Low Phosphate Concentrations Research Articles

Preliminary results of fisheries research in the Laptev Sea at RV «Professor Levanidov» in September 2019

In September 2019, the R/V “Professor Levanidov” carried out comprehensive studies in the Laptev Sea from 140 ° to 110 ° E at depth range of 60–950 m. High positive anomalies of air and surface water temperatures were noted up to + 2 + 3 °C and +1.5 °C respectively with an abnormally low sea ice cover. Cold Arctic and desalinated waters of the river flow dominated in the upper 200 m layer. Atlantic waters were traced in the layer from 180–300 m to the bottom, and their core — at a maximum temperature of + 1.6 + 1.92 °C at 240–280 m. By the time of the survey, the active vegetation of phytoplankton had finished. Oxygen saturation in the almost the entire surface layer was below 100 %. Primary production was limited by mineral nitrogen concentration. Low concentrations of phosphates, nitrates and silicates in the intermediate waters indicated the penetration of Atlantic waters. The average biomass of plankton on the shelf was 239,8 mg/m3, on the slope — 374,2 mg/m3. Copepods and chaetognaths with considerable contribution of hyperiids and euphausiids dominated in plankton by weight. The bulk of zooplankton was large organisms, while small and medium fractions were 22 and 10 %. Brittle star, starfish, and shrimp dominated in trawl benthos. 25 demersal fish species of 14 families were found in bottom trawl catches. Zoarcidae, Liparidae and Gadidae were the most diverse. Polar cod was recorded along the entire route with highest concentrations in Atlantic waters at depths of 217–460 m. Copepods dominated in the diet of Polar cod juveniles, while that of adults was supplemented by large pelagic crustaceans and fish with high level of cannibalism. Greenland halibut was found along the entire continental slope at 270–750 m while the largest catches were in its central and western parts. The increase of halibut number and occurrence was due to the wide distribution of Atlantic waters on the slope. The main component of its diet was Polar cod.

Photosynthetic performance and stevioside concentration are improved by the arbuscular mycorrhizal symbiosis in Stevia rebaudiana under different phosphate concentrations.

In plants, phosphorus (P) uptake occurs via arbuscular mycorrhizal (AM) symbiosis and through plant roots. The phosphate concentration is known to affect colonization by AM fungi, and the effect depends on the plant species. Stevia rebaudiana plants are valuable sources of sweetener compounds called steviol glycosides (SGs), and the principal components of SGs are stevioside and rebaudioside A. However, a detailed analysis describing the effect of the phosphate concentration on the colonization of AM fungi in the roots and the relationship of these factors to the accumulation of SGs and photochemical performance has not been performed; such an analysis was the aim of this study. The results indicated that low phosphate concentrations (20 and 200 µM KH2PO4) induced a high percentage of colonization by Rhizophagus irregularis in the roots of S. rebaudiana, while high phosphate concentrations (500 and 1,000 µM KH2PO4) reduced colonization. The morphology of the colonization structure is a typical Arum-type mycorrhiza, and a mycorrhiza-specific phosphate transporter was identified. Colonization with low phosphate concentrations improved plant growth, chlorophyll and carotenoid concentration, and photochemical performance. The transcription of the genes that encode kaurene oxidase and glucosyltransferase (UGT74G1) was upregulated in colonized plants at 200 µM KH2PO4, which was consistent with the observed patterns of stevioside accumulation. In contrast, at 200 µM KH2PO4, the transcription of UGT76G1 and the accumulation of rebaudioside A were higher in noncolonized plants than in colonized plants. These results indicate that a low phosphate concentration improves mycorrhizal colonization and modulates the stevioside and rebaudioside A concentration by regulating the transcription of the genes that encode kaurene oxidase and glucosyltransferases, which are involved in stevioside and rebaudioside A synthesis in S. rebaudiana.

Separation of monoclonal antibody monomer-dimer mixtures by gradient elution with ceramic hydroxyapatite.

