MICROBIAL DIVERSITY OF THE MINERAL-MICROBIAL CONTINUUM IN LAVA CAVES: IMPLICATIONS FOR NITROGEN CYCLING

Lava caves contain diverse microbial communities and secondary mineral deposits of potential importance to biogeochemical cycling. We investigated these deposits in nine caves at Lava Beds National Monument, California, USA, as part of BRAILLE (Biologic and Resource Analog Investigations in Low Light Environments), a research project sponsored by NASA’s PSTAR program. Samples were collected aseptically from secondary mineral deposits including mineral crusts, polyps, coralloids, fingers, and cauliflower morphologies. More microbial appearing deposits sampled included white, tan, and yellow microbial mats, and a soft, silica-rich material termed ooze. Extracted DNA was sequenced with 16S rRNA gene sequencing targeting Archaea and Bacteria. Our results showed a surprising amount of diversity in the more mineral deposits and some differences among the different sample types. Actinobacteria were more prominent in mineral-dominated features (30%) compared with more microbial deposits (20%), with the most abundant genera being Crossiella, Rubrobacter, and Pseudonocardia. Nitrospira, known for its oxidation of nitrite to nitrate, was most abundant in the more microbial deposits. Bacterial diversity was most distinct in ooze deposits, with the Nitrospira, wb1-P19 (nitrite oxidizer), subgroup_2, and MND1 being most abundant. Archaea were dominated by the Woesearcheaeceae, Nitrosotaleaceae, and Nitrosopumilaceae, Nitrocosmicus, and Nitrososphaeraceae, most of which are newly described ammonia oxidizers. Across the bacterial and archaeal diversity documented in mineral and microbial features, evidence suggests a significant role in nitrogen cycling. Prior research in lava caves has concentrated on microbial mat diversity, with more limited studies of mineral deposits. Our results suggest that mineral and microbial deposits are important parts of lava cave ecosystems and biogeochemical cycling and may provide insights as to the best candidates for sampling in the search for life in lava caves documented on extraterrestrial bodies within our Solar System.