Influence of clay minerals on the morphology of fungal pellets

The objective of this study was to assess the influence of clay minerals on the morphology of mycelial pellets produced under submerged conditions as a prelude to the development of biomineral sorbents for toxic metals. The macro- and microscopic morphology of fungal pellets of melanin-containing microfungi Cladosporium cladosporioides, C. herbarum and Humicola grisea grown in liquid clay-containing medium was studied using scanning electron microscopy. It was found that the inclusion of clay minerals (bentonite, palygorskite and kaolinite) in the liquid medium influenced size, shape and structure of the mycelial pellets produced. In general, a reduction of pellet size, an increase in the length of surface hyphae of the pellets (except for H. grisea), and a reduction in exopolymer production were observed with increasing clay mineral concentrations up to 5% (w/v). For C. herbarum, an increasing concentration of bentonite changed the normal pelleted growth form to diffuse star-like growth, which was also observed for C. cladosporioides in the presence of kaolinite. The presence of bentonite and kaolinite in the medium for H. grisea and palygorskyte for C. cladosporioides led to the formation of numerous bulbous hyphae in pellets. However, bentonite decreased such bulbous growth in C. herbarum compared to control pellets which grew as bulbous mycelium. The dynamics of C. cladosporioides growth and pellet formation in the presence of bentonite (0.5% (w/v)) was investigated. It was found that the clay particles were involved in the formation of pellet structure at all stages of fungal growth. The porosity of a growing pellet (evaluated as the ratio of surface hyphae length to pellet radius) increased during early growth (17-24 h) and then decreased up to the stationary phase. A general model of the structure of a fungal pellet grown in clay-containing medium is proposed. The pellets consist of three main layers: a central core, with densely packed mycelium aggregated with solid clay minerals or a matrix of clay/polysaccharides; a middle layer with looser mycelium mixed with clay mineral flakes; and an outer, or 'hairy' zone, with loose hyphae surrounded by clay mineral flakes. The relevance of these studies to fungal growth and morphology, and to the development of biomineral sorbents for metal removal is discussed.