Diversity of multicellular magnetotactic prokaryotes in the intertidal zone of Huiquan Bay, Qingdao

Multicellular magnetotactic prokaryotes (MMPs) are a group of aggregates composed of 10-100 gram-negative cells synthesizing intracellular magnetic crystals. Two morphotypes of MMPs have been identified, including several species of globally distributed spherical mulberry-like MMPs (sMMPs), and ellipsoidal pineapple-like MMPs (eMMPs). We recently collected MMPs from the intertidal zone of Huiquan Bay, Qingdao. Optical microscopy showed that there were two types of MMPs in the area, including sMMPs and eMMPs. We observed the size of eMMPs was 9.25 ± 0.79 × 7.48 ± 0.79 μm (n = 24), and the average diameter of sMMPs was 5 ± 0.66 μm (n = 24). Transmission electron microscopy showed that these MMPs contained three sizes of bullet-shaped crystals in parallel chains or clusters. The length and width ratios of the sizes of these magnetosomes were 4.16±0.64, 3.07±0.29 and 2.51±0.36 (n=44). The 16S rRNA gene of micromanipulation-purified sMMPs and eMMPs were cloned and sequenced. Phylogenetic analysis based on the 16S rRNA gene sequence revealed that 565 sequences of MMPs belonged to 16 OTUs, affiliated with Deltaproteobacteria. Four OTUs displayed >3.48% sequence divergence and two OTUs displayed >7.26% sequence divergence with respect to previously reported MMPs. This result suggested that they represented six new species and two novel genera. These results indicated that the intertidal zone of Huiquan Bay has a high diversity of MMPs that bio-mineralize iron crystals and play an important role in iron cycling in such a complex environment. These observations provide a new perspective of the diversity of MMPs in general and expand knowledge of the occurrence of MMPs in the Huiquan Bay.

Keywords: Intertidal zone, Diversity, Multicellular magnetotactic prokaryotes (MMPs),  16S rRNA gene, Magnetosomes