Effect of K+ on the membrane functions of an alkalophilic Bacillus

Effects of temperature, pH, and salinity on the growth kinetics of Pseudomonas sp. NB-1, a newly isolated cold-tolerant, alkali-resistant, and high-efficiency nitrobenzene-degrading bacterium

ABSTRACTStrain NB-1, which can efficiently degrade nitrobenzene, was identified as Pseudomonas frederiksbergensis. NB-1 was resistant to cold and alkali with the widest temperature (4-35 °C) and pH (5-11) adaptive range, compared with other reported nitrobenzene-degrading microorganisms. Based on the Haldane-Andrews model, the real maximum specific growth rate μ_m', specific affinity a_A, and inhibition coefficient K_i were used in response surface methodology (RSM) simultaneously for the first time to guide NB-1 to treat nitrobenzene wastewater. According to the RSM model, the environmental factors (temperature, pH, salinity) corresponding to the optimal values of μ_m', a_A, and K_i were determined. By comparing the specific growth rates corresponding to the optimal values of μ_m', a_A, and K_i, respectively, the optimum growth conditions of NB-1 were determined under different nitrobenzene concentrations. The study of μ_m', a_A, and K_i by RSM provided a new approach for a more accurate optimization of biological wastewater treatment conditions.

Uptake and degradation of EDTA by Escherichia coli

It was found that Escherichia coli exhibited a growth by utilization of Fe(III)EDTA as a sole nitrogen source. No significant growth was detected when Fe(III)EDTA was replaced by EDTA complexes with other metal ions such as Ca(2+), Co(2+), Cu(2+), Mg(2+), Mn(2+), and Zn(2+). When EDTA uptake was measured in the presence of various ions, it was remarkable only when Fe(3+) was present. The cell extract of E. coli exhibited a significant degradation of EDTA only in the presence of Fe(3+). It is likely that the capability of E. coli for the growth by utilization of Fe(III)EDTA results from the Fe(3+)-dependent uptake and degradation of EDTA.

Bioenergetics of alkaliphilic Bacillus spp

Alkaliphilic microorganisms are widely distributed in nature. Among them, several aerobic alkaliphilic Bacillus spp. have been studied in terms of their mechanisms of physiological adaptation under an extremely alkaline condition. On the basis of chemiosmotic theories, neutrophiles produce H+ electrochemical potential (deltap), which is the sum of transmembrane pH gradient (deltapH) (alkaline, inside) and membrane potential (deltapsi) (negative, inside), for active transport of solutes, motility, and ATP synthesis. In the case of alkaliphiles, it seems that Mitchell's chemiosmotic theories alone cannot explain clearly their positive H+ electrochemical potential (deltap) across the membrane because these bacteria exhibit deltaph in a direction opposite to that in neutrophiles, which seems to be causing extensively negative to produce energy, theoretically. Nevertheless, it is reported that ATP synthesis is more rapid at high alkaline pH than at near neutral pH in the facultative alkaliphile Bacillus pseudofirmus OF4. The respiratory system of alkaliphilic microorganisms might have an important role in compensating the reversed transmembrane pH gradient by means of ATP synthesis. To understand the function of the respiratory system in alkaliphiles, several respiratory components in alkaliphilic Bacillus spp. were isolated and characterized. In these studies, respiratory components of alkaliphiles exhibiting several unique characteristics are identified. These characteristics may have an important role in obtaining energy in alkaline environments. Information obtained from bioenergetics studies of alkaliphiles will reveal new important findings on general energy coupling phenomena.

Energetic problems of extremely alkaliphilic aerobes

Over a decade of work on extremely alkaliphilic Bacillus species has clarified the extraordinary capacity that these bacteria have for regulating their cytoplasmic pH during growth at pH values well over 10. However, a variety of interesting energetic problems related to their Na(+)-dependent pH homeostatic mechanism are yet to be solved. They include: (1) the clarification of how cell surface layers play a role in a category of alkaliphiles for which this is the case; (2) identification of the putative, electrogenic Na+/H+ antiporter(s) that, in at least some alkaliphiles, may completely account for a cytoplasmic pH that is over 2 pH units lower than the external pH; (3) the determination of whether specific modules or accessory proteins are essential for the efficacy of such antiporters; (4) the mechanistic basis for the increase in the transmembrane electrical potential at the high external pH values at which the potential-consuming antiporter(s) must be most active; and (5) an explanation for the Na(+)-specificity of pH homeostasis in the extremely alkaliphilic bacilli as opposed to the almost equivalent efficacy of K+ for pH homeostasis in at least some non-alkaliphilic aerobes. The current status of such studies and future strategies will be outlined for this central area of alkaliphile energetics. Also considered, will be strategies to elucidate the basis for robust H(+)-coupled oxidative phosphorylation by alkaliphiles at pH values over 10. The maintenance of a cytoplasmic pH over 2 units below the high external pH results in a low bulk electrochemical proton gradient (delta p). To bypass this low delta p, Na(+)-coupling is used for solute uptake even by alkaliphiles that are mesophiles from environments that are not especially Na(+)-rich. This indicates that these bacteria indeed experience a low delta p, to which such coupling is an adaptation. Possible reasons and mechanisms for using a H(+)-coupled rather than a Na(+)-coupled ATP synthase under such circumstances will be discussed.

