Biogenicity and Characterization of Moonmilk in the Grotta Nera (Majella National Park, Abruzzi, Central Italy)

Abstract

Observations and hypotheses on the possible influence of unidentified calcifying bacteria on moonmilk speleothem formation in the Grotta Nera are reported for the first time. The Majella Massif hosts a complex karst system of several caves; the accessible Grotta Nera is the most interesting one. Despite its name, the cave is characterized by particularly abundant ivory-white deposits of moonmilk. Two samples of moonmilk were analyzed to determine the geochemistry, fabric, depositional setting, and extent of biogenicity. For this, we combined geochemical, scanning electron microscopic, microbiological, and in vitro precipitation studies. X-ray diffraction of the moonmilk deposits gave clear evidence for the presence of calcite. Scanning electron microscopy showed that moonmilk in the Grotta Nera consists of a network of calcite fibers oriented in all directions, resembling a felted mat. The cultivation on specific medium of moonmilk and drip-water samples showed the presence of fungi, actinomycetes, and other bacteria, but the dominant cultivable microorganisms were bacteria, which produced significant crystallization. Examination of Gram-stained smears taken from the fifteen different colony types showed that the majority (66.7%) of the bacterial isolates were Gram-negative. Single small rods and rod chains were the most common bacteria isolated from the Grotta Nera. None of the molds isolated from the Grotta Nera samples were able to precipitate CaCO 3 crystals, suggesting a major bacterial contribution to moonmilk deposition in the cave. Bacteria were capable of precipitating CaCO 3 on B-4 solid medium at 15 (cave temperature), 22, and 32 uC. The calcifying bacteria isolated from the Grotta Nera showed a greater capability to solubilize CaCO 3 than those associated with consolidated stalactites sampled from previously studied caves. The electron microscopy and microbiological evidences, together with the geochemistry and environmental data, allowed us to postulate the biogenic nature of the moonmilk in the Grotta Nera Cave.

Calcite moonmilk of microbial origin in the Etruscan Tomba degli Scudi in Tarquinia, Italy

“…We demonstrated for the first time that a hypogeal tomb, the Tomba degli Scudi , contains moonmilk. In Italy, moonmilk has been described in caves in the Italian Alps 22 , 23 , in the Grotta Nera in the Appennini 24 – 27 , and in the Su Bentu cave in Sardinia 28 . A world map depicting all caves that have been analysed with molecular biological methods was published in 2012 29 .…”

Origin of abundant moonmilk deposits in a subsurface granitic environment

“…However, its origin is a controversial issue that has generated debate during the last decade. The formation of NFC has been ascribed either to physicochemical processes, biotic processes or even a combination of both (Onac & Ghergari, ; Verrecchia & Verrecchia, ; Borsato et al ., ; Cañaveras et al ., ; Cacchio et al ., ).…”

Assessment of the Potential Role of Streptomyces in Cave Moonmilk Formation

“…A biogenic origin has been hypothesized for a number of speleothems, including coralloids (Banks et al, 2010), pool fingers (Melim et al, 2001), ferromanganese deposits (Northup et al, 2003; Spilde et al, 2005), helictites (Tisato et al, 2015), and moonmilk (Canaveras et al, 2006). Unlike typical mineral deposits, moonmilk is present as a soft and pasty precipitate on cave surfaces and within pools (Richter et al, 2008; Cacchio et al, 2014). The origin of moonmilk has been controversial for many years due to the complex mineralogy, the atypical crystalline morphology, and also the size of its crystals (Verrecchia and Verrecchia, 1994; Canaveras et al, 1999, 2006; Bindschedler et al, 2010, 2014).…”