rRNA and Poly-β-Hydroxybutyrate Dynamics in Bioreactors Subjected to Feast and Famine Cycles

Abstract

Feast and famine cycles are common in activated sludge wastewater treatment systems, and they select for bacteria that accumulate storage compounds, such as poly-␤-hydroxybutyrate (PHB). Previous studies have shown that variations in influent substrate concentrations force bacteria to accumulate high levels of rRNA compared to the levels in bacteria grown in chemostats. Therefore, it can be hypothesized that bacteria accumulate more rRNA when they are subjected to feast and famine cycles. However, PHB-accumulating bacteria can form biomass (grow) throughout a feast and famine cycle and thus have a lower peak biomass formation rate during the cycle. Consequently, PHB-accumulating bacteria may accumulate less rRNA when they are subjected to feast and famine cycles than bacteria that are not capable of PHB accumulation. These hypotheses were tested with Wautersia eutropha H16 (wild type) and W. eutropha PHB-4 (a mutant not capable of accumulating PHB) grown in chemostat and semibatch reactors. For both strains, the cellular RNA level was higher when the organism was grown in semibatch reactors than when it was grown in chemostats, and the specific biomass formation rates during the feast phase were linearly related to the cellular RNA levels for cultures. Although the two strains exhibited maximum uptake rates when they were grown in semibatch reactors, the wild-type strain responded much more rapidly to the addition of fresh medium than the mutant responded. Furthermore, the chemostat-grown mutant culture was unable to exhibit maximum substrate uptake rates when it was subjected to pulse-wise addition of fresh medium. These data show that the ability to accumulate PHB does not prevent bacteria from accumulating high levels of rRNA when they are subjected to feast and famine cycles. Our results also demonstrate that the ability to accumulate PHB makes the bacteria more responsive to sudden increases in substrate concentrations, which explains their ecological advantage.rRNA-targeted hybridization techniques, such as fluorescence in situ hybridization and oligonucleotide membrane hybridization, are increasingly used to study microbial population diversity and dynamics in wastewater treatment systems (34). Although the temporal trends determined by these two techniques often agree with each other, their evaluations of population abundance may differ considerably. For example, Oerther and coworkers (19) reported that the Acinetobacter spp. population accounted for up to 43% of the total rRNA, while this population accounted for only up to 4.4% of the total number of cells in a municipal activated sludge system, suggesting that the cellular rRNA levels for this population were relatively high. In a related study, an opposite trend was found for the Gordonia spp. population in the same activated sludge treatment system (20). Some of the discrepancies can probably be explained by variation in the regulation of rRNA between species. However, the reactor conditions, specific metabolic functions, and ecological interactions...

Molecular insight into activated sludge producing polyhydroxyalkanoates under aerobic–anaerobic conditions

“…Applying such a sensitive approach as real-time PCR enabled the discovery of considerable changes during the process. Generally, the differences in the RNA level are correlated well with biomass formation rate [29]. Our results (Fig.…”

Bacterial community assembly in activated sludge: mapping beta diversity across environmental variables

“…The presence of these organisms has also been linked to influent compositions. For examples, members of the Gordoniaceae are present at high lipid loading rates (Frigon, Muyzer, van Loosdrecht, & Raskin, ); and members of the Thiotrichaceae are present at high volatile fatty acid concentration and in the presence of sulfides as they are capable of mixotrophic growth (Martins et al., ). Although the links between the influent composition and other environmental factors is not established for all the plant‐specific highly abundant populations, these observations reported here demand further investigations of their role in activated sludge microbial communities.…”

Modelling using rRNA-structured biomass models

“…A survey of activated sludge structured-biomass models indicates the following specific objectives for the developed models: † implementing a fundamental metabolic yield coefficients (the ATP/NADH2 ratio in the oxidative phosphorylation) (Smolders et al 1994;Beun et al 2000;Dicks et al 2001) † predicting transient conditions under the dynamic conditions of activated sludge processes or batch tests (Daigger & Grady 1982b;Smolders et al 1995aSmolders et al , 1995bGrady et al 1996;Beun et al 2000;Lavallé e et al 2002;Vanrolleghem et al 2004;Frigon et al 2006) † predicting microbial activities in different processes (Oerther et al 2001;Frigon et al 2002a;2002b;Stroot et al 2005;Simpson et al 2006) † interfacing with microbial population dynamics results obtained by molecular techniques Gilbride et al 2006) RNA-based models have been developed with all these specific objectives in prospect.…”

“…used the rRNA/rDNA ratio to characterize the metabolic activity of Pseudomonas abietaniphila in activated sludge.Using a genetic knockout mutantFrigon et al (2006) observed that the reactor configuration and the ability to produce PHB had an impact on the rRNA level for a pure culture growing in an acetate-fed reactor. Because the intracellular concentration of rRNA varied with process configuration or with the loading as well as with the growth rate, these observations showed that one pseudo steady state can not hold for different process configurations or for different steady states (i.e.…”