Role of Chemotaxis in the Ecology of Denitrifiers

Abstract

A modification of the Adler capillary assay was used to evaluate the chemotactic responses of several denitrifiers to nitrate and nitrite. Strong positive chemotaxis was observed to NO 3 − and NO 2 − by soil isolates of Pseudomonas aeruginosa, Pseudomonas fluorescens , and Pseudomonas stutzeri , with the peak response occurring at 10 −3 M for both attractants. In addition, a strong chemoattraction to serine (peak response at 10 −2 M), tryptone, and a soil extract, but not to NH 4 + , was observed for all denitrifiers tested. Chemotaxis was not dependent on a previous growth on NO 3 − , NO 2 − , or a soil extract, and the chemoattraction to NO 3 − occurred when the bacteria were grown aerobically or anaerobically. However, the best response to NO 3 − was usually observed when the cells were grown aerobically with 10 mM NO 3 − in the growth medium. Capillary tubes containing 10 − 3 M NO 3 − submerged into soil-water mixtures elicited a significant chemotactic response to NO 3 − by the indigenous soil microflora, the majority of which were Pseudomonas spp. A chemotactic strain of P. fluorescens also was shown to survive significantly better in aerobic and anaerobic soils than was a nonmotile strain of the same species. Both strains had equal growth rates in liquid cultures. Thus, chemotaxis may be one mechanism by which denitrifiers successfully compete for available NO 3 − and NO 2 − , and which may facilitate the survival of naturally occurring populations of some denitrifiers.