锰氧化细菌的分离鉴定及其锰氧化特性的分析

利用选择性培养基对锰矿样品进行分离、筛选, 得到一株高效锰氧化细菌(MN1405)。经形态特征、生理生化特征以及16S rRNA 基因序列分析, 将菌株MN1405 鉴定为Arthrobacter echigonensis。在培养条件下, MN1405 对培养基中的锰离子去除率可达93.38%, 且其培养所获得的培养液也具有良好的除锰效果。     

Distribution and mobility of manganese in the hyperaccumulator plant Phytolacca acinosa Roxb. (Phytolaccaceae)

The distribution and mobility of manganese (Mn) in the hyperaccumulator plant species Phytolacca acinosa Roxb. (Phytolaccaceae) were investigated in a hydroponic system. The plants were exposed to 2 or 5 mM Mn for up to 28 days. For any given plant, the Mn content in the mature leaves (nos. 5–9) was always higher than that in the old (nos. 1–4) and young leaves (nos. 10–14). Within the different parts of a leaf, Mn was preferentially accumulated in the leaf marginal area, where the observed level was threefold higher than that in the midrib. Cross-sectional analysis of the leaf revealed that the concentration of Mn was higher in the leaf epidermis than in the mesophyll. Cell fractionation analysis with P. acinosa leaves showed that most of the Mn (78.4%) was present in the final supernatant fraction (following centrifugation at 20,000 g for 45 min). The distribution of Mn in the leaves of P. acinosa was controlled mainly by the transpiration rate. Our investigation demonstrated that Mn was readily transported from the roots to shoots of P. acinosa but that it could not be remobilized readily after it reached leaves.     

Chemical forms of manganese in the leaves of manganese hyperaccumulator Phytolacca acinosa Roxb. (Phytolaccaceae)

Phytolacca acinosa Roxb. is a Mn hyperaccumulating plant. In the present study, the chemical forms of Mn in the leaves of P. acinosa were investigated using chemical analyses and X-ray absorption spectroscopy (XAS). P. acinosa plants were grown hydroponically with 2 mM Mn for 28 days. About 80% of the Mn in the leaves of P. acinosa was found in the supernatant fraction after centrifugation at 20,000g for 45 min. The supernatant fraction was then used to identify the chemical forms of Mn. Gel filtration analysis (Sephadex G-10) showed that oxalate and Mn appeared in the same fraction of the supernatant and the molar ratio of oxalic acid to Mn was 1.12, indicating that there was sufficient oxalic acid in P. acinosa leaves to complex Mn. XAS was employed to investigate the chemical species of Mn in leaves of P. acinosa. Results showed that Mn in leaves was bivalent and almost 90% of the total Mn was Mn-oxalate. The oxalate concentration in the leaves of P. acinosa was not affected by increasing Mn concentration in the solution, suggesting that oxalate biosynthesis was not induced by Mn.     

商陆对锰污染土壤的修复实验研究

商陆是在中国境内发现的多年生、草本型锰超积累植物。通过室内土培试验,评价商陆对土壤中锰的去除潜力,确定最佳收获时间,以期达到最佳的重金属污染土壤植物修复效果。结果表明:商陆能将土壤中的锰转运到地上部位,叶片中Mn含量最高,平均值为17 043 mg/kg DW,远远大于茎和根的锰含量均值;单株的平均富集量在浓度为500 mg/kg DW时达最高,一棵商陆可富集平均13 mg的Mn;动态修复中确定的最佳收获时间为60 d,不同时间收获的商陆地下部分生物量差异不明显(P0.05),地上部分则差异较大。连续收获不改变其锰生物富集能力。这表明商陆对锰有较强的富集能力,是一种优良的修复锰污染土壤的物种,对土壤重金属污染的治理及植物修复领域数据库的完善具有重要意义。     

Enhancing tonoplast Cd/H antiport activity increases Cd,Zn, and Mn tolerance,and impacts root/shoot Cd partitioning in <Emphasis Type="Italic">Nicotiana tabacum </Emphasis>L.

