Estimation of the Yield Coefficient of Pseudomonas sp. Strain DP-4 with a Low Substrate (2,4-Dichlorophenol [DCP]) Concentration in a Mineral Medium from Which Uncharacterized Organic Compounds Were Eliminated by a Non-DCP-Degrading Organism

Abstract

The yield coefficient (YC) of Pseudomonas sp. strain DP-4, a 2,4-dichlorophenol (DCP)-degrading organism, was estimated from the number of CFU produced at the expense of 1 unit amount of DCP at low concentrations. At a low concentration of DCP, the YC can be overestimated in pure culture, because DP-4 assimilated not only DCP but also uncharacterized organic compounds contaminating a mineral salt medium. The concentration of these uncharacterized organic compounds was nutritionally equivalent to 0.7 g of DCP-C ml ؊1 . A mixed culture with non-DCP-degrading organisms resulted in elimination of ca. 99.9% of the uncharacterized organic compounds, and then DP-4 assimilated only DCP as a substrate. In a mixed culture, DP-4 degraded an initial concentration of 0.1 to 10 g of C ml of DCP ؊1 and the number of CFU of DP-4 increased. In the mixed culture, DCP at an initial concentration of 0.07 g of C ml ؊1 was degraded. However, the number of CFU of DP-4 did not increase. DCP at an extremely low initial concentration of 0.01 g of C ml ؊1 was not degraded in mixed culture even by a high density, 10 5 CFU ml ؊1, of DP-4. When glucose was added to this mixed culture to a final concentration of 1 g of C ml ؊1 , the initial concentration of 0.01 g of C ml of DCP ؊1 was degraded. These results suggested that DP-4 required cosubstrates to degrade DCP at an extremely low initial concentration of 0.01 g of C ml ؊1 . The YCs of DP-4 at the expense of DCP alone decreased discontinuously with the decrease of the initial concentration of DCP, i.e., 1.5, 0.19, or 0 CFU per pg of DCP-C when 0.7 to 10, 0.1 to 0.5, or 0.07 g of C ml of DCP ؊1 was degraded, respectively. In this study, we developed a new method to eliminate uncharacterized organic compounds, and we estimated the YC of DP-4 at the expense of DCP as a sole source of carbon.

Kinetics and Yields of Pesticide Biodegradation at Low Substrate Concentrations and under Conditions Restricting Assimilable Organic Carbon

“…The consequence of these multiphasic kinetics is that extrapolation of kinetic data measured at higher concentrations may significantly under-or overestimate observed kinetics at low, environmentally relevant concentrations (10,11). Comparatively few studies have investigated microbial growth and yield of bacterial strains at low substrate concentrations (12,13). Several authors have reported threshold concentrations (typically less than 10 g liter Ϫ1 ) below which degradation continues in a nongrowth regime (9,11,14), but these conclusions are based solely on observed first-order substrate utilization kinetics; direct measurements of microbial growth or yield at these low concentrations have not been reported.…”

“…Experience in our lab suggests that experiments conducted with mineral medium in glassware treated according to standard laboratory practices can contain up to 100-fold more AOC than experiments conducted with more rigorously treated glassware (15). In fact, it can be assumed that growth experiments that were conducted in glassware and mineral medium prepared according to standard laboratory procedures may have contained up to 1 mg liter Ϫ1 of contaminating AOC as the extent of growth in negative controls could not be distinguished from growth in experiments conducted with up to 1 mg liter Ϫ1 of substrate (12,16,17). In effect, this implies that any biodegradation assay conducted at low substrate concentrations (below approximately 1 mg liter Ϫ1 ) is a mixed-substrate assay, which will inevitably have profound effects on both substrate utilization kinetics (18)(19)(20) and yield measurements (12,21).…”

“…In fact, it can be assumed that growth experiments that were conducted in glassware and mineral medium prepared according to standard laboratory procedures may have contained up to 1 mg liter Ϫ1 of contaminating AOC as the extent of growth in negative controls could not be distinguished from growth in experiments conducted with up to 1 mg liter Ϫ1 of substrate (12,16,17). In effect, this implies that any biodegradation assay conducted at low substrate concentrations (below approximately 1 mg liter Ϫ1 ) is a mixed-substrate assay, which will inevitably have profound effects on both substrate utilization kinetics (18)(19)(20) and yield measurements (12,21). Therefore, experiments designed to quantify substrate utilization kinetics and microbial growth yields at low substrate concentrations should include appropriate measures to minimize the interfering effects of contaminating AOC.…”

A Model Framework to Describe Growth-Linked Biodegradation of Trace-Level Pollutants in the Presence of Coincidental Carbon Substrates and Microbes

“…Coincidental microbes may compete with pesticide degraders for coincidental carbon substrates which could similarly effect the kinetics of growth-linked pesticide biodegradation. 20,24,25 Studies on survival or die-off of pesticide degrader strains 26,27 have not explicitly considered the compounding effects of coincidental carbon substrates or coincidental microbes on the kinetics of pesticide biodegradation or the growth of the pesticide degrader strain. Several researchers have documented the ability of coincidental carbon to support the growth of specific pesticide degrading strains.…”

“…Several researchers have documented the ability of coincidental carbon to support the growth of specific pesticide degrading strains. 17,20,24 However, only a fraction of the coincidental carbon is typically accessible to the strains. 15 The uncertainty of how much of the coincidental carbon is available for the pesticide degrader makes it more complex to explain these experimental results.…”