Geomicrobiology of secondary mineral deposits from underexplored lava tubes from Galapagos Islands (Ecuador)

The Galapagos Islands are an archipelago of volcanic islands located in the Pacific Ocean, 960 km west of the Ecuador’s coast. An outstanding aspect of this unique ecosystem on Earth is its extensive underground network of lava tubes, plenty of secondary mineral deposits (speleothems), such as coralloid-type speleothems and moonmilk deposits, which origin is associated with microbial activity in many cases. In this study, molecular biology techniques were performed to identify the bacterial communities associated with moonmilk and coralloid speleothems from two lava tubes of Santa Cruz Island, in Galapagos. In addition, FESEM-EDS, micro-computed X-ray tomography and mineralogical analyses were carried out to better understand the composition of these speleothems and their interactions with microorganisms. Moonmilk deposits, displaying pasty and soft texture, were composed of calcite and clay minerals, whereas coralloid speleothems consisted of goethite and amorphous silica, particularly opal-A. Microbial cells, especially filamentous bacteria, were abundant in both secondary minerals and were found in close association with extracellular polymeric substances (EPS) and mineral deposits rich in Si, Ca or Fe. Furthermore, fossilized bacteria, reticulated filaments and microboring were observed by FESEM. The analysis of 16S rDNA revealed the presence of different bacterial philotypes, some of them associated with the genus Aciditerrimonas, which includes iron-reducing thermoacidophilic species from solfataric soils.