Sequestration mechanisms that prevent high concentrations of free metal ions from persisting in metabolically active compartments of cells are thought to be central in tolerance of plants to high levels of divalent cation metals. Expression of AtCAX2 or AtCAX4, which encode divalent cation/proton antiporters, in Nicotiana tabacum cv. KY14 results in enhanced Cd- and Zn-selective transport into root tonoplast vesicles, and enhanced Cd accumulation in roots of plants exposed to moderate, 0.02 μM Cd in solution culture (Korenkov et al. in Planta 225:403–411, 2007). Here we investigated effects of expressing AtCAX2 and AtCAX4 in the same lines on tolerance to growth with near-incipient toxicity levels of Cd, Zn and Mn. Less growth inhibition (higher tolerance) to all three metals was observed in 35S::AtCAX2 and FS3::AtCAX4 expressing plants. Consistent with the tolerance observed for Cd was the finding that while root tonoplast vesicle proton pump activities of control and FS3AtCAX4 expressing plants grown in 3 μM Cd were similarly reduced, and vesicle proton leak was enhanced, root tonoplast vesicle antiporter activity of these plants remained elevated above that in controls. We suggest that CAX antiporters, unlike tonoplast proton pump and membrane integrity, are not negatively impacted by high Cd, and that supplementation of tonoplast with AtCAX compensates somewhat for reduced tonoplast proton pump and proton leak, and thereby results in sufficient vacuolar Cd sequestration to provide higher tolerance. Results are consistent with the view that CAX2 and CAX4 antiporters of tonoplast play a role in tolerance to high, toxic levels of Cd, Zn, and Mn in tobacco.     

Sublethal nickel toxicity shuts off manganese oxidation and pellicle biofilm formation in Pseudomonas putida GB-1

In sediments, the bioavailability and toxicity of Ni are strongly influenced by its sorption to manganese (Mn) oxides, which largely originate from the redox metabolism of microbes. However, microbes are concurrently susceptible to the toxic effects of Ni, which establishes complex interactions between toxicity and redox processes. This study measured the effect of Ni on growth, pellicle biofilm formation and oxidation of the Mn-oxidizing bacteria Pseudomonas putida GB-1. In liquid media, Ni exposure decreased the intrinsic growth rate but allowed growth to the stationary phase in all intermediate treatments. Manganese oxidation was 67% less than control for bacteria exposed to 5 μM Ni and completely ceased in all treatments above 50 μM. Pellicle biofilm development decreased exponentially with Ni concentration (maximum 92% reduction) and was replaced by planktonic growth in higher Ni treatments. In solid media assays, growth was unaffected by Ni exposure, but Mn oxidation completely ceased in treatments above 10 μM of Ni. Our results show that sublethal Ni concentrations substantially alter Mn oxidation rates and pellicle biofilm development in P. putida GB-1, which has implications for toxic metal bioavailability to the entire benthic community and the environmental consequences of metal contamination.     

Contaminant Removal of Domestic Wastewater by Constructed Wetlands： Effects of Plant Species

A comparative study of the efficiency of contaminant removal between five emergent plant species and between vegetated and unvegetated wetlands was conducted in small-scale （2.0 m×1.0 m×0.7 m, lengthxwidthxdepth） constructed wetlands for domestic wastewater treatment in order to evaluate the decontaminated effects of different wetland plants. There was generally a significant difference in the removal of total nitrogen （TN） and total phosphorus （TP）, but no significant difference in the removal of organic matter between vegetated and unvegetated wetlands. Wetlands planted with Canna indica Linn., Pennisetum purpureum Schum., and Phragmites communis Trin. had generally higher removal rates for TN and TP than wetlands planted with other species. Plant growth and fine root （root diameter ≤ 3 mm） biomass were related to removal efficiency. Fine root biomass rather than the mass of the entire root system played an important role in wastewater treatment. Removal efficiency varied with season and plant growth. Wetlands vegetated by P. purpureum significantly outperformed wetlands with other plants in May and June, whereas wetlands vegetated by P. communis and C. indica demonstrated higher removal efficiency from August to December. These findings suggest that abundance of fine roots is an important factor to consider in selecting for highly effective wetland plants. It also suggested that a plant community consisting of multiple plant species with different seasonal growth patterns and root characteristics may be able to enhance wetland performance.     

Bioremediation of Cadmium-Contaminated Soil through Cultivation of Maize Inoculated with Plant Growth–Promoting Rhizobacteria

A pot experiment was conducted to study the effect of single and co-inoculation of Bacillus mycoides and Micrococcus roseus strains, indigenous to heavy metal (HM)–contaminated soils, on the growth and essential-nutrient and Cd uptake of maize in a soil polluted with 100 and 200 mg Cd kg^?1. Increasing Cd levels significantly decreased shoot and root dry weights, and shoot P, Fe, Zn, and Mn uptake. All bacterial treatments significantly increased biomass and shoot nutrient uptake of plant compared with control in the soil polluted with Cd. Inoculation of plants with B. mycoides and consortium of two bacteria significantly increased, whereas M. roseus significantly decreased, shoot and root Cd uptake, and Cd transfer and translocation factors compared with control in Cd-polluted conditions. The results showed that B. mycoides and consortium of two bacteria had an effective role in phytoextraction and M. roseus was the most effective treatment in phytostabilization of Cd.     