Modeling the chromatographic separations of proteins at manufacturing scale is important since downstream processing costs are often dominant. At such scales, the columns are highly overloaded heightening the challenge of predicting performance. In this work, the separation of a monoclonal antibody monomer-dimer mixture is conducted by gradient elution chromatography with ceramic hydroxyapatite (CHT) columns Type I and Type II under overloaded conditions. Phosphate gradients are shown to be preferable over sodium chloride gradients since the latter result in undesirable pH transitions generated within the column itself. Using sodium phosphate gradients separation is obtained with both CHT types, achieving approximately 90% recovery at 99% monomer purity starting with a mixture containing 30% dimer at total protein loads up to 30 mg/mL. Because of its higher binding capacity, even higher loadings can be obtained with CHT Type I without monomer breakthrough. A hybrid model is developed to describe the separation. The model, based on an empirical description of two-component, competitive isotherms at low sodium phosphate concentration coupled with the stoichiometric displacement model at higher sodium phosphate concentrations, is in good agreement with the experiments using the linear driving force (LDF) approximation to describe adsorption/desorption kinetics. The same LDF rate coefficient predicts the separation at loadings between 0.8 and 30 mg/mL. The model developed in this work can be used as a general tool to optimize operating conditions, understand what factors limit performance, and compare different operating modes.

The PHO pathway regulates white-opaque switching and sexual mating in the human fungal pathogen Candida albicans.

The opportunistic fungal pathogen Candida albicans is able to switch among several morphological phenotypes in response to environmental changes. White-opaque transition is a typical phenotypic switching system involved in the regulation of pathogenesis and sexual reproduction in C. albicans. Under regular laboratory culture conditions, to undergo white-to-opaque switching, cells must first undergo homozygosis at the mating-type locus (MTLa/a or α/α) since the a1/α2 heterodimer represses the expression of the Wor1 master regulator of switching in MTLa/α heterozygous strains. In this study, we report the roles of the PHO pathway of phosphate metabolism in the regulation of white-opaque switching and sexual mating in C. albicans. We find that deletion of the PHO pathway genes PHO81, PHO80, PHO2, and PHO4 induces the opaque phenotype in MTLa/α heterozygous cells. Low concentrations of external phosphate are conducive for the opaque phenotype in both MTL homozygous and heterozygous strains. Moreover, phosphate starvation can also increase the mating efficiency in C. albicans. Consistently, the pho80/pho80 mutant mimics an artificial phosphate starvation state and mates efficiently at both lower and higher phosphate concentrations. Our study establishes a link between the PHO pathway and white-opaque epigenetic switching in C. albicans.

Phosphate starvation responses in crop roots: from well-known players to novel candidates

Phosphorus (P) is a major essential macronutrient for plants, and its availability in the soil is scarce and not renewable. Molecular responses to low concentrations of inorganic phosphate (Pi) are better understood in Arabidopsis, whereas several information gaps still exist in crops. In addition, rice has invaluable socio-economic importance and is a crop model system for studies on Pi starvation responses due to its high tolerance to Pi deprivation. Thus, unraveling the mechanisms related to Pi starvation in rice is extremely important for plant breeding and agronomic productivity and sustainability. In this review, we discuss the recent progress and remaining gaps of knowledge on the molecular mechanisms of sensing and signaling Pi starvation in rice. By exploring publicly available transcriptomic data, we propose new functions on Pi metabolism for previously known genes and pinpoint attention-worthy genes with unknown function, although evidently regulated under Pi starvation. Orthologs of the promising Pi-responsive genes with unclear function identified in rice roots were also discussed in the context of other crops. New strategies to increase global food productivity by improving Pi uptake in rice and possibly in other crops are proposed.

Role of root exudates on assimilation of phosphorus in young and old Arabidopsis thaliana plants.

The role of root exudates has long been recognized for its potential to improve nutrient use efficiency in cropping systems. However, studies addressing the variability of root exudates involved in phosphorus solubilization across plant developmental stages remain scarce. Here, we grew Arabidopsis thaliana seedlings in sterile liquid culture with a low, medium, or high concentration of phosphate and measured the composition of the root exudate at seedling, vegetative, and bolting stages. The exudates changed in response to the incremental addition of phosphorus, starting from the vegetative stage. Specific metabolites decreased in relation to phosphate concentration supplementation at specific stages of development. Some of those metabolites were tested for their phosphate solubilizing activity, and 3-hydroxypropionic acid, malic acid, and nicotinic acid were able to solubilize calcium phosphate from both solid and liquid media. In summary, our data suggest that plants can release distinct compounds to deal with phosphorus deficiency needs influenced by the phosphorus nutritional status at varying developmental stages.

