UTILIZATION OF DISSOLVED NITROGEN BY HETEROTROPHIC BACTERIOPLANKTON -A COMPARISON OF 3 ECOSYSTEMS

The contributions of different organic and inorganic nitrogen and orga nic carbon sources to heterotrophic bacterioplankton in batch cultures of oceanic, estuarine, and eutrophic riverine environments were compa red. The importance of the studied compounds was surprisingly similar among the three ecosystems. Dissolved combined amino acids (DCAA) were most significant, sustaining from 10 to 45% of the bacterial carbon d emands and from 42 to 112% of the bacterial nitrogen demands. Dissolve d free amino acids (DFAA) supplied 2 to 7% of the carbon and 6 to 24% of the nitrogen incorporated into the bacterial biomass, while dissolv ed DNA (D-DNA) sustained less than 5 and 12% of the carbon and nitroge n requirements, respectively. Ammonium was the second most important s ource of nitrogen, meeting from 13 to 45% of the bacterial demand in t he oceanic and estuarine cultures and up to 270% of the demand in rive rine cultures. Nitrate was taken up in the oceanic cultures (uptake eq ualed up to 46% of the nitrogen demand) but was released in the two ot hers. Assimilation of DCAA, DFAA, and D-DNA combined supplied 43% of t he carbon demand of the bacteria in the oceanic cultures, while approx imately 25% of the carbon requirements were met by the three substrate s at the two other sites. Assimilation of nitrogen from DCAA, DFAA, D- DNA, NH4+, and NO3-, on the other hand, exceeded production of particu late organic nitrogen in one culture at 27 h and in all cultures over the entire incubation period (50 h). These results suggest that the st udied nutrient sources may fully support the nitrogen needs but only p artially support the carbon needs of microbial communities of geograph ically different ecosystems. Furthermore, a comparison of the initial concentrations of the different substrates indicated that relative poo l sizes of the substrates seemed to influence which substrates were pr imarily being utilized by the bacteria.