Effects of inoculation of a plant growth promoting rhizobacterium <Emphasis Type="Italic">Burkholderia</Emphasis> sp. D54 on plant growth and metal uptake by a hyperaccumulator <Emphasis Type="Italic">Sedum alfredii</Emphasis> Hance grown on multiple metal contaminated soil

Batch experiments were designed to characterize a multiple metal resistant bacterium Burkholderia sp. D54 isolated from metal contaminated soils in the Dabaoshan Mine in South China, and a follow-up experiment was conducted to investigate the effects of inoculating the isolate on plant growth and metal uptake by Sedum alfredii Hance grown on soils collected from a heavily contaminated paddy field in Daxing County, Guangxi Zhuang Automounous Region, Southwest China. Our experiments showed that strain D54 produced indole acetic acid (IAA), siderophores, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, and solubilizing inorganic phosphate and solubilized insoluble metal bearing minerals. Bacterial inoculation significantly enhanced S. alfredii biomass production, and increased both shoot and root Cd concentration, but induced little variation in root/shoot Pb concentration and shoot Zn concentration. Despite this, the total shoot and root uptake of Cd, Pb and Zn in S. alfredii inoculated with D54 increased greatly compared to the non-inoculated controls. It was concluded that inoculation with strain D54 could help S. alfredii grow better on metal contaminated soils, produce more biomass, and remove more metals from soil, which implies improved efficiency of phytoextraction from metal contaminated soil. The knowledge gained from the present experiments constitutes an important advancement in understanding of the interaction between plant growth-promoting bacteria and hyperaccumulators with regard to plant ability to grow and remove the multiple heavy metals from soils.     

Mixed planting with a leguminous plant outperforms bacteria in promoting growth of a metal remediating plant through histidine synthesis

The effectiveness of plant growth promoting bacteria (PGPB) in improving metal phytoremediation is still limited by stunted plant growth under high soil metal concentrations. Meanwhile, mixed planting with leguminous plants is known to improve yield in nutrient deficient soils but the use of a metal tolerant legume to enhance metal tolerance of a phytoremediator has not been explored. We compared the use of Pseudomonas brassicacearum, Rhizobium leguminosarum, and the metal tolerant leguminous plant Vicia sativa to promote the growth of Brassica juncea in soil contaminated with 400 mg Zn kg^–1, and used synchrotron based microfocus X-ray absorption spectroscopy to probe Zn speciation in plant roots. B. juncea grew better when planted with V. sativa than when inoculated with PGPB. By combining PGPB with mixed planting, B. juncea recovered full growth while also achieving soil remediation efficiency of >75%, the maximum ever demonstrated for B. juncea. μXANES analysis of V. sativa suggested possible root exudation of the Zn chelates histidine and cysteine were responsible for reducing Zn toxicity. We propose the exploration of a legume-assisted-phytoremediation system as a more effective alternative to PGPB for Zn bioremediation.     

Establishment of Sweet Birch on Surface Mine Spoil as Influenced by Mycorrhizal Inoculation and Fertility

Induced mycorrhization of sweet birch (Betula lenta L.) by Pisolithus tinctorius (Pers.) Coker & Couch, as influenced by substrate fertility, was evaluated for its effects on seedling growth and physiology. Following a brief period in seed flats, seedlings were transplanted to mine spoil where they resided for 30 months, and three nutrition regimes were imposed throughout the study by application of differing nutrient solution concentrations. High fertility suppressed mycorrhizal formation by P. tinctorius but promoted that of other mycobionts. Pisolithus mycorrhization induced substantial aboveground and belowground growth as indicated by dimensions and mass for the former and mass and length for the latter but favoring root over shoot growth overall. Furthermore, these mycorrhizae were frequently able to compensate for the growth stimulation of higher nutrient additions. Measurements of xylem pressure potential and soil water potential indicated that water uptake was enhanced by P. tinctorius during simulated drought episodes of two durations and in subsequent recovery periods. Inoculated seedlings had higher foliar concentrations of critical nutrients, especially N, and lower concentrations of potentially phytotoxic metallic elements, particularly Mn, than uninoculated seedlings, although the latter response was absent in high fertility. Spoil analyses clearly revealed the influence of the nutrition regimes but also the effects of seedling uptake on substrate chemistry, and reinforced the findings of the foliar analysis concerning suppression of metal uptake by P. tinctorius. Collectively, these results suggest that P. tinctorius can provide sweet birch an array of physiological benefits that will permit this tree species to flourish on harsh substrates such as surface mine spoils without heavy application of chemical fertilizers.     

