Distribution of Culturable Phosphate-Solubilizing Bacteria in Soil Aggregates and Their Potential for Phosphorus Acquisition.

Deciphering distribution patterns of phosphate-solubilizing bacteria (PSB) and phosphorus-cycling-related genes in soils is important to evaluate phosphorus (P) transformation. However, the linkage between PSB number and P-cycling-related gene abundance in soils, especially soil aggregates, remains largely unknown. Here, we estimated the numbers of PSB and abundances of P-cycling-related genes (i.e., gcd and bpp) in soil aggregates under different fertilization regimes as well as P-solubilizing performance and plant-growth-promoting ability of PSB. We found that tricalcium phosphate-solubilizing bacteria, phytate-degrading bacteria, and gcd and bpp abundances were more abundant in silt plus clay (silt+clay; <53 µm) than in macroaggregate (250 to 2000 µm) and microaggregate (53 to 250 µm). Fertilization treatment and aggregate fractionation showed distinct effects on PSB number and P-cycling-related gene abundance. We found significantly negative correlation between gcd gene abundance and tricalcium phosphate-solubilizing bacterial number (Col-CaP) and dramatically positive correlation between bpp gene abundance and phytate-degrading bacterial number (Col-Phy). P fractions were responsible for PSB number and P-cycling-related gene abundance. The isolated Pseudomonas sp. strain PSB-2 and Arthrobacter sp. strain PSB-5 exhibited good performances for solubilizing tricalcium phosphate. The inoculation of Pseudomonas sp. PSB-2 could significantly enhance plant fresh weight, plant dry weight, and plant height. Our results emphasized distinct distribution characteristics of PSB and P-cycling-related genes in soil aggregates and deciphered a close linkage between PSB number and P-cycling-related gene abundance. Our findings might guide the isolation of PSB from agricultural soils and provide a candidate plant-growth-promoting bacterium for agro-ecosystems. IMPORTANCE Phosphate-solubilizing bacteria are responsible for inorganic P solubilization and organic P mineralization. Elucidating the linkage between phosphate-solubilizing bacterial number and P-cycling-related gene abundance is important to isolate plant-growth-promoting bacteria for agro-ecosystems. Our findings reveal differentiating strategies of phosphate-solubilizing bacteria in soil aggregates, and the deciphered P fractions show strong effects on distribution patterns of phosphate-solubilizing bacteria and P-cycling-related genes. Additionally, we isolated phosphate-solubilizing bacteria with good plant-growth-promoting ability. This study enriches our knowledge of P cycling in soil aggregates and might guide the production and management of farmland.

Dredging alleviates cyanobacterial blooms by weakening diversity maintenance of bacterioplankton community.

Disentangling ecological mechanisms behind dredging is meaningful to implement environmental policy for improving water quality. However, environmental adaptation and community assembly processes of bacterioplankton in response to dredging disturbance are poorly understood. Based on Illumine MiSeq sequencing and multiple statistical analyses, we estimated interactions, functions, environmental breadths, phylogenetic signals, phylogenetic clustering, and ecological assembly processes of bacterioplankton community before and after dredging. We found distinct change in community composition, comparable decreases in diversity, functional redundancy and conflicting interaction, relatively low phylogenetic clustering, and relatively weak environmental adaptation after dredging. The bacterioplankton community assembly was affected by both stochastic and deterministic processes before dredging, but dominated by stochasticity after dredging. Sediment total phosphorus was a decisive factor in balancing determinism and stochasticity for bacterioplankton community assembly before and after dredging. Consequently, taxonomic and phylogenetic α-diversities of bacterioplankton exhibited higher contributions to the water trophic level represented by chlorophyl α before dredging than afterwards. Our results emphasized bacterioplankton in response to environmental changes caused by dredging, with nutrient loss and ecological drift playing important roles. These findings extend knowledge of contribution of bacterioplankton diversity to water trophic level and decipher mechanisms of bacterioplankton diversity maintenance in response to dredging, which is useful for guiding mitigation of cyanobacterial blooms.

