Observations on a magnetotactic bacteria-grazing ciliate in sediment from the intertidal zone of Huiquan Bay, China

Abstract : Magnetotactic bacteria (MTB) are a group of prokaryotes having the ability to orient and swim along geomagnetic field lines because they contain intracellular magnetosomes, which are synthesized through a biomineralization process. Magnetosomes have recently also been found in unicellular eukaryotes, which are referred to as magnetically responsive protists (MRPs). The magnetosomes have three origins in MRPs. In this study we characterized a MTB-grazing ciliated MRP that was magnetically collected from intertidal sediment of Huiquan Bay, China. Based on 18S rRNA gene sequence analysis, the ciliated MRP was tentatively identified as Uronemella parafilificum HQ. Using transmission electron microscopy, we observed that magnetosomes having 2-3 shapes were randomly distributed within this ciliate. Energy-dispersive X-ray spectroscopy and high-resolution transmission electron microscopy images of the magnetosomes were consistent with them being composed of magnetite. Magnetosomes having the same shape and mineral composition were also detected in MTB that occurred in the same environment as the ciliated MRP. Statistical analysis showed that the size and shape of the magnetosomes in the ciliated MRP were similar to those in MTB. The results suggest that this ciliated MRP can graze, ingest, and digest various types of MTB. It is certainly worth noting that this is the first record of MRPs in Asian aquatic sediment and suggesting they might be widely distributed. These results also support the assertion that MRPs probably contribute to the ecological cycles of iron, and expand possibilities for research into the mechanism of magnetoreception in eukaryotes.