The effect of metal diffusion and supply limitations on conditional stability constants determined for durum wheat roots

Conditional stability constants (log K), and binding site densities (Γ_max) for dissolved metals and biota are important input parameters for the Biotic Ligand Model. However, determination of these binding parameters is likely to be influenced by solution kinetics because roots have a large metal-binding capacity and can accumulate metals rapidly. The aim of this study was to determine if the rate of free metal ion diffusion to the root surface, and amount of metal in the bulk solution, is sufficient to accommodate the maximum root–metal accumulation capacity. The extent to which these kinetic limitations affect the magnitude of log K and Γ_max values was also assessed. Seven day old hydroponically grown durum wheat (Triticum turgidum L. var durum, cv ‘Arcola’) were exposed to solutions with p{Cu²⁺}s ranging from 10.54 to 2.26 (~20 °C, pH = 6.0, ionic strength = 0.03 M). Exposure solutions were prepared with and without the metal buffer nitrilotriacetic acid (NTA) so that the total amount of metal in the exposure solution, and net flux of metal to the root, could be varied. The results demonstrate that NTA enhances Cu accumulation at exposure p{Cu²⁺}s between 10 and 6. Comparison of the diffusive flux to the root with the metal flux into the root, for (−NTA) and (+NTA) Cu exposures, showed that the flux of the un-buffered free metal ion to the root was not large enough to accommodate the maximum Cu binding capacity between 10 and 6 p{Cu²⁺} in solution. The total amount of Cu in solution may have limited uptake for exposure p{Cu²⁺}s of 10.01 and 9.01, but the background concentrations of Cu in the control plants prevented definitive conclusions from being made within this exposure range. Similar results were found for Mn and Ni. For Cd, which had lower background concentrations in the roots, the amount of metal in solution did not limit uptake until a p{Cd²⁺} of 10.01. Limiting the supply of Cu²⁺ to the root (i.e. low {Cu²⁺}s with no NTA) caused only a moderate bias in Γ_max values, but suppressed the log K value by 3.44 log units. The log K values for Cd, Mn and Ni, in the absence of NTA, were more similar than expected, which suggests that the kinetics of free ion re-supply to the root surface limited metal uptake, as it did for Cu. Section Editor: T. B. Kinraide     

Remediation of AMD Contaminated Soil by Two Types of Reeds

Acid mine drainage (AMD) adversely impacts many regions in the world. The interactions among citric acid (CA), rhizosphere bacteria and metal uptake in different types of Phragmites australis cultured in spiked AMD contaminated soil were investigated. Compared with non-contaminated reeds cultured under the same conditions, wild reeds harvested from a contaminated site accumulated more metals into tissues. Rhizosphere iron oxidizing bacteria (Fe(II)OB) enhanced the development of Fe plaque but had no significant impact on the formation of Mn and Al plaque on the root surface of either reeds. Plaque may restrain the accumulation of Fe and Mn into tissues of reeds. CA inhibited the growth of Fe(II)OB, reduced the formation of metal plaque and significantly elevated metal accumulations into both underground and aboveground biomass of reeds. The concentrations of Fe, Al and Mn were higher in belowground organs than aboveground tissues. The roots contained 0.28 ± 0.01 mg/g Mn, 3.09 ± 0.51 mg/g Al, 94.47 ± 5.75 mg/g Fe, while the stems accumulated 0.19 ± 0.01 mg/g Mn, 1.34 ± 0.02 mg/g Al, 10.32 ± 0.60 mg/g Fe in wild reeds cultured in soil added with 33,616 ppm CA. Further field investigations may be required to study the effect of CA to enhance phytoremediation of metals from real AMD contaminated sites.     