Dredging mitigates cyanobacterial bloom in eutrophic Lake Nanhu: Shifts in associations between the bacterioplankton community and sediment biogeochemistry.

Cyanobacterial blooms are a worldwide environmental problem, which is partly attributed to their access to excessive nitrogen (N) and phosphorus (P). Preventing the blooms by reducing N and P from internal inputs is viewed as a challenge. To evaluate the effects of dredging on cyanobacterial abundances and bacterioplankton communities, water and sediment samples were collected from eutrophic Lake Nanhu (Wuhan, China) before dredging (2017) and after dredging (2018). After dredging, significant decreases were observed for sediment nutrients (e.g., C, N, and P sources); C-, N-, P-, and S-cycling-related enzyme activity; N- and P-cycling-related gene abundance; microbial abundance; and dramatic changes were observed in the composition of the sediment microbial community. The release rates of nutrient including nitrogen, phosphorus, and organic matter decreased after dredging, and sediment biogeochemistry was closely correlated to nutrient release rates. Additionally, our observations and analyses indicated that the abundance and diversity of the bacterioplankton community decreased significantly, the composition and interaction of the bacterioplankton community dramatically changed, and the bacterioplankton community function (e.g., N, P-cycling-related enzymes and proteins) down regulated after dredging. Water and sediment physicochemical factors explained 72.28% variation in bacterioplankton community composition, and these physicochemical factors were significantly correlated with diversity, composition, and function of bacterioplankton community. Our findings emphasized that cyanobacterial blooms in freshwater ecosystems were closely correlated with noncyanobacterial bacterioplankton that were largely conserved at the phylum level, with Proteobacteria, Actinobacteria, and Bacteroidetes as the main taxa. To our knowledge, this is the first report clarifying the mechanism of cyanobacterial blooms mitigation by dredging, via changing the association between the bacterioplankton community and sediment biogeochemistry. Our findings are of significance and indicate that dredging is effective for mitigating cyanobacterial blooms.

Responses of the rhizosphere bacterial community in acidic crop soil to pH: Changes in diversity, composition, interaction, and function.

Soil pH is an important predictor of bacterial community composition and diversity. Examining the effects of pH on diversity, structure, interaction, and function of rhizosphere bacterial communities in acidic crop soils provide valuable information for knowing potential role of rhizosphere bacteria in crop yield. Here, we collected soils from artificial greenhouses and applied Illumina Miseq sequencing, quantitative PCR techniques, multiple ecological analysis methods, including topological analysis and functional profiling to analyze our data and validate our hypotheses. We found that the soil physicochemical properties, species diversity, and rhizosphere bacterial community composition were significantly affected by the degree of soil acidification (pH < 5.5 and pH > 5.5) but not vegetation type. Additionally, bacterial absolute abundance increased with higher pH. The 18 soil samples were clustered into two distinct groups of pH < 5.5 and pH > 5.5 at the OTU level, and soil pH had more of an effect on bacterial community composition compared to the other physicochemical variables. In addition, rhizosphere bacteria might presented relatively less competition for survival in pH < 5.5 soils, and bacterial community functions, including nutrient (i.e., carbon, nitrogen, phosphorus, and sulphur) cycling-related enzymes and proteins, were downregulated in more acidic soils (pH < 5.5) based on sequence analysis. To our knowledge, this report is the first to show that pH is a key factor affecting the diversity, structure, interaction, and function of rhizosphere bacterial communities in acidic crop soil in artificial greenhouses. Our findings emphasize that community function and structure of rhizosphere bacteria are closely correlated in more acidic soils, and the decreased crop yield may be correlated with attenuation of the function of the rhizosphere bacterial community.

Alkaline phosphatase-harboring bacterial community and multiple enzyme activity contribute to phosphorus transformation during vegetable waste and chicken manure composting.

