Modelling the kinetics of biodegradation of anionic surfactants by biofilm bacteria from polluted riverine sites: A comparison of five classes of surfactant at three sites

Abstract

The capacity of epilithic bacteria from different riverine sites to biodegrade representatives of five classes of anionic surfactants, was quantified by analysis of biodegradation kinetics in die-away tests. The sampling sites in the River Ely, South Wales were located near a sewage treatment plant outfall. Indigenous stones from the riverbed were removed from sites immediately beneath the outfall (BO), and at sites 15 m upstream (BU) and 50 m downstream (BD). The five surfactants used were sodium dodecyl sulphate (representing primary linear alkyl sulphates, PLAS), Empicol R ESB3 S (alkyl ethoxy sulphates, AES), potassium undecyl-2-sulphate (secondary linear alkyl sulphates, SLAS), sodium 1-dodecane sulphonate (primary alkane sulphonates, PAΣ) and 5-phenyl dodecane sulphonate (linear alkyl benzene sulphonates, LABΣ). Irrespective of sampling site, the order of biodegradability assessed as the reciprocal of half-time for surfactant disappearance was PLAS > AES > SLAS > PAΣ > LABΣ. Within each surfactant class, the half-time decreased in the order BU > BD > BO. Non-linear regression analysis of the die-away data to several kinetic models which combined disappearance of surfactant by zero, first order or Michaelis-Menten kinetics, with growth (exponential or logistic) of competent cells, yielded as best fit the model involving exponential growth of competent cells with first-order disappearance of substrate. This model generated two biologically-significant parameters: K 1 which is equivalent to first-order rate constant in the die-away test at t = 0 (i.e. before growth distorts the population from its indigenous composition); and r, the specific growth rate of the biodegrading population in the die-away flask. For a given site, K 1 decreased in the order PLAS > AES > SLAS > PAΣ > LABΣ, reflecting their relative biodegradability. Values of r for the primary sulphate esters (PLAS and AES) were always greater than values for the other surfactants, especially the sulphonates (PAΣ and LABΣ), indicating that different sub-populations existed for the biodegradation of these classes. Moreover, lack of inter-site variation in this pattern suggested similar distributions of surfactant-biodegrading populations at each site. Values of { K 1 viable-cell count }, which is a measure of surfactant-biodegrading activity per unit of population, fell into two groups: higher values (i.e. > 10 −9h −1 cfu −1 ml) with little inter-site variation (∼3-fold) for sulphated surfactants, and lower values (mostly < 10 −10 h −1 cfu −1 ml) with much greater inter-site variation (> 30-fold) for sulphonates. These results indicated that the capacity to biodegrade sulphated surfactants was more widely distributed in bacteria than was the capacity to biodegrade sulphonates. This situation may have evolved as a result of selective pressure from exposure of bacteria to environments in which natural analogues of sulphated surfactants, but not of aryl sulphonates or LABΣ, are commonly present.