Cycling of biologically available nitrogen in riverine humic substances between marine bacteria, a heterotrophic nanoflagellate and a photosynthetic dinoflagellate

The effects of freshwater dissolved organic matter (DOM) on the growth of a community of coastal marine bacteria, a heterotrophic flagellate (Cafeteri a roenbergensis) and an autotrophic dinoflagellate (Prorocentrum minimum) w ere studied in an experimental system incubated under laboratory conditions . The DOM used was in the form of riverine-isolated humic substances (HS). The addition of HS increased bacterial growth, which in turn increased grow th of C. roenbergensis. P. minimum attained higher abundance, higher chloro phyll a content per cell and a higher cellular nitrogen (N) content when gr own with HS addition. In the treatment with P. minimum and bacteria approxi mately 35% of the humic-associated N was utilized by the organisms, as indi cated by the increase of particulate N in P. minimum and bacteria cells. Th ere was no net accumulation of inorganic N in any treatment. indicating tha t bacteria acted as a sink for N when utilizing the I-IS as substrate. More over, grazing activity by C. roenbergensis did not cause any significant ac cumulation of inorganic N in treatments with C. roenbergensis and bacteria, suggesting that bacteria used the inorganic N released by the grazers. Thu s. the increased growth of P. minimum with HS present was probably not caus ed by bacterial mineralization of inorganic nitrogen, but could have been c aused by the algae using dissolved organic nitrogen compounds.