Combining a modelling with a genetic approach in establishing associations between genetic and physiological effects in relation to phosphorus uptake

The Pup1 locus confers tolerance to phosphorus (P) deficiency in rice (Oryza sativa L.). Transferring the Pup1 locus to an intolerant genotype increased P uptake by a factor 3 to 4. Lines with the Pup1 locus maintained higher root growth rates under P deficiency, but only as they started to diverge from intolerant lines in P uptake. It was thus not possible to determine if differences in root growth preceded and caused differences in P uptake or whether high root growth was the result of higher external P uptake efficiency (P influx per root size). The purpose of this paper is to review experimental evidence on the effect of Pup1 in light of recent results in modelling cause-and-effect relations between root growth, external efficiency and P uptake. Model simulations suggested that only very small changes in factors enhancing root growth were needed to explain the effect of Pup1 on P uptake. A 22% increase in root fineness or in internal P utilization efficiency (root dry matter per root P) was sufficient to triple P uptake . External root efficiency had to increase by 33 to account for the effect of Pup1. However, the most noticeable effect of increases in external efficiency was a subsequent stimulation of root growth that contributed eight times more to final P uptake compared to the change in external efficiency. Comparisons of model simulations with empirical data suggested that measured differences in external efficiency between Nipponbare and NIL-Pup1 were sufficiently large to account for the increase in P uptake. A segregation analysis using several pairs of contrasting NILs (at the Pup1 locus) further confirmed this as Pup1 co-segregated with external efficiency but not with seedling root growth or internal efficiency.     

Formation of cadmium sulfide nanoparticles mediates cadmium resistance and light utilization of the deep-sea bacterium Idiomarina sp. OT37-5b

Heavy metal is one of the major factors threatening the survival of microorganisms. Here, a deep-sea bacterium designated Idiomarina sp. OT37-5b possessing strong cadmium (Cd) tolerance was isolated from a typical hydrothermal vent. Both the Cd-resistance and removal efficiency of Idiomarina sp. OT37-5b were significantly promoted by the supplement of cysteine and meanwhile large amount of CdS nanoparticles were observed. Production of H₂S from cysteine catalysed by methionine gamma-lyase was further demonstrated to contribute to the formation of CdS nanoparticles. Proteomic results showed the addition of cysteine effectively enhanced the efflux of Cd, improved the activities of reactive oxygen species scavenging enzymes, and thereby boosted the nitrogen reduction and energy production of Idiomarina sp. OT37-5b. Notably, the existence of CdS nanoparticles obviously promoted the growth of Idiomarina sp. OT37-5b when exposed to light, indicating this bacterium might grab light energy through CdS nanoparticles. Proteomic analysis revealed the expression levels of essential components for light utilization including electron transport, cytochrome complex and F-type ATPase were significantly up-regulated, which strongly suggested the formation of CdS nanoparticles promoted light utilization and energy production. Our results provide a good model to investigate the uncovered mechanisms of self-photosensitization of nonphotosynthetic bacteria for light-to-chemical production in the deep biosphere.     

Changes in Shewanella putrefaciens CN32 Membrane Stability upon Growth in the Presence of Soluble Mn(II), V(IV), and U(VI)

In natural reducing environments, such as anoxic sediments and soils, bacteria may be exposed to high concentrations of soluble transition metals. The aim of this study was to identify physiological and biochemical adaptations of Shewanella putrefaciens CN32 membranes to soluble Mn(II), V(IV), and U(VI). We assessed responses of CN32 to these metals, in aerobic and anaerobic cultures, by means of membrane fluidity and fatty acid composition assays. During aerobic growth, all metals had a stabilizing effect on fluidity, while under anoxic conditions this was only observed for bacteria treated with U(VI). Membrane gel-to-fluid phase transition temperatures were higher under anaerobic conditions and were not affected by the metal treatments. Fatty acid desaturation demonstrated linear correlation with significant increases in membrane fluidity, despite metal treatments that did not significantly alter fatty acid chemistry. Scanning transmission X-ray microscopy (STXM) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at Mn 2p- and V 2p-edges revealed that both Mn(II) and V(IV) were associated with CN32 membranes, with V(IV) associating as VO²⁺ under anoxic conditions only. The results of this study indicate that the bacterial growth environment greatly impacts membrane chemistry and stability, with overall implications for in vitro as well as in situ studies. Supplemental materials are available for this article. Please go to the publisher's online edition of Geomicrobiology Journal to view the supplemental file.     

