Xanthomonas campestris pathovar campestris (Xcc) is the causative agent of crucifer black rot disease, which causes severe losses in agricultural yield world-wide. This bacterium is a model organism for studying plant-bacteria interactions. We sequenced the complete genome of Xcc 8004 (5,148,708 bp), which is highly conserved relative to that of Xcc ATCC 33913. Comparative genomics analysis indicated that, in addition to a significant genomic-scale rearrangement cross the replication axis between two IS1478 elements, loss and acquisition of blocks of genes, rather than point mutations, constitute the main genetic variation between the two Xcc strains. Screening of a high-density transposon insertional mutant library (16,512 clones) of Xcc 8004 against a host plant (Brassica oleraceae) identified 75 nonredundant, single-copy insertions in protein-coding sequences (CDSs) and intergenic regions. In addition to known virulence factors, full virulence was found to require several additional metabolic pathways and regulatory systems, such as fatty acid degradation, type IV secretion system, cell signaling, and amino acids and nucleotide metabolism. Among the identified pathogenicity-related genes, three of unknown function were found in Xcc 8004-specific chromosomal segments, revealing a direct correlation between genomic dynamics and Xcc virulence. The present combination of comparative and functional genomic analyses provides valuable information about the genetic basis of Xcc pathogenicity, which may offer novel insight toward the development of efficient methods for prevention of this important plant disease.

Arsenic is a toxic metalloid and its pollution has become a global environmental problem. This paper reviewed the current knowledge on the speciation, toxicity and metabolism of arsenic in microalgae. A number of arsenic species are present in various microalgae. Due to the great toxicity of inorganic arsenic, microalgae may undergo different processes to reduce the arsenic toxicity, including cell surface binding, arsenite [As(III)] oxidation, arsenate [As(V)] reduction, methylation, transformation into arsenosugars or arsenolipids, chelation of As(III) with glutathione and phytochelatins, as well as excretion from cells. Several genes and enzymes involved in arsenic transformations have been identified and characterized. Many factors, especially nutrient elements (e.g., nitrogen and phosphorus) in cells and in culture, affect arsenic metabolic pathways of microalgae. Arsenic metabolism in the unicellular algae has gained considerable interest because these processes control not only the effectiveness of arsenic phycoremediation, but also the risk of arsenic contamination in algal products. Future research need to focus on (1) the regulative mechanisms of arsenic absorption, biotransformation and excretion at molecular level; (2) the effects of intracellular nutrient dynamics on arsenic speciation; (3) the impacts of culture regime on the arsenic metabolism in microalgae; (4) the transfer of arsenic species across aquatic food web in order to better evaluate the roles of microalgae in arsenic cycling.

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Four strains (Enterobacter sp. EnHy-401, Arthrobacter sp.ArHy-505, Azotobacter sp.AzHy-510 and Enterobacter sp.EnHy-402) which have the ability to solubilize tricalcium phosphate (TCP) were used to study the mechanism of P-solubilization. It was found that three phosphate solubilizing bacteria (EnHy-401, ArHy-505 and AzHy-510) producing exopolysaccharide (EPS) have a stronger ability for P-solubilization than isolate EnHy-402 without EPS production, of those, the strain EnHy-401 with the highest EPS production and efficient organic acids on P-solubilization had a stronger capacity for Psolubilization than the others. Further studies demonstrated that addition of EPS into medium could increase the amount of phosphorus solubilized by organic acid, but failed to release phosphorus from TCP alone. The synergistic effects of EPS and organic acid on TCP solubilization varied with the origin and the concentration of EPS in medium. EPS produced by EnHy-401 was most effective in promoting phosphorus release at an optimal concentration in medium. The increase of P-solubilization brought by EPS attributed to the participation of EPS led to the change in homeostasis of P-solubilization, pushing it towards P dissolved by holding free phosphorus in the medium, consequently resulting in greater phosphorus released from insoluble phosphate. We therefore suggest that EPS with ability of phosphorus-holding may be a novel important factor in the microbial dissolution of TCP except for organic acid.