Stimulatory Effect of Xenobiotics on Oxidative Electron Transport of Chemolithotrophic Nitrifying Bacteria Used as Biosensing Element

Abstract

Electron transport chain (ETCh) of ammonium (AOB) and nitrite oxidizing bacteria (NOB) participates in oxidation of ammonium to nitrate (nitrification). Operation of ETCh may be perturbed by a range of water-soluble xenobiotics. Therefore, consortia of nitrifying bacteria may be used as a biosensor to detect water contamination. A surprising feature of this system is an increase of oxygen consumption, detected in the presence of certain inhibitors of ETCh. Thus, to shed light on the mechanism of this effect (and other differences between inhibitors) we monitored separately respiration of the bacteria of the first (AOB - Nitrosomonas) and second (NOB -Nitrobacter) stages of nitrification. Furthermore, we measured plasma membrane potential and the level of reduction of NAD(P)H. We propose a novel model of ETCh in NOB to explain the role of reverse electron transport in the stimulation of oxygen consumption (previously attributed to hormesis).

Modeling of Nitrification Kinetics in a Respirometric Biosensor under Suboptimal Conditions

“…Initially, the bacteria were isolated after the activated sludge inoculation to the selection media specific for nitrifiers according to the protocol described by Mac Donald and Spokes [21]. The consortium of nitrifiers was characterized previously using genetic and biochemical methods [1,12]. Next, consortia of nitrifying bacteria were grown in an 8 L laboratory-scale reactor as a fed-batch culture in physicochemical conditions described previously.…”

“…The biomass from this bioreactor was then immobilized in polyurethane sponges (length 100 mm, diameter 25 mm) located horizontally in polystyrene tubes (120 mm long, diameter 25 mm) in a flow system in ABTOW [1]. The layout of ABTOW, its calibration and its operation, were described by Woznica et al [1,2,12,13]. Briefly, the bacterial biofilms were grown in bioreactors for a minimum of 14 days to obtain their stable structure and physiology.…”

“…The pH of the medium was regulated with sodium hydroxide and sulfuric acid. The measurements were recorded in 5 s intervals, and the ABTOW activity was represented as a function of time (v(t)) using quasi-Newton estimation [1,2,12,13].…”

“…The advantage of nitrifying bacteria is the high consumption of oxygen in their metabolism (especially during ammonium oxidation), with a simultaneous slight increase in biomass which results from the low energy efficiency of the nitrification processes. This allows for high metabolic stability of the microorganisms used as the biological element of the biosensor for a relatively long period of time [9][10][11][12].…”

“…The Automatic Biodetector of Water Toxicity (ABTOW), developed by our group, is an example of this approach. The dynamics of biofilm development, optimal working conditions and the response of ABTOW to different xenobiotics have been determined empirically [1,2,12,13]. The optimal state may be disturbed by the introduction of examined water to ABTOW, resulting in a false positive signal.…”