TOXIC EFFECTS OF METALS ON MICROBIAL ORGANIC CARBON ASSIMILATION AND NITRATE REMOVAL

Ground and surface water-quality is critical in Northwest Arkansas because they support unique ecosystems, provide drinking water, and are sources of tourism revenue. Literature on the coevolution of labile organic matter, dissolved nutrients, and metal fluxes in relation to the epikarst microbiota is limited in the region. Two experiments were conducted to assess microbial response to dissolved organic carbon (DOC), nutrients, and dissolved metals. A gravel and spring-water sample were collected and portioned into 1 L mason jar microcosms to assess potential microbial nutrient utilization at the spring site. Sodium acetate (CH₃COONa), potassium nitrate (KNO₃), and a suit of metals were added to the microcosms, and they were allowed to incubate in a dark- temperature-controlled environment for 11 and 4 weeks, respectively. Isotopically labeled-acetate (Ch₃¹³COONa) and nitrate (K¹⁵NO₃) were added to the microcosms as stable isotopic tracers of microbial biological processes. Water samples were collected weekly from the microcosms without metal additions, and daily from microcosms with metal additions. Samples were analyzed for dissolved oxygen (DO) concentration, dissolved inorganic carbon (DIC) concentration, metal concentration, ¹³C-DIC and ¹⁵N-NO₃ isotopic compositions. In microcosms without metal additions, DO decreased to below 2 mg/L, while δ¹³C-DIC became increasingly isotopically enriched as DIC concentration increased, and δ¹⁵N-NO₃ data showed clear indication of denitrification activity. Microcosms with metal additions showed no clear indication of denitrification activity, and DO concentrations remained aerobic throughout the experiment. Microcosms with dissolved metal concentrations above 10 µg/L displayed inhibitory effects on microbial processes. These processes included DOC assimilation and denitrification. DIC production increased and δ¹³C-DIC became more enriched after 2 days in microcosms with <1 µg/L dissolved metals. The results of this study indicate water-quality degraded as dissolved nutrients and organic carbon were removed less efficiently when dissolved metal concentrations exceed 10 µg/L. Increased metal fluxes into groundwater-surface water systems have major implications for water quality evolution in epikarst.