Effect of water quality variation on fish assemblages in an anthropogenically impacted tropical estuary, Colombian Pacific

In tropical estuaries, fish diversity varies spatially and temporally due to behavioral processes such as reproductive migrations, predator avoidance, and foraging, which are affected by water quality. Eutrophication is one of the main factors affecting water quality in estuaries. The objective of this study was to determine variation in fish assemblage explained by fluctuating water quality in the Buenaventura Bay. Fish were captured using artisanal trawl nets during the wet, dry, and transitional seasons at four sampling sites. Additionally, alkalinity; phosphate, nitrite, and nitrate concentrations; dissolved oxygen; pH; temperature; and suspended solids were measured. Multivariate analysis was used to assess the effect of water quality on fish assemblage. In Buenaventura Bay, the assemblage composition of Pseudupeneus grandisquamis, Daector dowi, and Citharichthys gilberti was affected by nitrate concentration. Moreover, large fish biomasses were associated with high nitrite concentration, intermediate salinity, and low dissolved oxygen, suggesting that these estuaries are dominated by species tolerant to poor water quality. Species richness was associated with low nitrate and phosphate concentrations, more suitable water quality indicators, and intermediate temperatures. These results suggest that the deteriorating water quality of estuaries as a result of the anthropogenic impact could increase dominance and decrease richness, resulting in structural changes of fish assemblages.

Electrohydromodulation for phosphate recovery from wastewater

The mining of phosphate rock to produce fertilizer, and subsequent disposal of the municipal wastewater containing a significant fraction of the phosphate applied agriculturally, has led to environmental impacts of phosphate mining as well as surface water eutrophication. However, recovery of phosphate (H2PO4-, HPO42-, PO43-) from wastewater, either chemically or biologically, has not been sufficiently economical at low phosphate concentrations in raw wastewater (~2.4 mg-P l−1) to motivate widespread adoption. In this work, we demonstrate phosphate recovery directly from raw and mineral-spiked septic tank effluent by electrochemical pH shifting, termed electrohydromodulation (EHM), without expensive chemical addition. EHM using a CMI-7000 multivalent cation exchange membrane (MCEM) at 5 V at 1.347 mA/cm2 resulted in the highest efficiency at lowest energy consumption. 94 and 95% phosphate were recovered from mineral-spiked septic tank effluent (simulating advanced oxidation-based direct potable reuse water), and raw septic tank effluent, at an energy demand of 1.046 kWh/m3 (3.960 kWh/1000 Gal) and 1.863 kWh/m3 (7.054 kWh/1000 Gal), respectively, low despite use of non-toxic, inexpensive graphite electrodes. Results from scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray powder diffraction (XRD) indicated that recovered precipitates consisted principally of amorphous calcium phosphate (ACP), with minor amounts of amorphous calcium carbonate (ACC), having an overall Ca:P ratio of 2. CO2 stripping at low pH prior to pH shifting was found to minimize competitive calcite precipitation to improve product purity. Electrodes used intermittently, with polarity reversal, over 1.5 years of experiments were intact from fouling.

Bulking and floatation of the anammox-HAP granule caused by low phosphate concentration in the anammox reactor of expanded granular sludge bed (EGSB)

The effect of phosphate concentration on the anammox-HAP process was investigated in this work. A high total nitrogen removal efficiency (>82.6%) and a stable total phosphate removal efficiency (>56.2%) was achieved in reactor with the phosphate concentration over 11.4 mg L−1. However, a phosphate concentration below to 5.7 mg L−1, a floatation of sludge occurred caused the deterioration of process. A new understanding for the floatation was divided into three stages: the stable stage, bulking stage and floating stage. First, anammox biofilm coupled with HAP for granulation in the stable stage. Second, the aggregation of bulking sludge resulted in changes in viscoelastic properties of the sludge. Third, the floatation resulted from unreleasable gas bubbles in the granules wrapped a high concentrations of slime layer proteins. Overall, this paper suggests that a control strategy was a sufficient supply of phosphate for the stable operation.

