Uptake of dissolved organic and inorganic nitrogen in microalgae-dominated sediment: comparing dark and light in situ and ex situ additions of 15N

Microbial communities within bare intertidal sediment have an active role in uptake of inorganic and organic nitrogen as it is transported through estuaries. N-15-labeled dissolved inorganic nitrogen (DIN, NH4+; 250 mu mol l(-1), 500 mu mol l(-1)) and dissolved organic nitrogen (DON, algal-derived; 125 mu mol l(-1), 250 mu mol l(-1)) were applied to diatom-dominated sandy intertidal sediment under light and dark conditions to investigate short-term N uptake (24 h). Two experiments compared uptake in intact sediments (in situ) and homogenized slurries (ex situ). In both experiments, N uptake was similar in light and dark conditions, and benthic microalgae (BMA) dominated both biomass and DIN and DON uptake over heterotrophic bacteria. Substantially lower uptake of DON than DIN occurred for both experiments, likely because organic molecules require extra cellular processing before uptake by BMA. Compared to intact sediments, sediment slurries had higher N uptake into sediment organic matter (3-36x), lower bacterial biomass (13.6 +/- 3.5% versus 41.1 +/- 7.6% intact) and low bacterial contribution to N-15 uptake (14 +/- 0.8% versus 14 +/- 3.0%). Differences are likely due to shifts within the microbial community and sediment environment caused by sediment homogenization or incubation effects. Consistently, uptake rates within slurries were greater than within intact sediments, and patterns of significant differences among treatments were different. Slurry incubations are therefore not reliable for quantification or comparison of in situ uptake rates across different N substrates, but biomarkers appear robust between the 2 methods, indicating low bacterial contribution to N uptake in BMA-dominated sediment.