Paralytic shellfish poisoning (PSP) toxins are potent neurotoxins mainly produced by dinoflagellates and being concentrated in bivalves through food web transfer. Increasing number of findings of toxin-producing bacteria in the cells of dinoflagellate such as Alexandriumtamarense supports the hypothesis of the bacterial origin of PSP toxins. Evidence that there are specific symbiosis bacterial taxa associated with the phytoplankton indicates the presence of specific selective mechanisms between them, and implies that the symbiosis bacteria have some vital function to the benefit of the dinoflagellates. Studies on the role of toxin-producing symbiosis bacteria in the marine ecosystem are considered to be becoming more important. Although toxigenic bacteria could be isolated from toxic dinoflagellates, it was not clearly proven whether the isolated bacterial strains based on culture-dependent manner and the corresponding intracellular bacteria were the same because of microbial unculturability. This paper aims to demonstrate the biodiversity of the symbiotic bacteria associated with toxic dinoflagellate A. tamarense using the culture-independent high-throughput pyrosequencing method, as well as the phylogenetic analysis based on 16S rDNA sequences of the symbiotic cultivable bacteria strains isolated from toxic Alexander tamarense.

Potassium (K) reduces the deleterious effects of drought stress on plants. However, this mitigation has been studied mainly in the aboveground plant pathways, while the effect of K on root-soil interactions in the underground part is still underexplored. Here, we conducted the experiments to investigate how K enhances plant resistance and tolerance to drought by controlling rhizosphere processes. Three culture methods (sand, water, and soil) evaluated two rapeseed cultivars’ root morphology, root exudates, soil nutrients, and microbial community structure under different K supply levels and water conditions to construct a defensive network of the underground part. We found that K supply increased the root length and density and the organic acids secretion. The organic acids were significantly associated with the available potassium decomposition, in order of formic acid > malonic acid > lactic acid > oxalic acid > citric acid. However, the mitigation had the hormesis effect, as the appropriate range of K facilitated the morphological characteristic and physiological function of the root system with increases of supply levels, while the excessive input of K could hinder the plant growth. The positive effect of K-fertilizer on soil pH, available phosphorus and available potassium content, and microbial diversity index was more significant under the water stress. The rhizosphere nutrients and pH further promoted the microbial community development by the structural equation modeling, while the non-rhizosphere nutrients had an indirect negative effect on microbes. In short, K application could alleviate drought stress on the growth and development of plants by regulating the morphology and secretion of roots and soil ecosystems.

Background: Halometabolites, an important group of natural products, generally require halogenases for their biosynthesis. Actinomycetes from the Arctic Ocean have rarely been investigated for halogenases and their gene clusters associated, albeit great potential of halometabolite production has been predicted. Therefore, we initiated this research on the screening of halogenases from Arctic marine actinomycetes isolates to explore their genetic potential of halometabolite biosynthesis. Results: Nine halogenase genes were discovered from sixty Arctic marine actinomycetes using in-house designed or previously reported PCR primers. Four representative genotypes were further cloned to obtain full coding regions through genome walking. The resulting halogenases were predicted to be involved in halogenation of indole groups, antitumor agent ansamitocin-like substrates, or unknown peptide-like compounds. Genome sequencing revealed a potential gene cluster containing the halogenase predicted to catalyze peptide-like compounds. However, the gene cluster was probably silent under the current conditions. Conclusions: PCR-based screening of halogenase genes is a powerful and efficient tool to conduct bioprospecting of halometabolite-producing actinomycetes from the Arctic. Genome sequencing can also identify cryptic gene clusters potentially producing new halometabolites, which might be easily missed by traditional isolation and chemical characterization. In addition, our study indicates that great genetic potential of new halometabolites can be expected from mostly untapped actinomycetes from the polar regions.

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