La(III)-bentonite/chitosan composite: A new type adsorbent for rapid removal of phosphate from water bodies

It is highly imperative to remove phosphorus from water bodies because the excessive phosphorus content can cause eutrophication of water bodies. In this work, we prepare a kind of environmentally friendly and low-cost adsorbent comprising lanthanum (La)-doped bentonite and chitosan, with a simple, facile and scalable method combining direct calcination and subsequent blending. The optimal adsorbent, La-doped bentonite/chitosan-5, La-BC-5 hereinafter, can rapidly reach equilibrium in 20 min and provide 93.2% adsorption efficiency (initial phosphate concentration, 50 mg/L); and as high as 99.7% could be removed in 5 min under low phosphate concentration (2 mg/L). Fitting result indicates the phosphate adsorption isotherm fits the Freundlich model, meanwhile the adsorptive kinetics follows a pseudo-second-order model. Anti-interference result for some typical anionic species including NO3−, SO42−, Cl−, indicates 97.5% removal efficiency could be achieved under coexisting phosphate concentration of 5 mg/L. Meanwhile, reusability experiment reveals that removal efficiency decreases to 89.1% after five regeneration cycles. Notably, practical water adsorption experiment manifests the La-BC-5 can efficiently reduce the phosphate concentration from 0.30 to 0.07 mg/L. The favorable properties with rapid adsorption, excellent anti-interference for coexisting anions and reusability foresee the enormous potentials for practical phosphate removal in water bodies.

Study on removal of phosphorus as struvite from synthetic wastewater using a pilot-scale electrodialysis system with magnesium anode

Struvite precipitation may become ineffective in removing phosphorus due to the low concentration of phosphate in the liquid. In this study, electrolysis with a magnesium anode was applied to recovering phosphorus and ammonia as struvite from wastewater. A novel electrodialysis process (ED) with a magnesium anode was developed, and its feasibility to treat synthetic wastewater with low phosphate concentration was demonstrated in a pilot-scale experimental system. To achieve high phosphate removal efficiency in the product stream, the optimal initial pH and flow rate were found to be 8.8 and 200 L h−1, respectively, for the ED system at a constant current of 0.1 A. The pilot-scale ED system under the consecutive batch mode removed 65% phosphate from the synthetic wastewater containning 10 mg L-1P, and the phosphate concentration in the product stream was kept at 30 mg L−1 after 280 min. The running cost of the ED system was estimated to be $31.27 kg−1 P for synthetic wastewater with 10 mg L−1 P, mainly resulting from the cost of the loss of the magnesium anode. The precipitates generated from the product stream were confirmed as struvite by XRD analysis.

Surface Functional Group Engineering of CeO2 Particles for Enhanced Phosphate Adsorption.

Surface functional groups play a dominating role in determining the adsorption performance of metal oxide particles. The ability to manipulate the surface functional groups is vital in designing an effective adsorbent for water decontamination. In this study, a facile method is proposed for tuning the amount of the surface hydroxyl groups of CeO2 particles. The volume of water added during the ethylene glycol-mediated solvothermal synthesis of CeO2 particles can be used to adjust the amount of surface hydroxyl groups. By simple reduction in the volume of water, the number of surface hydroxyl groups of CeO2 particles can be increased and the phosphate adsorption capacity can be greatly improved. Our results show that the obtained CeO2 particles have high phosphate adsorption capacity at low phosphate concentrations, fast adsorption kinetics, and the ability to achieve an ultralow phosphate concentration in the real sewage effluent. This study provides an effective strategy for designing highly effective metal oxide adsorbents through surface functional group engineering.

Simultaneous removal of nitrate, phosphorous and cadmium using a novel multifunctional biomaterial immobilized aerobic strain Proteobacteria Cupriavidus H29

A novel biomaterial FeCl3/CaCl2/KH2PO4 modified municipal sludge biochar (FCPC) was synthesized. And the impacts of critical factors such as HRT, temperature and C/N ratio on simultaneous denitrification, dephosphorization and Cd(II) removal were investigated. Results show that the highest nitrate removal efficiency reached 92.22% (8.49 mg·L−1·h−1) in test group A and approximately 100% (9.19 mg·L−1·h−1) in test group B. Very low phosphate concentrations (approximately 2.50 mg/L) were detected in the effluent. The average removal efficiency of Cd(II) reached 86.40% (4.42 mg·L−1·h−1) in experimental group A and 90.15% (4.61 mg·L−1·h−1) in experimental group B. Gas emissions and biological precipitation in the bioreactors were monitored, further to confirming contaminant removal mechanisms. Additionally, Cupriavidus H29 was found to contribute dominantly to the FCPC bioreactor activity.

