USING PAIRED GEOCHEMISTRY AND METAGENOMICS TO EXPLORE SOIL CRUSTS AS ANCIENT TERRESTRIAL ANALOGUES

Biological soil crusts (BSCs) are resilient, symbiotic communities of cyanobacteria, fungi, and algae that thrive in dry ecosystems. Because of their composition and tendency to grow in exclusion of advanced land plant communities, they are often invoked as a modern analogue for the development of early terrestrial ecosystems. Research on their metagenomics has identified a common suite of cyanobacteria and other bacteria that dominate their metabolisms. Geochemical work on BSCs has focused primarily on C and N cycling, with some research into trace metals and potential biosignatures of similar, ancient communities in the terrestrial rock record. Often, these two veins of BSC research are separate, limiting interpretations about links between metabolisms and elemental cycling, as well as posing challenges for evaluating critically potential biosignatures (e.g., redox-sensitive metals). Here, we analyzed a BSC community from the Colorado Plateau near Moab, UT for both geochemistry and metagenomics, allowing a more holistic interpretation of the potential biosignatures in the sediment. 10cm cores from a 3x3m grid were collected and stored on dry ice until analysis. Metagenomic data were collected using a FastPREP 16S RNA extraction and sequencing, and 16S data were analyzed using Mothur and GAST. The BSC community was similar to others from this region, with the six most abundant phyla being Acidobacteria, Proteobacteria, Bacteroidetes, Verrumicrobia, Actinobacteria, and Cyanobacteria. Alpha and beta diversity varied based on depth an immature versus mature BSC, and comparisons between BSCs of similar morphology showed highly dissimilar communities. Major and trace element geochemistry was measured via ICP-MS by ALS Minerals. P, Cu, Cr, Mo, Cd, Mg, and U were enriched in soil crusts compared to un-crusted sandy substrate. Al, Ca, Na, Fe, Mn, and V were consistently depleted in soil crust compared to the substrate. Ongoing work will analyze cm-scale depth profiles for mm-scale elemental shifts within and under the BSC, but these early results suggest the potential for a multi-elemental biosignature for the presence/absence of BSCs in the geologic record as well as potentially being able to link the geochemical results to more detailed community composition.