extracellular polymeric substances (EPS) and clay minerals promote the aggregation of mineral particles. The aggregation process is influenced by various crystal structures of clay minerals. In this study, we conducted a 30-day incubation by using Rhodotorula mucilaginosa (Rho, a fungus with a high ability to produce EPS) and two types of clay minerals, i.e., the layered montmorillonite (Mt) and the fibrous palygorskite (Pal). The D50 size of Mt consistently increased from 55.6 μm (Mt@CK) to 127.0 μm (Mt@30d). Meanwhile, evident yellow-red growth clusters gradually appeared after the addition of montmorillonite. This phenomenon is likely attributed to the promotion of fungal growth by the layered structure of montmorillonite. In contrast, after the addition of palygorskite, the D50 size of Pal slightly decreased from 50.6 μm (Pal@CK) to 33.9 μm (Pal@10d). This decrease was likely due to the sorption of relatively small fibrous palygorskite particles that restricted fungal nutrient intake. In addition, ATR-FTIR spectra confirmed the intense adsorption of EPS onto the montmorillonite particles. Evident shifts were clearly observed in the FTIR spectra of the Mt@20d and Mt@30d samples. Moreover, the intense symmetric COO^– stretching vibrations (1318 cm^–1) indicated that a polysaccharide–montmorillonite complex was formed. At the genetic level, the STE7 and STE11 genes (which facilitate EPS secretion) exhibited higher levels of upregulation in the treatment with the addition of montmorillonite than in that with palygorskite. These findings provide new insights into the roles of different clay minerals during the fungal-assisted aggregation process.