EVALUATION OF HETEROTROPHY IN IN SERPENTINITE-ASSOCIATED WATERS FROM THE COAST RANGE OPHIOLITE, NORTHERN CALIFORNIA, USA

The deep biosphere in cold, dark sub-seafloor ultramafic rocks (i.e., those rocks rich in Fe and Mg) is stressed by exceedingly high pH, transient, if any, inorganic carbon availability, and little known organic carbon inventories. As a test of heterotrophic carbon use, serpentinite-associated waters (from groundwater sampling wells and associated surface seepages in tectonically uplifted mantle units in ophiolites) were tested for differences with respect to aqueous geochemistry and performance in EcoPlates™ - Biolog Inc. This work focuses on the Coast Range Ophiolite, CA field locations for water sampling.

Characteristics of each sampling site are presented (pH, mineral substrate, Ca²⁺/Mg²⁺ ratio, aqueous metal loads, etc.). Complementary EcoPlate™ results [prefabricated 96-well plates, seeded with triplicate experiments for determining microbiological community response to difference organic carbon sources; a triplicate control experiment with just water is built in to the plate also] are also presented. We found that waters from selected California groundwater wells (7 discrete wells) and related surface seeps (5 hydrologically connected sites) sourced in serpentinites were analyzed. EcoPlate™ average well-color development (AWCD), which reflects microbial activities averaged per plate (as in Garland and Mills, 1991), differs across sites. Correlations of AWCD with environmental data (such as pH, oxidation-reduction potential or ORP, Ca²⁺/Mg²⁺ ratio, and Fe contents) are evaluated.

Clarifying the geochemical-biological relationships that bear out in these analyses informs discourse on the energetic limits of life in serpentinizing systems, with relevance to ultramafic-hosted life on continents and in the seabed