THE MICROBE-MINERAL INTERFACE IN CAVES: A MOONMILK PERSPECTIVE

While geomicrobiology has received substantial interest in the last decade, one unresolved issue is the involvement of microbes in the formation of speleothems in caves. Moonmilk is a widely distributed, secondary formation and refers to the very hydrated white spongy/pasty or powdery masses found coating walls and speleothems in caves. It is often described as having a cottage cheese-like consistency and may be composed of several carbonate minerals. The wet pasty forms of moonmilk are so striking that some special explanation for their origin seems to be necessary, since calcite in cave environments usually has a completely different habit, hard and crystalline. It is generally accepted that moonmilk might be the by-product of the life-cycle of microbes, although the questions remain as to what organisms are involved and whether the organisms identified are actively involved or simply buried during mineral precipitation. X-Ray Diffraction Analysis was used to confirm the mineralogy of calcite moonmilk samples from Entrance and Exit Caves, Tasmania. Environmental scanning electron microscopy revealed crystalline character associated with, and encrusting hyphal microbes. The structure of the crystalline material appeared distinct from that of a reference CaCO3 sample obtained from ceiling rock of Entrance Cave. To determine composition of microbial communities, culture-dependent and -independent techniques were used. Isolations were dominated by the Actinobacteria, particularly the sub-order Pseudonocardineae.. Other taxa represented included alpha, beta and gamma Proteobacteria, Cytophaga-Flexibacter-Bacteroides and Firmicutes. Denaturing Gradient Gel Electrophoresis and 16S rDNA clone library analysis revealed communities dominated by the alpha and beta Proteobacteria, Cytophaga-Flexibacter-Bacteroides and to a lesser extent, the Actinobacteria. A major part of this study was concerned with method development due to the well recognised difficulties of culturing microbes and extracting DNA from calcite matrices. The description of the composition of microbial communities in moonmilk is an important starting point and provides a focus for fundamental studies concerning how these populations interact with geological processes in the formation of moonmilk.