Uptake of dissolved inorganic nitrogen, urea and amino acids in the Scheldt estuary: comparison of organic carbon and nitrogen uptake

Abstract

Uptake of dissolved ammonium, nitrate, nitrite, urea and amino acids was studied in the Scheldt estuary in different seasons over a salinity gradient. The importance of inorganic nitrogen sources was compared to that of urea and amino acids and the relative use of urea and amino acid nitrogen and carbon was studied. Urea and amino acids constituted up to 43 and 29% of total nitrogen uptake, respectively, and were of similar importance as inorganic substrates. Ammonium oxidation by nitrifiers and ammonium uptake by algae and bacteria were of similar magnitude. In January, April and November, amino acids constituted a source of both nitrogen and carbon while urea mainly constituted a source of carbon. During the summer months, amino acids were used mainly as a source for nitrogen, while urea was a source for both carbon and nitrogen; urea was rarely used as nitrogen substrate alone. Due to this seasonal uncoupling of nitrogen and carbon assimilation, dually labeled substrates are necessary to assess the importance of urea and amino acids as substrates for organic nitrogen and carbon.

Incorporation of nitrogen from amino acids and urea by benthic microbes: role of bacteria versus algae and coupled incorporation of carbon

“…The full incorporation of 15 N combined with incorporation of ~50% of the 13 C indicates that EAAO accounted for at least 50% of total microbial 15 N incorporation. These results are consistent with previous reports on the importance of EAAO in water column studies (Pantoja & Lee 1994, Mulholland et al 1998, 2002, Andersson et al 2006 and for the first time show the importance of EAAO in microbial nitrogen incorporation in sediment. Although extracellular oxidation of amino acids and subsequent resynthesis of amino acids inside the cells seems inefficient, it may actually be more economical than direct uptake of intact amino acids, which requires more energy than uptake of NH 4 + (Antia et al 1991 and references therein).…”

“…5) confirms that part of the AA-13 C was transformed to 13 C-DIC. This uncoupled behavior of nitrogen and carbon from amino acids is consistent with results from previous studies on algal cultures (Algeus 1948, Stephens & North 1971, Palenik & Morel 1990, Antia et al 1991 and coastal waters (Schell 1974, Mulholland et al 2002, Andersson et al 2006, and conversion of amino acid carbon to DIC in a coastal sediment was reported by Christensen & Blackburn (1980).…”

“…These results are comparable with those of Lund & Blackburn (1989), who measured production of 14 C-DIC after addition of 14 C-urea to a coastal marine sediment but no incorporation of 14 C. Release of 13 C-or 14 C-DIC following addition of 13 C-or 14 C-urea was also observed for algal cultures (Price & Harrison 1988, Antia et al 1991 and references therein), bacteria from estuarine waters (Jørgensen 2006) and a natural planktonic microbial community (Tamminen & Irmisch 1996). Moreover, uncoupled uptake of 15 N and 13 C from dual-labeled urea has been reported for coastal waters (Mulholland et al 2004, Fan & Glibert 2005, Andersson et al 2006.…”

“…Studies on algal cultures have shown that incorporation of urea-N is uncoupled from urea-C (Price & Harrison 1988, Antia et al 1991, while both coupled and uncoupled uptake of nitrogen and carbon has been reported for amino acids (Algeus 1948, Stephens & North 1971, Palenik & Morel 1990, Antia et al 1991. For natural aquatic microbial communities, uptake of N and C from organic compounds has been studied using duallabeled urea (Mulholland et al 2004, Fan & Glibert 2005, Andersson et al 2006) and amino acids (Schell 1974, Zehr et al 1985, Mulholland et al 2002, Andersson et al 2006). In general, these studies found that amino acids can supply both N and C for microbial growth, while urea serves as a source of N only.…”

Benthic metabolism and nitrogen cycling in a sub-tropical coastal embayment: spatial and seasonal variation and controlling factors

“…The large return of DIN in winter is most likely due to much smaller DON uptake coupled with excretion of DIN by macrofauna, while the lower summer returns may reflect assimilation and retention of N by microbes. The predominance of dark DON uptake throughout the study and the association of DON fluxes with Component 1 (Table 2) suggest that heterotrophic organisms were utilising DON compounds as either a carbon or nitrogen substrate (Andersson et al 2006). Uptake of DON was most pronounced in Cluster 3 (Table 3) and tended to increase during the second half of the night, which suggests that endogenous DON may become depleted within the benthos during the dark, leading microbes to remove dissolved substrates from the water column.…”