Development of Fe0/Fe3O4 composites with tunable properties facilitated by Fe2+ for phosphate removal from river water

Phosphorus (P) has been overused for decades in agriculture and industry, such that excess phosphorus inevitably enters natural waters, especially river water, via runoff. Trace phosphate in natural water will cause eutrophication, and it is important to achieve low concentration phosphate removal effectively because of the increasingly stringent standards. In this study, a new technology, using Fe0/Fe3O4 composites facilitated by Fe2+ for low concentration phosphate removal was developed. The amount of Fe3O4 on the Fe0 surface was tuned and Fe0/Fe3O4-2 was proved to have best performance in phosphate removal and iron corrosion. The initial Fe2+ concentration of 0.5 mg/L was chosen for effective phosphate removal and meeting an acceptable total iron concentration below 0.3 mg/L after treatment. The generated iron corrosion products showed a high phosphate removal capacity and favorable selectivity towards phosphate in the presence of co-existing anions and NOM. The phosphate in real river water was decreased from 0.4 mg-P/L to 0.010 mg-P/L by using 0.5 g/L Fe0/Fe3O4-2 facilitated by 0.5 mg/L Fe2+. Adsorption, co-precipitation and precipitation contributed to phosphate removal, and the main removal mechanism was adsorption, which included electrostatic attraction and inner-sphere complexation. This study highlights a promising new method for the removal of low concentrations of phosphate from river water.

New Developments in the Treatment of X-Linked Hypophosphataemia: Implications for Clinical Management.

X-linked hypophosphataemia (XLH) is due to mutations in phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and represents the most common heritable form of rickets. In this condition, the hormone fibroblast growth factor 23 (FGF23) is produced in excessive amounts for still unknown reasons, and causes renal phosphate wasting and suppression of 1,25-dihydroxyvitamin D, leading to low serum phosphate concentrations. Prolonged hypophosphataemia decreases apoptosis of hypertrophic chondrocytes in growth plates (causing rickets) and decreases mineralisation of existing bone (causing osteomalacia). In contrast to historical conventional treatment with oral phosphate supplements and active vitamin D for the last 50 years, the new anti-FGF23 antibody treatment (burosumab) targets the primary pathology by blocking FGF23, thereby restoring phosphate homeostasis. In this review, we describe the changes in treatment monitoring, treatment targets and long-term treatment goals, including future opportunities and challenges in the treatment of XLH in children.

Removal of phosphate from wastewater by modified bentonite entrapped in Ca-alginate beads

Methods of removing phosphate from wastewater with a low phosphate concentration are of great environmental significance. In this study, immobilized beads were prepared by entrapping modified bentonite powder in calcium-alginate (Al-NaBT-CA), and the potential of the beads for phosphate removal from wastewater was investigated. The effects of pH (1–10) and initial phosphate concentration (0.5–50 mg/L) were also examined in batch experiments with Al-NaBT-CA beads. The optimum pH value for phosphate removal by Al-NaBT-CA beads was pH 3. In addition, a high initial phosphate concentration promoted phosphate adsorption. Adsorption kinetics showed that the adsorption of phosphate using beads followed a pseudo-second-order kinetic model (R2 = 0.98–0.99). The adsorption isotherm data was well fitted by the Sips adsorption model. The maximum phosphate adsorption capacity of the Al-NaBT-CA beads was 15.77 mg/g, which was slightly less than that of the modified powder. The specific surface area of the Al-NaBT-CA beads was 17.01 m2/g, and their average pore size was 13.41 nm. Scanning electron microscopy suggested that the high inner porosity and rough outer surface of the beads facilitated phosphate transfer.