Succinate:quinone oxidoreductase (complex II) containing a single heme b in facultative alkaliphilic Bacillus sp. strain YN-2000

Succinate:quinone oxidoreductase (EC 1.3.5.1) was first purified from the facultative alkaliphilic Bacillus sp. strain YN-2000 in the presence of Triton X-100. The isolated enzyme showed high succinate-ubiquinone oxidoreductase activity at pH 8.5. The Km for ubiquinone 1 and the Vmax of the enzyme were determined to be about 5 microM and 48 micromol of ubiquinone 1 per min per mg, respectively. The catalytic activity of the enzyme was 50% inhibited by 9 microM 2-thenoyltrifluoroacetone or 0.8 microM 2-n-heptyl-4-hydroxyquinoline- N-oxide. The enzyme consisted of three kinds of subunits with molecular masses of 66, 26, and 15 kDa, respectively, and contained 1.28 mol of covalently bound flavin adenine dinucleotide, 0.9 mol of heme b, 1.35 mol of menaquinone, 8.3 mol of nonheme iron, and 7.5 mol of inorganic sulfide per mol of enzyme. The enzyme showed symmetrical alpha absorption peaks at 556.5 and 554 nm in the reduced state at room temperature and 77 K, respectively. The potentiometric analysis of the enzyme yielded an Em,7 of heme b of about -64 mV (n = 1). Furthermore, the content of the enzyme was increased up to fivefold when the bacterium was grown at pH 10 compared with pH 7. These results indicate that the succinate:quinone oxidoreductase with a single heme b is involved in the respiratory chain of the alkaliphile at a very alkaline pH.

Ammonium/urea-dependent generation of a proton electrochemical potential and synthesis of ATP in Bacillus pasteurii

The influence of ammonium and urea on the components of the proton electrochemical potential (delta p) and de novo synthesis of ATP was studied with Bacillus pasteurii ATCC 11859. In washed cells grown at high urea concentrations, a delta p of -56 +/- 29 mV, consisting of a membrane potential (delta psi) of -228 +/- 19 mV and of a transmembrane pH gradient (delta pH) equivalent to 172 +/- 38 mV, was measured. These cells contained only low amounts of potassium, and the addition of ammonium caused an immediate net decrease of both delta psi and delta pH, resulting in a net increase of delta p of about 49 mV and de novo synthesis of ATP. Addition of urea and its subsequent hydrolysis to ammonium by the cytosolic urease also caused an increase of delta p and ATP synthesis; a net initial increase of delta psi, accompanied by a slower decrease of delta pH in this case, was observed. Cells grown at low concentrations of urea contained high amounts of potassium and maintained a delta p of -113 +/- 26 mV, with a delta psi of -228 +/- 22 mV and a delta pH equivalent to 115 +/- 20 mV. Addition of ammonium to such cells resulted in the net decrease of delta psi and delta pH without a net increase in delta p or synthesis of ATP, whereas urea caused an increase of delta p and de novo synthesis of ATP, mainly because of a net increase of delta psi. The data reported in this work suggest that the ATP-generating system is coupled to urea hydrolysis via both an alkalinization of the cytoplasm by the ammonium generated in the urease reaction and a net increase of delta psi that is probably due to an efflux of ammonium ions. Furthermore, the findings of this study show that potassium ions are involved in the regulation of the intracellular pH and that ammonium ions may functionally replace potassium to a certain extent in reducing the membrane potential and alkalinizing the cytoplasm.

