In order to better understand the factors that modulate the size and reactivity of high molecular weight organic matter dissolved in seawater, fluorescently labeled dextrans were used as the model compounds whose molecular weight distributions were monitored by flow field-flow fractionation (flow FFF or FIFFF) during incubations in seawater matrices. Two fluorescein isothiocyanate (FITC) labeled dextrans (145k and 2M Da) were incubated in whole seawater (natural microbial population and natural dissolved organic matter (DOM) present), 0.02 pm filtered seawater (all microorganisms removed, but natural DOM 2815 Copyright 0 1997 by Marcel Dekker, Inc. Downloaded by [Athabasca University] at 14:39 06 June 2016 2816
RATANATHANAWONGS WILLIAMS AND KEILlargely unaltered), and UV-oxidized seawater (no microorganisms or natural DOM present). Flow FFF fractograms of the two FITC-dextrans incubated in UV-oxidized seawater showed no changes, signifying that the dextrans did not undergo any alteration or aggregation. The dextrans incubated in filtered seawater with natural DOM present resulted in fractograms that are shifted to higher retention times, consistent with aggregation of the destran and natural DOM.In the whole seawater incubations, the complex changes in the fractograms over time indicated that the dextrans underwent both aggregation with natural DOM and degradation by hcterotrophic microorganisms. The latter was confirmed by microscopic examination of the collected fractions after selectively staining the microorganisms with the fluorescent dye 4,6-diamidino-2-phenylindole (DAPI). The FITC dextrans were observed to be "attached" to the bacteria's outer cell membrane where they are subsequently hydrolyzed into small low molecular weight fragments by extracellular enzymes. Some of these degraded dextrans are assimilated by the bacteria and the rest (-70%) are released into the seawater. Approximately half of the released degraded dextrans are of high enough molecular weight to be retained by flow FFF at the conditions employed in these studies. The data presented in this paper illustrate that, when used as both a qualitative and semipreparative tool, flow FFF can provide information on the relationships between natural and model DOM and microorganisms that would be difficult or impossible to obtain using other methods.