SPATIAL AND TEMPORAL CONTROLS ON NITROGEN BIOGEOCHEMISTRY IN A PHOTOSYNTHETIC, REDOX STRATIFIED WETLAND BIOMAT

Engineered wetlands offer a unique opportunity to investigate fundamental biogeochemistry in a reproducible and controlled environment. For example, in wetlands constructed ~550 km apart in Orange County and Discovery Bay, CA, a geotextile lined shallow open water design selected for analogous benthic microbial communities. Phylogenetic inquiry revealed this thick (5 – 15 cm), photosynthetic ‘biomat’ assemblage to contain diatoms (predominantly Staurosira construens var. venter) complemented by a diverse array of heterotrophic bacteria. Previous work characterizing nitrogen loss pathways in the biomat demonstrated a compelling capacity for denitrification with potential contributions from anammox, despite receiving nitrate rich influent with minimal ammonium concentrations. Further inquiry revealed that biogenic sulfide production invoked a shift from canonical denitrification to DNRA, supporting anammox growth. Ongoing work is investigating the spatial distribution of nitrogen cycling activity within the biomat and the potential role of diatoms in regulating bacterial community structure and functionality. A series of field campaigns were performed to query diel shifts in geochemistry and microbial functionality in the biomat. Cores were collected in parallel with porewater sampling for genomic (16S rRNA gene and metagenomic/transcriptomic sequencing) and geochemical (carbohydrate extractions) inquiry in an attempt to couple porewater geochemistry and diatom exudate production with functional microbial community dynamics. Photosynthesis near the biomat/water interface caused dramatic redox shifts in the water column, where oxygen concentrations exceeded 700 uM during peak photosynthesis (pH ~10) and fell below 100 uM from respiration at night (pH ~7.5). Oxygen profiles within the biomat indicated that photosynthesis was limited to the upper ~5 mm, and the availability of electron acceptors (i.e., NO₃^-, NO₂^-, SO₄^2-) and donors (i.e., NH₄⁺, DOC) suggested that denitrification and anammox energetics were likely most favorable in the upper ~1 – 3 cm of the biomat. Phylogenetic and metagenomic data, in complement to functional biomat incubations in the laboratory, will serve to link geochemical observations with biological functionality in this unique engineered environment.