Comparison of mechanisms enhancing biodegradability of refractory lake water constituents’

Bacterial degradation of macromolecular DOM, aged under weakly photolytic conditions (daylight > 300 nm), was compared to that of the original compounds before and after provision of a cosubstrate. Bacteria, Pseudomonas sp., Flavobacterium sp., and Erkinia sp., and macromolecules (apparent mol wt 2 1,500) were isolated from the same lake water samples, one taken in winter and one in summer. Decomposition was determined by DOC measurements, since UV absorbance A 2,,D-300nm did not give an accurate indication of loss rates. Both aging and the pulse dose of nutrients enhanced the accessibility of the refractory DOM to Pseudomonas sp. and Erwinia sp., while Flavobacterium sp. utilized the original substances most effectively. The original macromolecules were decomposed 12-22%, aging increased the available portions by a further 51 O%, and provision of the cosubstrate, glutamic acid, enhanced degradation by 3-8% and by 20% (Erwinia sp. and DOM fraction, both isolated in summer). The constituents of the macromolecules in the winter sample rendered accessible by photolysis were energetically advantageous to Pseudomolzas sp. and Erwinia sp. The degradable portions of the original and aged macromolecules were consumed within the first week of incubation, and the remainder was persistent. Provision of a nutrient pulse did not result in the production of enzymes that might have enabled the bacteria to assimilate the refractory remainder. Both mechanisms are thought to reduce the longevity of the refractory DOM in a stepwise manner in natural waters. Dissolved organic matter (DOM) in lake water consists mainly of refractory, high molecular weight substances. The macromolecular DOM fraction was mineralized up to 30% in degradation experiments with pure cultures of bacteria (Stabel et al. 1979). Sederholm et al. (1974), who also used pure cultures of bacteria, reported changes in percentages of phenolic compounds in aquatic humic matter indicating both degradation and humification of the substances. Geller (1983a) investigated the biodegradation of the macromolecular and the oligomeric DOM fraction with respect to the seasonal compositional variability of the substances and of the natural bacterial populations. Regardless of season and community composition, the macromolecules, 50-70% of the dissolved organic carbon (DOC), were persistent. There was only one instance in which bacteria effectively decomposed the DOM fractions of the parent lake water