DISSOLVED ORGANIC-MATTER STUDIES IN ENCLOSED SYSTEMS - APPLICATION OFHYDROPHOBIC FRACTIONATION FOR THE ASSESSMENT OF ORGANIC NITROGEN DYNAMICS

The evolution of dissolved organic matter (DOM) in a non-axenic batch culture of the marine diatom Thalassiosira tumida was studied by hydro phobic fractionation during a three month experiment. DOM was fraction ated with XAD-2 resin into hydrophobic (acid and neutral, ''humic'') a nd hydrophilic fractions. The combined amino acid contents of unfracti onated filtered seawater, XAD-fractions and particulate material were determined during the growth, stationary and degradation phases of the culture, and variations related to changes in dissolved organic nitro gen (DON) in XAD-fractions, dissolved inorganic nitrogen, algal and ba cterial biomass. XAD-fractionation enabled the discrimination of simul taneously ocurring release and uptake of organic nitrogenous compounds : During the diatom growth there was a net increase of total DON conce ntrations, which was mostly accounted for by the hydrophilic fraction. A concurrent heterotrophic uptake of combined amino acids and other n on-amino acid organic nitrogen was discernible by the decrease of thei r concentrations in the hydrophobic fractions. In the stationary phase , during the prevailing net consumption of total DON, the production o f algal exudates could be detected in the hydrophobic fractions, while uptake mainly involved non-amino acid organic nitrogen from the hydro philic fraction. During the degradation phase, after two months part o f the particulate amino acid pool was transformed into hydrophilic DON , which in contrast to the stationary phase, was not adequate for supp orting sustained bacterial growth. This suggests that the generation o f recalcitrant substances may begin in the hydrophilic fraction of DOM . A slight increase of the hydrophobic acid fraction was indicative of the incipient formation of humic substances. XAD-2 was able to adsorb substances from fast changing DOM pools and thus should be a useful t ool in studies concerned with phytoplankton and bacterial dynamics.