The Living Cave Paste
Deep inside the limestone caves of the Garraf Massif, the walls are coated in a peculiar white, creamy substance known as moonmilk. When wet, it has a soft texture often compared to cream cheese; when dry, it becomes a crumbly powder. For centuries, its origin was a mystery, with some even speculating it was petrified moonlight. Modern analysis shows it is primarily composed of fine crystals of carbonate minerals, usually calcite. While moonmilk exists in caves worldwide, the deposits in Garraf are notable for the specific biochemical process that creates them.
The formation is not a simple geological process. It is the direct result of microbial life. The actors are chemolithotrophic bacteria, organisms that derive their energy from chemical reactions with inorganic materials, not from sunlight or organic matter. In the Garraf caves, these bacteria perform a specific reaction: they oxidize manganese. This chemical process releases the energy the bacteria need to live. As a waste product of their metabolism, they precipitate calcite, effectively building a mineral coating around themselves. Over generations, these microscopic mineral deposits accumulate to form the thick, paste-like layers of moonmilk seen on the cave walls.
A Microbiome of Rock-Builders
The identification of these highly specialized bacteria was made possible through 16S rRNA gene sequencing. This molecular technique allows scientists to identify and classify microbes by analyzing the genetic code for a component of their ribosomes, a universal piece of cellular machinery. The analysis showed a community of bacteria perfectly adapted to their dark and nutrient-poor environment, "eating" metal to survive.
The environment of the Garraf Park is essential to this process. The park covers over 12,820 hectares of limestone hills. This geology provides the raw material—calcium carbonate—that the bacteria utilize. Water percolating through the limestone becomes saturated with calcium and provides the medium for these reactions. The moonmilk itself is a complex ecosystem. While mostly calcite, it contains a significant amount of organic matter, primarily the bacteria that created it. The substance is surprisingly water-rich, and its consistency is determined by its moisture content, ranging from a viscous fluid to a soft solid. This process of biomineralization, where life creates geology, is an interaction between the living world and the inorganic planet.