Stimulatory effect of NH4+ on the transport of leucine and glucose in an anaerobic alkaliphile

An anaerobic alkaliphile, EP01, specifically requires NH4+ for the acceleration of amino acid and glucose transport [Koyama, N. (1988) FEBS Lett. 253, 187-189]. In this paper, we attempted to clarify how NH4+ is involved in the transport system. The bacterium acidifies the cytoplasm, which was suggested to result in NH4+ accumulation when NH4Cl was added to the medium. Increase of the NH4Cl concentration administered to the medium caused the acceleration of leucine and glucose transport, which was accompanied by an increase in the internal pH and the absolute internal concentration of NH4+, whereas a decrease in the concentration ratio of internal NH4+/external NH4+ was observed. The addition of 3 mM NH4Cl, which resulted in significant stimulation of leucine and glucose transport, raised the internal NH4+ concentration by 42 mM, but the internal pH only by 0.1 units. It seems more likely that leucine and glucose transport are accelerated depending on the increase in the internal NH4+ concentration rather than the increase in the internal pH. By the imposition of an inwardly directed Na+ gradient, the K(+)-loaded membrane vesicles accumulated leucine and glucose, indicating that a sodium chemical potential is available for active transport. The membrane of the bacterium exhibited a Na(+)-stimulated ATPase activity which was remarkably enhanced by the addition of NH4Cl, depending on its concentration, and was inhibited by vanadate. Leucine and glucose transport were inhibited by vanadate. Based on these results, we propose a mechanism in which NH4+ contributes internally to leucine and glucose transport, depending on its concentration, by the activation of a Na(+)-translocating ATPase responsible for the generation of a sodium chemical potential.

pH homeostasis and bioenergetic work in alkalophiles

A Na+/H+ antiporter catalyses coupled Na+ extrusion and H+ uptake across the membranes of extremely alkalophilic bacilli. This exchange is electrogenic, with H+ translocated inward greater than Na+ extruded. It is energized by the delta chi 2 component of the delta mu H+ that is established during primary proton pumping by the alkalophile respiratory chain complexes. These complexes abound in the membranes of extreme alkalophiles. Combined activity of the respiratory chain, the antiporter, and solute transport systems that are coupled to Na+ re-entry, allow the alkalophiles to maintain a cytoplasmic pH that is several pH units more acidic than optimal external pH values for growth. There is no compelling evidence for a specific and necessary role for any ion other than sodium in pH homeostasis, and although there is very high cytoplasmic buffering capacity in the alkaline range, active mechanisms for pH homeostasis are crucial. Energization of the antiporter as well as the proton translocating F1F0-ATPase that catalyses ATP synthesis in the extreme alkalophiles must accommodate the problem of the low net delta mu H+ and the very low concentrations of protons, per se, in the external medium. This problem is by-passed by other bioenergetic work functions, such as solute uptake or motility, that utilize sodium ions for energy-coupling in the place of protons.

The Na+ cycle of extreme alkalophiles: a secondary Na+/H+ antiporter and Na+/solute symporters

Extremely alkalophilic bacteria that grow optimally at pH 10.5 and above are generally aerobic bacilli that grow at mesophilic temperatures and moderate salt levels. The adaptations to alkalophily in these organisms may be distinguished from responses to combined challenges of high pH together with other stresses such as salinity or anaerobiosis. These alkalophiles all possess a simple and physiologically crucial Na+ cycle that accomplishes the key task of pH homeostasis. An electrogenic, secondary Na+/H+ antiporter is energized by the electrochemical proton gradient formed by the proton-pumping respiratory chain. The antiporter facilitates maintenance of a pHin that is two or more pH units lower than pHout at optimal pH values for growth. It also largely converts the initial electrochemical proton gradient formed by respiration into an electrochemical sodium gradient that energizes motility as well as a plethora of Na+ solute symporters. These symporters catalyze solute accumulation and, importantly, reentry of Na+. The extreme nonmarine alkalophiles exhibit no primary sodium pumping dependent upon either respiration or ATP. ATP synthesis is not part of their Na+ cycle. Rather, the specific details of oxidative phosphorylation in these organisms are an interesting analogue of the same process in mitochondria, and may utilize some common features to optimize energy transduction.

Leucine transport system in a facultatively alkalophilic Bacillus

Some characterizations of the leucine transport system in a facultative alkalophile, which is able to grow over a wide pH range from 7.0 to 10.5, were attempted. Although the direction of a transmembrane pH gradient of the bacterium below pH 8.2 is opposite to that above pH 8.2 (N. Koyama and Y. Nosoh (1985) Biochim. Biophys. Acta 812, 206-212), leucine transport is likely to be driven only by sodium electrochemical potential irrespective of the external pH. It was suggested that histidine and sulfhydryl groups in the leucine transporter are involved in the translocation mechanism and the pK value of the histidine residue involved is approximately 7.0.