DON as a source of bioavailable nitrogen for phytoplankton

Abstract. Relative to inorganic nitrogen, concentrations of dissolved organic nitrogen (DON) are often high, even in regions believed to be nitrogen-limited. The persistence of these high concentrations led to the view that the DON pool was largely refractory and therefore unimportant to plankton nutrition. What DON is utilized was believed to fuel bacterial production. More recent work, however, indicates that rates into and out of the DON pool can be large, and that the constancy in concentration is a function of tightly coupled production and consumption processes. Evidence is also accumulating that indicates phytoplankton, including a number of harmful species, may obtain a substantial part of their nitrogen nutrition from organic compounds. Ongoing research is investigating ways to discriminate between autotrophic and heterotrophic utilization, as well as a number of mechanisms such as cell surface enzymes and photochemical decomposition, that could facilitate phytoplankton use of DON components.