Colonization of biofilms by bacteria capable of biodegrading sodium dodecyl sulphate (SDS) at clean and polluted riverine sites.

Biofilm formation on presterile slate-discs placed at a pristine source site and at three polluted sites located upstream from, at, and down-stream from a sewage-works outfall in a South Wales river, was monitored over 14 days. Viable bacterial cell densities in biofilms increased with time at all sites but more rapidly at the polluted sites, and stabilized after 1-4 days at levels equal to those of stones indigenous to the corresponding sites, and similar to those seen in other comparable pristine and polluted rivers. Biofilm cell densities were elevated about 1,000-fold at the sewage works outfall compared with the site immediately upstream. Downstream from the outfall, the cell densities were intermediate between the upstream and outfall values. Epilithon resuspended from the slates during colonization was tested for its capacity to biodegrade the surfactant sodium dodecyl sulphate (SDS) in die-away tests. No biodegradation of this common pollutant was observed for the samples from the pristine source. In contrast, even after only 1 day, all polluted sites produced samples capable of biodegrading SDS. Longer exposure in the river led to more rapid onset of biodegradation in the die-away tests, indicating an adaptation mechanism(s). Die-away kinetics were fitted by computerized non-linear regression analysis to one of several models. The model of best fit involved biodegradation of SDS by a bacterial population growing at the expense of endogenous carbon. The regression parameter reflecting SDS-degrading activity of the epilithic samples increased markedly during Days 0-4 for all three polluted sites. The stabilized values (Days 4-14) increased from the upstream site to the outfall, then decreased to intermediate values downstream. Although this pattern corresponded to the changes in viable cell numbers, the effect of the sewage input was less marked for the SDS-degrading activities than for bacterial cell densities. In addition, there was little variation in growth characteristics throughout colonization at all three polluted sites. Collectively the results indicate that the observed adaptation during exposure in the river is attributable to colonization of the epilithon by an existing SDS-degrading population, rather than the acquisition or adaptation of this biodegradative capability.