Diel controls on nitrogen loss pathways in a photosynthetic wetland biomat

A thick ( 10cm), benthic biomat reproducibly colonizes shallow open water treatment wetlands. Photosynthesis in the top centimeter of the biomat causes pronounced diel shifts in water column geochemistry, particularly with respect to pH ( 7.5 - 10) and dissolved oxygen ( 1 - 22 mg/L). Previous work characterizing nitrogen loss in this diffuse and stratified biomat revealed denitrification as the dominant pathway with contributions from anammox. However, it remains uncertain how coupled photosynthesis/respiration controls substrate availability for these nitrogen loss pathways and where they occur within the biomat. It is hypothesized that (i) denitrification activity is greatest at night and near the top of the biomat (e.g., beneath the oxic/anoxic interface) where algal exudate production can support heterotrophic growth but oxygen and light penetration are limited; (ii) anammox activity is spatially consistent with denitrification but greatest at dawn, when maximal inorganic carbon, nitrite, and ammonium are available from respiration, partial denitrification, and DNRA, respectively. Water column and biomat porewater samples collected under day and night conditions at the Prado Wetlands in Orange County, CA were analyzed for nitrogen species and dissolved organic carbon concentration and optical properties. Biomat samples were collected from the same depths as porewater to assess how the expression of nitrogen cycling genes responds to chemical changes over diel cycles. Porewater nitrogen speciation suggests that nitrate is fully attenuated within the first two centimeters of the vertical biomat profile; similar trends hold for nitrite but with an increase near the top during dark periods. In parallel with photosynthesis, pulses of organic matter accumulate in the oxic zone of the biomat during the day, followed by respiratory scavenging in the dark. Through coupling the energetics of denitrification and anammox with gene expression and dissolved organic matter quality, this work expands our understanding of coupled carbon and nitrogen cycling in stratified benthic sediments, as well as how organic matter properties relate to denitrification and anammox activity.