[Mechanism and Application of Plant Growth-Promoting Bacteria in Heavy Metal Bioremediation].

Heavy metal contamination is one of the main factors causing ecological and environmental degradation. Soil contamination by heavy metals decreases soil quality, reduces agricultural productivity and quality, and even threatens human health. Therefore, optimizing remediation strategies for soils polluted with heavy metals is of great significance for high-yield, good-quality, and sustainable agriculture. Numerous domestic and foreign scholars have carried out a large number of studies on the phytoremediation of heavy metal contaminated soils. However, the remediation efficiency may be restricted by soil and climatic/environmental conditions. The synergistic remediation of microorganisms and plants is considered an effective means to improve metal remediation efficiency under environmental stresses. Metal-resistant plant growth-promoting bacteria (PGPB) not only promote plant growth and its resistance to biotic (e.g., phytopathogens, etc.) and abiotic (e.g., drought, salinity, extreme temperatures, heavy metals, etc.) stresses but also alter meal bioavailability in soils and metal toxicity in plants, thereby improving phytoremediation efficiency. In this paper, the mechanisms involved in promoting plant growth and its stress tolerance, and affecting metal bioavailability by metal-resistant PGPB, were systematically summarized. Furthermore, research progress on the application of PGPB in ecological restoration in recent years was extensively reviewed.

[Identification and Characterization of a Hypothermic Alkaliphilic Aerobic Denitrifying Bacterium Pseudomonas monteilii Strain H97].

Low temperatures and high pH generally inhibit bio-denitrification. Thus, it is important to explore psychrotrophic and alkali-resistant microorganisms for nitrogen degradation. This study mainly focused on the identification of an alkaliphilic strain and preliminary exploration of its denitrification characteristics. Based on morphological observations, phospholipid fatty acids and 16S rRNA gene sequence analysis, strain H97, which was isolated from the winter paddy field in Guizhou province, was identified as Pseudomonas monteilii. Till date, there were few reports about the denitrification characteristics of Pseudomonas monteilii. The effects of environmental factors such as temperature, inoculation quantity, C/N ratio, initial pH, and carbon source were investigated using simulated wastewater. The optimum conditions for nitrate and total nitrogen removal by H97 were: inoculum size 1.5×106 CFU·(100 mL)-1; initial pH 9.0; C/N=15; 15℃; and sodium succinate as the carbon source. The nitrate and total nitrogen removal efficiencies were 97.69% and 96.32%, respectively, at optimum conditions with an initial nitrate nitrogen concentration of 50.0 mg·L-1. The temperature experiments indicated that the optimal temperature for highest nitrogen removal efficiency was 15℃, and that the strain H97 could survive in a wide range of 15-40℃. Additionally, the nitrate and total nitrogen efficiencies at the initial pH value of 7.0-11.0 were 91.21% and 79.10%, respectively, and the denitrification capacity then decreased to 64.75% at the initial pH 12.0. These results indicated that strain H97 showed cold and alkali resistance, which suggests an application potential for the treatment of alkaline nitrogen polluted water in the southern winter.

[Heterotrophic Nitrification and Aerobic Denitrification of the Hypothermia Aerobic Denitrification Bacterium: Arthrobacter arilaitensis].

High concentrations of ammonium, nitrate and nitrite nitrogen were employed to clarify the abilities of heterotrophic nitrification and aerobic denitrification of Arthrobacter arilaitensis strain Y-10. Meanwhile, by means of inoculating the strain suspension into the mixed ammonium and nitrate, ammonium and nitrite nitrogen simulated wastewater, we studied the simultaneous nitrification and denitrification ability of Arthrobacter arilaitensis strain Y-10. In addition, cell optical density was assayed in each nitrogen removal process to analyze the relationship of cell growth and nitrogen removal efficiency. The results showed that the hypothermia denitrification strain Arthrobacter arilaitensis Y-10 exhibited high nitrogen removal efficiency during heterotrophic nitrification and aerobic denitrification. The ammonium, nitrate and nitrite removal rates were 65.0%, 100% and 61.2% respectively when strain Y-10 was cultivated for 4 d at 15°C with initial ammonium, nitrate and nitrite nitrogen concentrations of 208.43 mg · L⁻¹, 201.16 mg · L⁻¹ and 194.33 mg · L⁻¹ and initial pH of 7.2. Nitrite nitrogen could only be accumulated in the medium containing nitrate nitrogen during heterotrophic nitrification and aerobic denitrification process. Additionally, the ammonium nitrogen was mainly removed in the inorganic nitrogen mixed synthetic wastewater. In short, Arthrobacter arilaitensis Y-10 could conduct nitrification and denitrification effectively under aerobic condition and the ammonium nitrogen removal rate was more than 80.0% in the inorganic nitrogen mixed synthetic wastewater.

[Efficiency and Mechanism of Capping with Purple Parent Rocks to Control Phosphorus Release from Sediments].

In order to study the efficiency and mechanism of capping with purple parent rocks to control phosphorus release from sediments, three kinds of purple parent rocks (Fei-xian-guan Formation, Peng-lai-zhen Formation and Sui-ning Formation) which distribute widely in Chongqing, and two types of rock (limestone and calcite) which have been found to effectively control the release of phosphorus from sediments, were selected as active covering materials to cap the sediments to conduct simulation experiment. The results indicated that: all three kinds of purple parent rocks showed strong inhibition of total phosphorus release from sediments to the overlying water, far better than limestone and calcite (P<0.05), and the highest removal rate of total phosphorus was 94.4% in the disposal of Fei-xian-guan Formation. Those five kinds of covering materials could promote the release and transformation of total phosphorus in sediments, which could significantly promote the in situ conversion of OP to Ca-P, while the three kinds of purple parent rocks were more conducive to convert the released phosphorus into inorganic phosphorus and organic phosphorus. At the same time, those five kinds of covering materials could also change the microbial community structure in sediments and overlying water, and the numbers of bacteria (labeled as PLFA16:0) were significantly negatively correlated with the content of total phosphorus in the overlying water.

