Abstract
Sediment microbial communities are an important sink for both organic and inorganic nitrogen (N), with microphytobenthos (MPB) biomass making the largest contribution to short-term N-assimilation and retention. Coastal waters are increasingly subject to anthropogenic nutrient enrichment, but the effect of nutrient enrichment on microbial assimilation, processing, and fate of MPB-derived N (MPB-N) remains poorly characterised. In this study, an MPB-dominated microbial community was labeled in situ with a pulse of 15NH4+-N. Laboratory core incubations of this labeled sediment under increasing nutrient concentrations (NH4+ and PO43−: ambient, 2 × ambient, 5 × ambient, and 10 × ambient) were used to investigate changes in the processing and flux pathways of the 15N-labeled MPB-N across 10.5 d under nutrient enrichment. Short-term retention of MPB-N by MPB was stimulated by nutrient addition, with higher 15N in MPB in the nutrient amended treatments (71–93%) than in the ambient treatment (38%) at 0.5 d After 10.5 d, the nutrient amended treatments had increased turnover of MPB-N out of MPB biomass into an uncharacterised pool of sediment ON (45–75%). Increased turnover of MPB-N likely resulted from decreased recycling of MPB-N between MPB and heterotrophic bacteria as inorganic nutrients were preferentially used as an N source and remineralisation of sediment ON decreased. Decreased breakdown of sediment ON reduced the efflux of MPB-N via DON in the amended (3.9–5.2%) versus the ambient treatment (10.9%). Exports of MPB-N to the water column were relatively small, accounting for a maximum of 14% of 15N exported from the sediment, and were predominantly exported DON and N2 (denitrification). Overall, there was considerable retention of MPB-N over 10.5 d, but increased nutrient loading shifted N from MPB biomass into other sediment ON.
Highlights
Estuaries have a considerable role in altering and processing terrestrial and riverine derived organic matter prior to export to the coastal oceans
By combining a pulse-chase application of the rare isotope 15N with the D/L-Ala-biomarker technique, we identified that increased nutrient availability as a pulsed addition of 5 to 50 μmol L − 1 NH4+ across three treatments: 1) stimulated initial retention of MPB-derived N (MPB-N) within MPB 2) increased microbial turnover of MPB-N into the uncharacterised sediment ON compartment as incubations progressed, and 3) decreased the amount of MPB-N lost via DON effluxes
Uptake of N is typically viewed as being MPB dominated, with N retained in MPB biomass (Evrard et al, 2008; Hardison et al, 2011; Sundbäck et al, 2006; Veuger et al, 2007) unless grazing causes trophic transfer of MPB-N (Eyre and Ferguson, 2002, 2005)
Summary
Estuaries have a considerable role in altering and processing terrestrial and riverine derived organic matter prior to export to the coastal oceans. Within shallow photic coastal settings, much of the organic matter processing is mediated within the sediments by the microbial community (Bauer et al, 2013). Microphytobenthos (MPB) contribute significantly to primary production and biomass by fixing carbon (C) and utilizing nitrogen (N) from the water column and porewater (Cook et al, 2004a; Dalsgaard, 2003; McGlathery et al, 2007). Duced by MPB, while MPB rely on inorganic sources or bacteriallyexcreted NH4+ from remineralisation of organic matter (Cook et al, 2007; Forehead et al, 2013). The fate and processing of MPBderived C (MPB-C) within sediments has been well described, with studies quantifying the incorporation of MPB-C into sediment organic matter and its loss from the sediment via efflux of primarily dissolved inorganic carbon (DIC) with some dissolved organic carbon (DOC) (Middelburg and Nieuwenhuize, 2001; Oakes et al, 2012; Oakes and Eyre, 2014, 2016)
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