Formation of high molecular weight material by free-living marine bacteria

Bacterial transformation of leucine into high molecular weight dissolved organic matter (HMW-DOM) material was studied in seawater cultures. Three different types of cultures were used: sodium azide poisoned cultures served as control, eukaryotic inhibitors to prevent bacterivory, and untreated cultures were used to determine the effect of grazing on bactena on the release of HMW-DOM. All these cultures were labeled wlth I4C-leucine and the distribution of incorporated label into particulate organic matter (POM) and I4C recovered in dissolved organic matter (DOM) was measured over a period of 10 d as well as the transformation into HMW-DOM. Fractionation of DOM was performed using dialysis membranes (molecular weight cutoff 50 000 daltons) and by gel filtration for substances with a molecular weight below l500 daltons. There was a shift to HMW-DOM discernible in grazed and ungrazed cultures and this DOM fraction accumulated to ca 6 % of the initially added leucine in the grazed culture within 8 d. Flagellates appeared to have only a minor influence on the formation of HMW-DOM but might be important in consuming bacterially derived HMW-DOM, representing a novel, nonpredatory way of carbon and energy transfer from bacteria to flagellates.