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 (Cafeteria roenbergensis) and an autotrophic dinoflagellate (Prorocentrum minimum) were 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 growth of C. roenbergensis. P. minimum attained higher abundance, higher chlorophyll a content per cell and a higher cellular nitrogen (N) content when grown with HS addition. In the treatment with P. minimum and bacteria approximately 35% of the humic-associated N was utilized by the organisms, as indicated by the increase of particulate N in P. minimum and bacteria cells. There was no net accumulation of inorganic N in any treatment. indicating that bacteria acted as a sink for N when utilizing the I-IS as substrate. Moreover, grazing activity by C. roenbergensis did not cause any significant accumulation of inorganic N in treatments with C. roenbergensis and bacteria, suggesting that bacteria used the inorganic N released by the grazers. Thus. the increased growth of P. minimum with HS present was probably not caused by bacterial mineralization of inorganic nitrogen, but could have been caused by the algae using dissolved organic nitrogen compounds.