Humic acid effects on 2-hydroxypyridine metabolism by starving Arthrobacter crystallopoietes cells

Bacterial strains amended to soil to facilitate bioremediation of sites contaminated with organic substances must be capable of surviving and functioning in the presence of a variety of soil organic and mineral components. Thus, the influence of humic acid on 2-hydroxypyridine catabolism, retention of 2-hydroxypyridine catabolic activity by whole cells, and hydrophobicity of the Arthrobacter crystallopoietes cells was examined. Humic acid, added to the starvation medium of the arthrobacter cells, which had been acclimated to catabolize 2-hydroxypyridine, resulted in increased stability of this metabolic activity compared to that of comparable cells starved in the absence of humic acid. For example, after 7 days starvation, cells incubated in the presence of 0.1% (wt/vol) humic acid oxidized 2-hydroxypyridine at a rate of 54.9 mu molh super(-1) compared to 14.6 mu molh super(-1) in the absence of humic acid. Although after 1 day starvation 2.5-fold more 2-hydroxypyridine-catabolizing activity was detected in the presence of humic acid than in its absence, this enzymatic activity declined to undetectable levels after 3 days starvation both with and without humic acid in the starvation medium. No effect of humic acid was noted on protein content of the cells. Hydrophobicity of the cells was not affected by humic acid during the first 4 days of starvation but after 7 days humic acid lessened the reduction in this property. Thus, changes in cell protein content and hydrophobicity did not explain the effect of humic acid on 2-hydroxypyridine catabolism of starving A. crystallopoietes cells.