Amino acid remineralization and organic matter lability in Chilean coastal sediments
Introduction
Particulate organic nitrogen compounds have been studied extensively in marine sedimentary systems because of their lability, their abundance in living organisms, and their importance as carbon and energy sources for primary and secondary producers (e.g., Lee, and Cronin, 1984, Henrichs et al., 1984, Burdige, and Martens, 1988, Whelan, and Emeis, 1992). In shallow, coastal areas, intensive mineralization of sedimentary organic nitrogen to dissolved inorganic nitrogen (DIN) compounds and subsequent transfer of this DIN to overlying waters usually accounts for 30–70% of the nitrogen demand of primary producers (Billen, 1978, Blackburn, 1986). Amino acids contribute most of the organic nitrogen in phytoplankton (Degens, 1970) and usually 20–50% of the total nitrogen in surface sediments (Henrichs et al., 1984, Burdige, and Martens, 1988, Whelan, and Emeis, 1992).
Because of the major contribution of amino acids to total dissolved organic nitrogen (DON) and the lability of these compounds to microbial mineralization, degradation of protein amino acids is thought to be the major source of inorganic nitrogen in sediments. This idea has been difficult to test quantitatively. Sedimentary protein amino acids are usually measured after strong acid hydrolysis and reported as total hydrolyzed amino acids (THAA); THAA include protein but also amino acids bound in some other chemical or physical matrix that are released during acid hydrolysis. For this reason, THAA measurements may not accurately reflect protein that is available for biological remineralization. Mayer and Rice (1992) explored this issue in a tidal mudflat in Maine and estimated that labile protein decomposition rates were higher than ammonification rates. They defined labile protein as that digested by proteases and measured by Coomassie Blue dye-binding (Mayer et al., 1986). Mayer and Rice suggested that the apparently lower ammonification rates might be caused by labile protein being resynthesized to a less labile form of protein and not converted to DIN. On the other hand, Burdige and Martens (1988), measuring THAA decay rates, found amino acid recycling to account for most (∼80%) of the total nitrogen regeneration in Cape Lookout Bight, NC.
We studied the distribution of sedimentary amino acids along a transect across the continental shelf off Concepción, Chile, and compared mineralization rates of THAA with rates of production of ammonium measured at the same sites and at the same time by Thamdrup and Canfield (1996). Our goals were to investigate the role of THAA degradation in the cycling of nitrogen in these sediments, and to evaluate whether THAA measurements are reliable estimates of available organic nitrogen for mineralization in surface sediments.
Section snippets
Study site and sampling
The coastal area off Central Chile undergoes strong seasonal upwelling (e.g., Ahumada and Chuecas, 1979) that results in high primary production ranging from 1 to 10 gC m−2 d−1 (Pantoja et al., 1987, Ahumada, 1991, Fossing et al., 1995), comparable to rates measured in the highly productive Peruvian upwelling area (40–100 m maximum depth) (Walsh, 1981). The sediments below this upwelling area contain dense populations of bacteria, which can compose up to 90% of the benthic biomass. Most of this
Amino acid concentrations in sediments and Thioploca
Surface sediment THAA concentrations were generally higher at the shallow (<100 m), near shore stations (6, 7 and 26) ranging from 350 to 1400 μmol THAA-N gdw−1 (Fig. 1). THAA concentrations decreased to low values (<100 μmol THAA-N gdw−1) below 5–10 cm depth in most stations. However, at Station 18 with its thick Thioploca mat, THAA concentrations were much lower throughout the core, usually less than 50 μmol THAA-N gdw−1. Farther offshore (Station 40) in ∼1000 m water depth, THAA
Acknowledgements
We thank B. B. Jørgensen, V. A. Gallardo and the scientific personnel and crew of the 1994 Thioploca Cruise. D. Canfield, T. Ferdelman, H. Fossing and B. Thamdrup helped obtain sediment samples and the TOC and TN data. Useful discussions with R. Aller and J. Middelburg are highly appreciated. Comments from an anonymous reviewer improved the quality of the manuscript. This study was supported by the U.S. National Science Foundation, the Max Planck Institute for Marine Microbiology, Germany, and
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Present address: Department of Oceanography, University of Concepción, PO Box 160-C, Concepción, Chile.