###### Abstract  
Lava caves are home to a stunning display of secondary mineral speleothems, such as moonmilk and coralloids, as well as highly visible microbial mats. These features contain diverse and under-characterized groups of bacteria. The role of these bacteria in the formation of secondary mineral speleothems is just beginning to be investigated. The lava caves of the Big Island of Hawai'i and in El Malpais National Monument, New Mexico (USA), share many morphologically similar speleothems. This study focused on investigating the overlap in bacteria across a wide range of speleothems in these two geographically distant sites. Through scanning electron microscopy (SEM) and 168 rRNA gene analysis, we found that Hawaiian caves have a greater alpha diversity and beta diversity separated by cave and speleothem type. Many Actinobacteria were in higher abundance in New Mexico caves, while Hawaiian caves contained more bacteria that are unclassified at the genus and species level. Discovering the diversity in bacteria in these secondary speleothems will assist in identifying cave secondary mineral formations that may be good candidates for finding life on extraterrestrial bodies. Hawai 'i, New Mexico, lava caves, bacteria; geomicrobiology; scanning electron microscopy; Mars; microbial ecology; Actinobacteria +
Footnote †: journal: Appl. Sci. 0024 14 6500 0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-0000-0000-0000-00000-00000-00000-00000-00000-00000-00000-0000-00000-00000-0  
secondary minerals such as coralloids [11; 12; 13], while other investigations of microbial diversity in lava caves included a study of sediment from Mt. Erebus, Antarctica [14]; coralloid, crusts, and moon milk from the Galapagos Islands, Ecuador [15]; opaline stromatolites in Mexico [16]; ooze, microbial mats, and moonmilk from the Canary Islands, Spain [17]; and a variety of secondary mineral deposits in Craters of the Moon National Monument and Preserve, USA [18]. Together, these studies are starting to paint a picture of the incredible diversity of organisms in lava caves and beginning to elucidate the roles these organisms play in the cave ecosystem, especially in the creation of secondary mineral deposits. Few studies, however, have investigated multiple morphology types from a single cave. As with lava caves elsewhere in the world, Hawaiian and New Mexican lava caves have been fruitful research sites for investigating a variety of different microbial mats; secondary mineral deposits, including copper silicates and calcite moonmilk; along with wall features such as gold-colored mineral veins, coralloids, and ooze. The indication of microbial activity in various secondary mineral deposits suggests that microbial presence and geomicrobiological interactions may contribute to the formation of these secondary minerals found in caves [10; 19]. We hypothesize that six commonly occurring and highly visible features (i.e., moonmilk, mineral crusts, coralloids, spheroids, microbial mats, and ooze), found in lava caves in New Mexico and Hawai'i, contain microbial life that may influence the physical and mineralogical differences between these features. Based on this hypothesis, we posit that several secondary morphologies (Figure 1) could provide the basis for scientists to effectively design rovers and software to identify viable targets for sampling in lava caves on Mars and other extraterrestrial bodies. [PERSON] et al. [20] proposed that one of the best places to search for evidence of extant or extinct life on Mars was in the subsurface. Such caves could have formed during Mars' history when volcanism was more active, and the climate was wetter and warmer. Subsurface environments would also provide refuge from the extreme conditions on the surface of Mars. Over the last decades, many lava-cave openings have been detected on the surface of Mars [21; 22]. Techniques and equipment to conduct these studies of subsurface environments on Mars to investigate these targets are underway.