USING ANALYSES OF DISSOLVED ORGANIC MATTER IN WATERS FROM VOLCANIC CAVES TO IDENTIFY PREVAILING MICROBIAL METABOLISMS

Terrestrial volcanic caves provide a valuable terrestrial analog for lava tubes on Mars and the Moon. Volcanic caves on Earth host abundant and diverse microbial life in the presence of liquid water, and morphologically and chemically diverse secondary mineral deposits (speleothems) that may preserve records of paleoenvironmental changes as well as signatures of past and present life. We studied the volcanic caves at Lava Beds National Monument (Lava Beds, N. California USA) as a Martian analog site, as part of the multi-disciplinary NASA BRAILLE (Biologic and Resource Analog Investigations in Low Light Environments) Project. Cave water in the form of ceiling drips and floor puddles was collected from seven caves at Lava Beds. We quantified and characterized dissolved organic matter (DOM) in the waters to constrain its role in microbial metabolism and elemental cycling within the caves. Dissolved organic carbon (DOC) and nitrogen (DON) in cave waters were measured using Shimadzu TOC/TN Analyzer, absorbance and fluorescence spectra of DOM were acquired using Horiba Aqualog Fluorometer, and molecular weight of DOM was determined using size exclusion chromatography. The cave waters were enriched in DOC (12±8 mg/L) with a molar C:N ratio of 2 to 22; this observation is surprising as it indicates a net lack of heterotrophic metabolism. It was further determined that the cave waters contained aromatic DOM with SUVA₂₅₄ ranging between 1.2 and 2.9 L/mg.m, terrestrially-derived and humified as indicated by humification index ranging between 7 and 26. Cave water (both drips and puddles) contained DOM of 100 Da, and 5,000 Da; in contrast, surface water overlying the caves contained 5,000 Da and 40,000 Da DOM, indicative of a plant-origin. We hypothesize that plant-derived DOM is drawn into caves with recharge water and remains unused due to insufficient heterotrophy within the caves. In contrast, 100 Da DOM may have formed by cave microbes via chemolithoautotrophic metabolism where inorganic carbon is fixed into simpler and smaller compounds. We interpret our findings to indicate that heterotrophic metabolisms are negligible in the caves at Lava Beds and chemolithoautotrophy is an important pathway for element cycling in these low light settings rich in organic carbon.