Влияние концентрации фосфатов на спектральные свойства ионов тулия во фторфосфатных стеклах

The Jadd-Ofelt parameters for thulium ions in fluorophosphate glasses with different phosphate content are calculated. The proximity of the values of the parameters Ωt of glasses with a content of up to 10 mol% Ba(PO3)2 to the similar parameters of the fluoride glasses and a sharp increase in their values with a further increase in phosphates are shown. In the framework of the Jadd-Ofelt theory, the spectral-luminescent parameters of glasses are analyzed. The spontaneous transitions probabilities, branching coefficients (β), and radiation lifetimes (τrad) for various excited states of Tm3+ ions are obtained. It was found that, at low phosphate concentrations, glasses have a high radiation lifetime for the excited 3F4 laser level. It has been shown that the glasses with a content of up to 10 mol% Ba(PO3)2 are characterized by a wide flat gain spectrum, which makes them promising materials for creating broadband amplifiers.

Study on Preparation, Microstructure and Properties of Micro-Arc Oxidation Ceramic Coating on AZ91 Magnesium Alloy in Phosphate Electrolyte

The micro-arc oxidation behavior of AZ91 magnesium alloy in alkaline phosphate electrolyte was studied. The effect of phosphate on the microstructure and properties of micro-arc oxide film was investigated and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive spectrum (EDS) and salt spray test. The results show that phosphate concentration has an important effect on the surface morphology, phase structure, thickness and corrosion resistance of micro-arc oxidation film. With the increase of phosphate concentration from 5g/L to 30g/L, the film quality decreases, the surface roughness increases, the thickness decreases and the corrosion resistance decreases. So in phosphate electrolysis the lower phosphate concentration in the liquid system is helpful to the formation of better film quality.

Selective phosphate removal using layered double hydroxide/reduced graphene oxide (LDH/rGO) composite electrode in capacitive deionization

Phosphate removal is a critical issue in water treatment because excess levels of phosphate can cause severe eutrophication. Capacitive deionization (CDI), which has several advantages, such as simple, eco-friendly, and energy efficient operations, has gained attention as a potential alternative over conventional phosphate removal technologies like activated sludge, chemical precipitation, and adsorption processes. However, CDI suffers from a lack of selectivity for phosphate, resulting from non-selective anion removal of positively biased electrodes. Herein, the layered double hydroxide/reduced graphene oxide (LDH/rGO) composite electrode in the CDI process was examined for selective phosphate removal. LDH/rGO showed the selective phosphate removal performance with sustained phosphate removal efficiency even in the presence of excess chloride. In addition, the selective phosphate removal in the CDI process with the LDH/rGO was successfully demonstrated in the simulated water, fabricated by adding a significantly low concentration of phosphate (0.4 mg∙L−1) into real river water matrix (Han River, Seoul, Korea). This result was explained by the high electrochemical selectivity of the LDH/rGO for phosphate.

Enzymatic characterization and regulation of gene expression of PhoK alkaline phosphatase in Sphingobium sp. strain TCM1.

Sphingobium sp. strain TCM1 can significantly degrade chlorinated organophosphorus flame retardants, such as tris(2-chloroethyl) phosphate. The PhoK of strain TCM1 (Sb-PhoK) is the main alkaline phosphatase (APase) that catalyzes the last step in the degradation pathway. Here, we purified and characterized Sb-PhoK produced in E. coli, and analyzed the regulation of Sb-phoK gene expression in strain TCM1. The recombinant Sb-PhoK was produced in the mature form, lacking a putative signal peptide, and formed a homodimer. Purified Sb-PhoK exhibited 384U/mg of specific activity at 37°C. The optimum temperature was 50°C, and Sb-PhoK was completely inactivated when incubated at 60°C for 10min. The optimum pH was 10, with stability observed at pH6.0-10.5. Sb-PhoK was suggested to contain two Ca2+ and one Zn2+ per subunit, but excess addition of Zn2+ into the reaction mixture markedly inhibited the enzyme activity. Sb-PhoK showed phosphatase activity against various phosphorylated compounds, except for bis(p-nitrophenyl) phosphate, indicating that it is a phosphomonoesterase with broad substrate specificity. The Km and kcat for p-nitrophenyl phosphate were 2.31mM and 1270s-1, respectively, under optimal conditions. The enzyme was strongly inhibited by vanadate, dithiothreitol, and SDS, but was highly resistant to urea and Triton X-100. Sb-phoK gene expression was regulated by the inorganic phosphate concentration in culture medium, and was induced at a low inorganic phosphate concentration. The deletion of Sb-phoB gene resulted in no induction of Sb-phoK gene even at a low inorganic phosphate concentration, confirming that Sb-PhoK is a member of Pho regulon.