[Isolation, Identification and Characteristics of a Rhodopseudomonas with High Ammonia-nitrogen Removal Efficiency].

A strain of photosynthetic bacterium named psb1 capable of ammonia-nitrogen degradation was isolated from a swamp in Yunnan. The psb1 was similar to Rhodopseudomonas sp. according to its cell morphological properties and absorption spectrum analysis of living cells. The alignment result of 16S rDNA amplification sequence with specific primers of photosynthetic bacteria showed that the homology between strain psb1 and Rhodopseudomonas sp. was 99%, and the alignment results of protein sequences of bacterial chlorophyll Y subunit showed that the strain psb1 and Rhodopseudomonas palustris were the most similar, with a similarity of 99%. But there was a great difference in the biological properties of the strains psb1 and Rhodopseudomonas palustris according to physiological biochemical characteristics and main fatty acid analysis. For example, strain psb1 could not utilize glucose and mannitol as carbon source, and had specific fatty acid C18:1ω6c. The results of single factor test showed that:the optimal growth was obtained at pH 7.0 and 40℃, the optimal nitrogen source was yeast extract. The optimal conditions for ammonia nitrogen biodegradation were as following:anaerobic, light, initial pH 6.0-7.0, temperature 30℃, inoculation volume 0.4%. Under that cultural condition, the degradation rate of ammonia nitrogen in wastewater could reach 99%. The results indicated that strain psb1 might be a novel bacterium in genus Rhodopseudomonas with high ammonia removal efficiency, and can be applied in the bioremediation of polluted landscape water.

[Effects of PAHs Pollution on the Community Structure of Denitrifiers in a Typical Oilfield].

Agricultural soils in the oilfields have the potential risk of PAHs (polycyclic aromatic hydrocarbons) pollution, and the denitrification process with nitrate as the terminal electron acceptor might be important for soil PAHs elimination under anaerobic condition. In this study, 9 soil samples listed as JH-1 to JH-9 were collected from the JiangHan oilfield with a history of more than 50 years. Using the functional genes (nirK: Cu-nitrite reductase gene; nirS: cd1-nitrite reductase gene) involved in denitrification as biomarkers, the community structure of soil denitrifiers was investigated by quantitative-PCR and T-RFLP (terminal-restriction fragment length polymorphism) combined with clone library, and the relationship between soil properties and community structure of soil denitrifers was discussed. The result indicated that the copy numbers of nirK were higher than those of nirS in all soil samples, and the lowest copy numbers of nirK and nirS were both detected in the JH-4 with the highest PAHs content. Meanwhile, the correlation analysis also showed a negative correlation between the copy numbers of those functional genes and soil PAHs content (nirK: R2=0.54, P<0.05; nirS: R2=0.58, P<0.05). Furthermore, the result of T-RFLP indicated that the nirK community structures in different soil samples varied significantly, which was obviously unique in the sample (JH-4) with the highest PAHs content. The subsequent RDA (redundancy analysis) also demonstrated that soil PAHs content as well as the available nitrogen and phosphorus belonged to the most important factors affecting the nirK community structure in this oilfield soil. Compared with nirK, little variation was shown about the nirS community structure among the soil samples. However, the abundance of nirS-harboring pseudomonas had a remarkably positive relation with the soil PAHs content, which indicated that pseudomonas, a well known bacterial genus with strong ability to degrade organic pollutants, might be an essential driver for PAHs degradation via denitrification process in this oilfield soil.

[Effect of Different Purple Parent Rock on Removal Rates of Nitrogen, Phosphorus and Organics in Landscape Water].

In order to understand the impacts of physicochemical properties of purple parent rock on the removal rates of nitrogen, phosphorus and organics in landscape water systems, four types of purple parent rocks including Peng-lai-zhen Formation (S1) , Sha-xi-miao Formation (S2) , Fei-xian-guan Formation (S3) and Sui-ning Formation (S4) , which distribute widely in Chongqing, were selected and autoclaved, and added to unsterile landscape water collected from Chong-de Lake in Southwest University, and the landscape water only was used as control. And several indicators such as total nitrogen and phosphorus and so on of every disposal were investigated periodically. The results indicated that: (1) The highest removal rates of total nitrogen, total phosphorus and Ammonia nitrogen were observed in Sl, which were 45.1%, 62.3% and 90%, respectively; the highest removal rate of COD was 94.5% in S4; the ammonia nitrogen content in the purple parent rocks was not obviously changed before and after the experiments, which indicated that the adsorption of ammonia nitrogen on purple parent rock surface was not the main reason for the decrease of ammonia nitrogen in water. (2) Arsenate had inhibitory effect on the sulfate-reducing bacteria, while copper and magnesium had promoting effect on gram-negative bacteria. (3) The microbial diversity was positively correlated to total nitrogen in water. (4) Based on the PCA analyses of microbial community structure and environmental factors, the mineral elements released from parent rock affected the structure and composition of microbial community in the test water, and then influenced the removal rates of nitrogen, phosphorus and organics in water systems.