UTILIZATION OF ORGANIC NITROGEN-SOURCES BY 2 PHYTOPLANKTON SPECIES AND A BACTERIAL ISOLATE IN PURE AND MIXED CULTURES

Algal production of dissolved organic carbon and the regeneration of n utrients from dissolved organic carbon by bacteria are important aspec ts of nutrient cycling in the sea, especially when inorganic nitrogen is limiting. Dissolved free amino acids are a major carbon source for bacteria and can be used by phytoplankton as a nitrogen source. We exa mined the interactions between the phytoplankton species Emiliania hux leyi and Thalassiosira pseudonana and a bacterial isolate from the Nor th Sea. The organisms were cultured with eight different amino acids a nd a protein as the only nitrogen sources, in pure and mixed cultures. Of the two algae, only E. huxleyi was able to grow on amino acids. Th e bacterium MD1 used all substrates supplied, except serine. During gr owth of MD1 in pure culture, ammonium accumulated in the medium. Contr ary to the expectation, the percentage of ammonium regenerated from th e amino acids taken up showed no correlation with the substrate C/N ra tio. In mixed culture, the algae grew well in those cultures in which the bacteria grew well. The bacterial yields (cell number) were also h igher in mixed culture than in pure culture. In the cultures of MD1 an d T. pseudonana, the increase in bacterial yield (number of cells) ove r that of the pure culture was comparable to the bacterial yield in mi xed culture on a mineral medium. This result suggests that T. pseudona na excreted a more-or-less-constant amount of carbon. The bacterial yi elds in mixed cultures with E. huxleyi showed a smaller and less consi stent difference than those of the pure cultures of MD1. It is possibl e that the ability of E. huxleyi to use amino acids influenced the bac terial yield. The results suggest that interactions between algae and bacteria influence the regeneration of nitrogen from organic carbon an d that this influence differs from one species to another.