Diversity, Characterization And Culture Of Marine Magnetotactic Bacteria

Magnetotactic bacteria are Gram-negative and represent a diverse group of microorganisms with respect to morphology, physiology and phylogeny. They have a cosmopolitan distribution in water columns or sediments with horizontal chemical stratification. The highest numbers of magnetotactic bacteria are generally found at the oxic-anoxic transition zone or redoxcline. Here, we studied the diversity of magnetotactic bacteria in a seawater pond within an intertidal zone at Huiquan Bay in the China Sea. The pond is composed of a permanently submerged part and a low tide subregion.The magnetotactic bacteria collected from the permanently submerged part display diversity in morphology and taxonomy. Their abundance reached up to 105 cells/cm3. Their diverse morphotypes, including cocci, ovoids, rods, vibrios and spirilla were observed, and the coccoid shape was most abundant. The magnetosomes of magnetotactic bacteria arranged in one, two or multiple chains. Irregular arrangement of magnetosomes was also evident. Morphology of magnetosome crystals was homogenous in a given cell, but displayed various shapes, including square, cubo-octahedron, parallelepiped, elongated hexagonal prismatic, bullet-shaped and tooth-shaped in different cells. The morphological diversity of marine magnetotactic bacteria was confirmed by restriction fragment length polymorphism analysis (RFLP) and sequence analysis of 16S rDNA. At least 10 different taxonomic groups were identified. Among them, nine belonged toα-Proteobacteria and one exception wasγ-Proteobacteria. Compared to known axenic or uncultivated magnetotactic bacteria, the 16S rDNA sequences of most magnetotactic bacteria collected from the permanently submerged part exhibited low sequence identities ranging from 76.4% to 89.4%. These results suggest a specific geo-distribution of the magnetotactic bacteria collected from the China Sea.In contrast, we found a virtually homogenous population of ovoid-coccoid magnetotactic bacteria in the low tide subregion of the pond. The dimensions of the cells ranged from 1.8 to 2.3 mm in width and 2.0 to 2.8 mm in length. They were bilophotrichously flagellated and exhibited polar magnetotactic behaviour. Almost all cells contained two chains of magnetosomes. The results of statistic analysis in TEM show that sixty per cent of cells had a difference of one magnetosome between the two chains. The number of magnetosomes per cell ranged from 7 to 31 with average of 18 and the occurrence of 19 magnetosomes per cell was the highest. Most of the magnetosome crystals in cells had a rectangular projected shape with a size of 101 + 24 nm (mean with standard deviation) in length and 83 + 21 nm in width. That gives a value of shape factor (width/length) about 0.83 + 0.09. They are likely to be single-domain magnetite. Analysis by energy dispersive X-ray spectroscopy (EDXS) unambiguously identified the'iron-oxide'nature of the magnetosome crystals. In addition, selected area electron diffraction analysis of individual crystals confirmed the magnetite nature of the magnetosomes in cells. The result of the magnetic hysteresis loop measurement indicated the magnetic moment of a single magnetosome was 2.6×10^-13 emu. The morphological diversity of marine magnetotactic bacteria was confirmed by RFLP and sequence analysis of 16S rDNA. At least 3 different taxonomic groups were identified. Pair-wise comparison revealed that 16S rDNA sequence identity showed >98% sequence identity each of them. The combination of restriction RFLP and sequencing of cloned 16S rDNA genes from the low tide subregion samples as well as fluorescence in situ hybridization (FISH) revealed the presence of a homogenous population. Moreover, phylogenetic analysis indicated that the Qingdao Huiquan low tide magnetotactic bacteria belong to a new genus affiliated with the a-subclass of Proteobacteria. This finding suggests the adaptation of the magnetotactic bacterial population to the marine tide. In this study, a novel marine magnetospirillum magneticum was cultured in semi-solid medium. Examined by TEM, the dimensions of the cells ranged from 3 mm in length and 0.8 mm in width. The cells contained one chain of magnetosomes. These magnetosomes were irregular in shape and the distribution of magnetosomes was asymmetric. But the pure strain was not obtained at present. This work is been doing now.