The influence of an electric field on growth and trace metal content in aquatic plants

It is known that both natural and artificial electric fields (EF) affect plants physiological parameters as well as germination, growth and yield. The present article describes results of a preliminary experiment on the impact of electric field on aquatic plants biogeochemistry. The objective of the present study was the assessment of the influence exerted by the electric field on growth and trace metals content of Elodea canadensis. In a laboratory experiment plants were exposed to the field intensity of 54?kV m^?1 for 7?days. The plants length was measured and the content of Fe, Mn, Ni, Pb, and Zn was determined using atomic absorption spectrometry (AAS). Results showed that the application of electric field slightly enhanced the growth of E. canadensis shoots. The content of Mn and Ni was significantly lower, and Pb and Zn significantly higher in plants exposed to the electric filed, while Fe content did not differ between control and EF treatment. This provides a rationale for further studies on biological effects of electric field in trace metal contaminated waters and application of an electrically enhanced phytoremediation.     

Aluminum-mycorrhizal interactions in the physiology of pitch pine seedlings

Aluminum (Al) in the rhizosphere adversely affects plant nutrition and growth. Although many conifer species, and pitch pine (Pinus rigida) in particular, have evolved on acidic soils where soluble Al is often high, controlled environment studies often indicate that Al interferes with seedling growth and nutrient relations. Under normal field conditions, conifer roots grow in a symbiotic relationship with ectomycorrhizal fungi, and this association may modulate the effects of Al on root physiology. To investigate the influence of mycorrhizal infection on Al toxicity, pitch pine seedlings were grown with or without the ectomycorrhizal symbiont Pisolithus tinctorius and were exposed to low levels of Al in sand culture. Aluminum at 50 μM reduced nonmycorrhizal seedling growth and increased foliar Al concentrations, but did not alter photosynthetic gas exchange or other aspects of seedling nutrition. Nonmycorrhizal seedlings exposed to 200 μM Al exhibited decreased growth, increased transpiration rates, decreased water use efficiency, increased foliar Al and Na levels, and reduced foliar P concentrations. Seedlings inoculated with P. tinctorius exhibited unaltered growth, physiological function, and ionic relations when exposed to Al. The fungal symbiont evidently modulated ionic relations in the rhizosphere, reducing Al-P precipitation reactions, Al uptake, and subsequent root and shoot tissue Al exposure.     

Mycorrhiza and soil bacteria influence extractable iron and manganese in soil and uptake by soybean

Excess manganese (Mn) in soil is toxic to crops, but in some situations arbuscular mycorrhizal fungi (AMF) alleviate the toxic effects of Mn. Besides the increased phosphorus (P) uptake, mycorrhiza may affect the balance between Mn-reducing and Mn-oxidizing microorganisms in the mycorrhizosphere and affect the level of extractable Mn in soil. The aim of this work was to compare mycorrhizal and non-mycorrhizal plants that received extra P in relation to alleviation of Mn toxicity and the balance between Mn-oxidizing and Mn-reducing bacteria in the mycorrhizosphere. A clayey soil containing 508 mg kg⁻¹ of extractable Mn was fertilized with 30 mg kg⁻¹ (P1) or 45 mg kg⁻¹ (P2) of soluble P. Soybean (Glycine max L. Merrill, cv. IAC 8-2) plants at P1 level were non-inoculated (CP1) or inoculated with either Glomus etunicatum (GeP1) or G. macrocarpum (GmP1), while plants at P2 level were left non-inoculated (CP2). Plants were grown in a greenhouse and harvested after 80 days. In the mycorrhizosphere of the GmP1 and GeP1 plants a shift from Mn-oxidizing to Mn-reducing bacteria coincided with higher soil extractability of Mn and Fe. However, the occurrence of Mn-oxidizing/reducing bacteria in the (mycor)rhizosphere was unrelated to Mn toxicity in plants. Using 16S rDNA sequence homologies, the Mn-reducing isolates were consistent with the genus Streptomyces. The Mn-oxidizers were homologous with the genera Arthrobacter, Variovorax and Ralstonia. While CP1 plants showed Mn toxicity throughout the whole growth period, CP2 plants never did, in spite of having Fe and Mn shoot concentrations as high as in CP1 plants. Mycorrhizal plants showed Mn toxicity symptoms early in the growth period that were no longer visible in later growth stages. The shoot P concentration was almost twice as high in mycorrhizal plants compared with CP1 and CP2 plants. The shoot Mn and Fe concentrations and contents were lower in GmP1 and GeP1 plants compared with the CP2 treatment, even though levels of extractable metals increased in the soil when plants were mycorrhizal. This suggests that mycorrhiza protected its host plant from excessive uptake of Mn and Fe. In addition, higher tissue P concentrations may have facilitated internal detoxification of Mn in mycorrhizal plants. The exact mechanisms acting on alleviation of Mn toxicity in mycorrhizal plants should be further investigated.