Nature Notes

Volume XXVI - 1995

Moonmilk and Cave-dwelling Microbes
By John Roth

Microbes lie as far from charismatic megafauna such as deer, bears, and bobcats as you can get. Studying these "forsaken fauna" is difficult because you cannot see them. Their geologic equivalent is mud, but even with x-ray diffraction and other high tech methods, the small particle size of muds can challenge the most dedicated researcher. When combined in caves, however, microbes and muds can form sediments known as moonmilk.

cave formations
formations in Oregon Caves
Oregon Department of Transporation photo.

Even the name has the lure of mystery. Its origin is from the German Mannlimilch, meaning "little earth-man." European peasants used moonmilk for centuries to heal infected cuts in livestock. Some believed that gnomes put this substance in caves for people to use. The white mud seemed to kill infections and speed healing at supernatural rates. Like much of what is in folklore, there is more than a germ of truth in these tales. Much of the calcite moonmilk sampled by investigators contains actinomycetes, which are the main producers of antibiotics.

Moonmilk is a textural term for a very fine, white cave material that absorbs a lot of water. Wet moonmilk looks and feels soft and pasty, somewhat like white cream cheese, when rubbed between the fingers. Dry moonmilk resembles talcum or chalk powder, in that it feels hard and crumbly. Moonmilk often contains 40 to 70 percent water, while organic material may make it even more plastic and slippery.

It is likely that organic activity plays a role in the buildup of some moonmilk, especially the calcite kind found in Oregon Caves. Calcite moonmilk can contain such bacteria as Macromonas bipunctata, along with cyanobacteria, fungi, and green algae. This microflora probably assists in breaking down minerals in the wall rock and adding them to the moonmilk. Moreover, researchers have found that the longer it takes water to reach the cave, the more likely it is that some of the organics will be consumed enroute. In general, water dripping into the deeper parts of Oregon Caves has less organic content than water reaching shallower parts.

Humans have impacted bacteria in moonmilk, as well as other microbe populations in Oregon Caves. An inventory done around every survey point in the cave shows a marked decrease in "cave slime" (mostly actinomycetes bacteria) growing on walls near the cave trail. Decline in these organisms could well be the result of lint and other visitor-induced organics that find their way to cave walls. As a result, non-native bacteria adapted to a high energy food source outgrow and outcompete the slow growing cave slime adapted to low energy foods. Cave slime may also have suffered further adverse effects by visitors touching the cave walls, or through the past practice of spraying those walls with bleach to control exotic plants.

As an example of their value to resource management in the park, microbes have been utilized to reconstruct the size and shape of prehistoric entrances at Oregon Caves. Since the natural openings are now highly modified, an inventory of the directional orientation of popcorn-shaped speleothems was needed so that gates could be built with partial restoration of those conditions resembling prehistoric air flow. The cave inventory also showed that exotic microbes contributing to rounded vermiculations (or "clay worms") are more common near the main trail, while the more complex forms of these lines on cave walls are prevalent further from the trail. Analysis if the rounded vermiculations show high amounts of lint and exotic cyanobacteria. The rounded clay worms will be removed, as they appear to be largely caused by lint and artificial lights.

Deposition of lint, skin, and hair in Oregon Caves does not appear to impact native microbes as much as in some other caves administered by the National Park Service. Knowing this has allowed flexibility in the design of a new cave trail. Rather than having settling "ponds" and a foot-high, lint-trapping curb on both sides of the trail along its entire length, only certain areas will be curbed or have drainage concentrated. If these areas trap substantial lint and non-native organic runoff, then additional curbs, drains, and settling ponds will be added and the areas cleaned more frequently. This system will allow for a more natural flow of water and air across the trail, yet will trap lint and other human-associated organics where they might threaten cave biota. The result should be a better balance between allowing for visitor use and preservation of the monument's primary resource.

John Roth is the natural resources management specialist for Oregon Caves National Monument.

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