The loss of organic nitrogen during marine primary production may be significantly overestimated when using 15N substrates

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Abstract
Field studies indicate that natural phytoplankton populations may release very significant amounts (20 to 40%) of newly assimilated dissolved inorganic nitrogen (DIN) as dissolved organic nitrogen (DON). In laboratory cultures, however, it is usually possible to account for at least 90% of nitrogen added to the system as DIN plus cell nitrogen. Here we show that the bulk of the missing nitrogen may be accounted for as dissolved free and dissolved combined amino acids. In cultures (which usually have a biomass density at least an order of magnitude greater than is present in offshore waters), the contribution of DON to system nitrogen thus appears to be minor. It is proposed that this difference may be explained if the levels of DON represent equilibrium between release–leakage and transport back into the algae. It is demonstrated, using a dynamic model of algal nitrogen physiology, that this mechanism can explain both laboratory and field observations. Simulations of incubations with DI15N reproduce the reported levels of loss in field incubations. However, because of isotope disequilibria between system components the 15N protocol may significantly overestimate the net loss of nitrogen from the algal cells. The arguments apply equally to studies of bacterial production of DON and to questions concerning the release of other dissolved organics by healthy phytoplankton. The significance of dynamic equilibria between the organisms and the medium may be missed in laboratory studies conducted with high biomass cultures.