Changes in C:N ratios and chemical structures of estuarine humic substances during aging

DON as a source of bioavailable nitrogen for phytoplankton

“…Amino acids, amino sugars, ammonium (NH + 4 ), and nucleic acid bases comprise 46 to 53% of the N associated with humic acids and 45 to 59% of fulvic acids (Schnitzer, 1985) with the remaining approximately 50% of humic-N unidentified (Carlsson and Granéli, 1993). Previous work indicates that the C to N (C:N) ratio of aquatic humic substances, isolated with XAD resin, ranges from 18 to 30:1 for humic acids and 45 to 55:1 for fulvic acids, but can vary considerably (Thurman, 1985;See, 2003;See and Bronk, 2005). The C:N of humic substances isolated with macroporous resins, however, may not reflect the C:N ratios of humic substances in situ.…”

“…During the isolation procedure humic substances are acidified to a pH of 2, thus bombarding the solution with free protons. These free protons can bump off loosely associated amino groups such that humics isolated using resins have a C:N ratio higher than humics in natural waters (See and Bronk, 2005).…”

“…More recently, the uptake of laboratory-produced 15 N-labeled humic compounds by the >0.7 µm size fraction has been observed in both riverine and coastal ecosystems (Bronk et al, unpublished data), humic substances have been implicated as a potential source of C and N to the toxic dinoflagellate Alexandrium catenella (Doblin et al, 2000), and growth of another toxic dinoflagellate Alexandrium tamarense was shown to increase when exposed to humic substances (Gagnon et al, 2005). Uptake of humic-N into phytoplankton biomass was also measured directly using 15 N-labeled humic substances produced in the laboratory (See and Bronk, 2005). In this experiment, nonaxenic cultures of 17 recently isolated estuarine and coastal phytoplankton strains took up 15 N-labeled humic-N (See et al, 2006), however, high rates of humic-N uptake were not sustained over long periods of time suggesting that a finite pool of labile N is associated with these compounds (See et D. A. Bronk et al: Phytoplankton DON uptake 285 al., 2006).…”

“…guanosine monophosphate, adenosine triphosphate [ATP]), acetamide, and creatine. In addition, some researchers have examined the uptake of in situ produced 15 N-labeled DON derived from plankton cells grown on 15 NH + 4 or 15 NO − 3 (Bronk and Glibert, 1993;Veuger et al, 2004), from Trichodesmium using 15 N 2 gas (Bronk et al, 2004), or labeled humic material derived from Spartina alterniflora that was grown with 15 N-labeled NH + 4 added to the sediment (See and Bronk, 2005;See et al, 2006). Traditionally the inorganic forms were thought to exclusively support primary production.…”

“…Additional studies show that NH + 4 can be adsorbed to humics in the estuarine and nearshore environment (Rosenfeld, 1979;Gardner et al, 1991). This NH + 4 can then be released into the surrounding water column as the cation exchange sites on the humic compound are exposed to increased salt concentrations (Rosenfeld, 1979;Gardner et al, 1991 andSee, 2003;See and Bronk, 2005).…”