Comparison of mechanisms enhancing biodegradability of refractory lake water constituents1

Abstract

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 Erwinia sp., and macromolecules (apparent mol wt ≥ 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 A200–300nm 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 5–10%, 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 Pseudomonas 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.