AME 16:281-293 (1999)  -  doi:10.3354/ame016281

ABSTRACT: Short-term, diel and annual variations in sugar concentrations (hexoses and oligosaccharides) were determined in a shallow eutrophic lake (Lake Au, Switzerland) by high performance liquid chromatography-thermospray mass spectrometry (HPLC-TSMS), a technique previously not used for the quantitative determination of mono- and oligosaccharides in aquatic environments. Concentrations of total hexoses and total disaccharides were extremely low in comparison to previous studies and showed little variations on an annual time scale with values <1 to 48 nM (mean: 13 nM) and <1 to 77 nM (mean: 9 nM), respectively. The low values can be attributed to the reliability of the mass spectrometric method that used labelled internal standards, which were added immediately after sampling, and the use of hollow-fibre filtration to remove delicate organisms without damage from the soluble fraction. The low concentrations also indicate the existence of high affinity sugar uptake systems in aquatic microbial communities. Trisaccharide and tetrasaccharide concentrations were below the detection limit of 2 nM on most sampling dates. The total carbon content of the mono- and oligosaccharides represented 0.003 to 0.3% of the dissolved organic carbon (DOC). We observed neither a trend in sugar concentration over the year, nor a dependence on phytoplankton density, but transient pulses of sugar concentrations occurred on an hourly time scale, suggesting a tight coupling between release and uptake of sugars. Average decrease rates of oligosaccharides (maltose, maltotriose, maltotetraose) that were added as substrates to in situ exposed 5 l microcosms ranged between 27 and 50 nmol l^-1 h^-1. Both the high decrease rates and the absence of cleavage products in the course of the incubations support the assumption that uptake of intact oligosaccharides by microheterotrophs was responsible for the decrease in concentrations. Furthermore, the immediate decrease of the added maltooligosaccharides demonstrate that these sugars are frequently used substrates in lakewater. This could not be confirmed for 3-O-b-galactopyranosyl-D-arabinose, for which a decrease in concentration was only observed after 29 h. These experiments indicate that the sugar composition of oligosaccharides and their type of linkage are important features influencing their bioavailability to microheterotrophs.

KEY WORDS: Thermospray mass spectrometry · Dissolved free carbohydrates · DOC · Oligosaccharides · Sugar dynamics · Lakewater