Geomicrobiology of nitrogen in a coastal aquifer : isotopic and molecular methods to examine nitrification and denitrification in groundwater

: The flux excess nitrogen is deleterious to coastal waters, resulting in deterioration of the water quality, increases in harmful algal blooms and disease in commercial fish stocks. On Cape Cod a significant portion of this nitrogen enters coastal waters through groundwater systems. Here we use isotopic and molecular biological methods to identify where the process that may lead to nitrogen removal occur, if population of microorganisms are present to carry out these process and where, and what are the potential rates of activity associated with these populations through the upper four meters of a coastal groundwater system. We show different populations of ammonia-oxidizing organisms based on the ammonia monooxygenase gene (amoA). Two archaeal populations, one shallow and closely related to water column-like sequences and one deep closely related to estuarine-like sequences. Rates of nitrification in the upper 2 m of sediment are significant and similar to marine sediments (208-456 pmol/g sediment/d). Denitrification occurs in the upper meter, evidenced by decrease in nitrate and increase in both delta Nitrogen 18 (up to + 20.1%) and delta Oxygen 18 (up to + 11.7%). The N- and O-isotopes of nitrate in the upper meter occur in a ratio that is indicative of groundwater denitrification.