The objective of this study was to evaluate changes in phosphorus fractions during vegetable waste and chicken manure composting. High throughput sequencing, quantitative PCR, and multiple analysis methods were applied to investigate interconnections among phosphorus fractions, enzyme activity, and phoD-harboring bacterial community composition. We found the highest composting temperature reached 61 °C and phosphorus fractions presented significant differences during a 60-day composting. The content of plant-absorbable phosphorus, including water soluble phosphorus, available phosphorus, and citric acid phosphorus increased by 121%, 87%, and 63%, respectively. Additionally, phoD gene abundance significantly correlated with the activities of nine enzymes. Our findings emphasize that microbial activity plays an important role in phosphorus transformation during composting, and the final composting product could be good biological phosphorus fertilizer. To our knowledge, this is the first report indicating that enzyme activity, community composition and abundance of phoD-harboring bacteria have direct and indirect effects on phosphorus transformation during composting.

[Antioxidative function of katG gene in Rhizobium leguminosarum].

OBJECTIVE: Catalase-peroxidase KatG can protect bacteria from damage of reactive oxygen species. This study investigated the antioxidative function of catalase - peroxidase gene katG in Rhizobium leguminosarum 3841. METHODS: katG mutant strain of R. leguminosarum was constructed by homologous recombination. The wild type, katG mutant and complementary strain were challenged by oxidative stress and symbiotic ability. RESULTS: Under free - living conditions, the katG mutant exhibited no generation time extension. However, cells of the katG strain were deficient in consumption oj high concentrations of H2O2and were vulnerable after aquick exposure to H2O2. The real-time qRT-PCR results showec that katG was expressed independently of exogenous H2O2. In contrast, the katG mutant strain displayed higher expres, level of ohrB gene and lower expression level of grxC than the wild type. With regard to symbiotic capacities with Pisum sativum, the katG mutant was indistinguishable in root nodule nitrogenase activity and competition nodule ability from the wild type. However, katG gene was expressed significantly lower in bacteroids than that in free-living strains. Besides, the colonization of the pea rhizosphere by the katG mutant was impaired compared to that of the wild type. CONCLUSION: ThE deletion of katG had nosignificant effect in 3841 under the free-living and symbiosis condition but was essential ir antioxidation and colonization of the pea rhizosphere. Although katG could not be induced by H2O2, it still played acentra role in antioxidation and symbiotic nitrogen fixation by regulating the antioxidant genes such as ohrB and grxC.

Paenibacillus enshidis sp. nov., Isolated from the Nodules of Robinia pseudoacacia L.

A Gram-positive, motile, endospore-forming, rod-shaped bacterium, designated RP-207(T), was isolated from the nodules of Robinia pseudoacacia L. plants planted in Enshi District, Hubei, PR China. Phylogenetic analyses based on the 16S rRNA gene sequence showed that the novel strain was affiliated to the genus Paenibacillus, with its closest relatives being Paenibacillus xylanilyticus XIL14(T) (95.6%), Paenibacillus peoriae DSM8320(T) (95.3%) and Paenibacillus polymyxa DSM 36(T) (95.3%). The DNA G+C content was 47.0 mol%. DNA-DNA hybridization value between strain RP-207(T) and P. xylanilyticus XIL14(T) was 40.1%. The diamino acid found in the cell wall peptidoglycan was meso-diaminopimelic. The major polar lipids were phosphatidylglycerol, phosphatidylethanolamine, diphosphatidylglycerol, an unidentified amino-phospholipid and an unknown phospholipid. The predominant menaquinone was menaquinone-7 (MK-7), and the major fatty acid was anteiso-C15:0 and C16:0. On the basis of its physiological and biochemical characteristics and the level of DNA-DNA hybridization, strain RP-207(T) is considered to represent a novel species of the genus Paenibacillus, for which the name Paenibacillus enshidis sp. nov. is proposed. The type strain is RP-207(T) (=CCTCC AB 2013275(T) = KCTC 33519(T)).