MICROBIAL FLUXES OF FREE MONOSACCHARIDES AND TOTAL CARBOHYDRATES IN FRESH-WATER DETERMINED BY PAD-HPLC

A new sensitive pulsed amperometric detection (PAD) method for measure ments of mono- and disaccharides in nM concentrations was used in comb ination with high performance liquid chromatography (HPLC) to study fl uxes of dissolved free and combined carbohydrates (DFCHO and DCCHO) in lake water. In a diel study concentrations of individual free sacchar ides typically were 5-50 nM, while total DFCHO concentrations ranged f rom 67 to 224 nM. No diel trends in concentration changes were obvious . At in situ light-dark conditions, dominant DFCHO were galactose, glu cose, fructose and mannose/xylose. In addition to these saccharides, a n increased abundance of melibiose and arabinose was measured in a par allel dark incubation. In a 118 h laboratory incubation of 1.0 mu m fi ltered lake water, concentrations of DFCHO decreased from 194 nM (at 1 2 h) to a minimum of 54 nM (at 73 h). Dominant DFCHO were glucose, fru ctose and cellobiose. During the incubation DCCHO varied from 1.27 to 2.20 mu M. Glucose, galactose and cellobiose made up 40, 30 and 10 mol -%, respectively, of the DCCHO. Fructose was degraded during hydrolysi s of the DCCHO. A decline of DCCHO at 55 h was reflected in a simultan eous increase of DFCHO, but otherwise no similarities between the two saccharide pools were found. Increased DCCHO concentrations and a high assimilation of glucose and fructose that was net reflected in a decl ine of their concentrations, both indicate that carbohydrates were pro duced during the experiment. Polysaccharides were probably excreted by the bacteria. Net assimilation of glucose and fructose sustained 14-1 9% (diel study) and 32% (long-term study) of the net bacterial carbon requirement.