10:45 AM - 12:15 PM
[SEM15-P14] Isolation of microbial cells and observation of magnetic bacteria from sediments collected from the Kokubu River near Takasu, Kochi, Japan
Keywords:paleomagnetism, magnetic bacteria
Seabed sediments contain magnetic particles, which acquire natural remanent magnetisation (NRM) by orientating themselves according to the orientation and strength of the earth's magnetic field at the time of deposition, providing an almost continuous record of past variations in the earth's magnetic field. For example, some sediments are estimated to carry 20-30 % of the NRM in magnetite of magnetobacterial origin, pointing to their quantitative importance (e.g.Yamazaki,2012; Yamazaki and Ikehara.,2012). Although there has been a vigorous search for magnetic bacteria in diverse environments and morphological and phylogenetic studies of these bacteria, there has been no quantitative assessment of their contribution to the NRM acquired by the sediments.
In this study, sediment cores were collected from the bottom of the Kokubu River near Takasu, Kochi, in order to investigate a method for quantitatively evaluating the proportion of magnetic particles of magnetic bacterial origin contributing to the NRM acquired by sediments during the present era. To avoid sandy sediments, a push core with a diameter of approximately 5 cm was collected from the river bottom where muddy sediments were exposed, at a time and date selected when the tide level was negative.
The sediment cores were promptly permeated with a mixture of 1.5% NaCl and 10% formalin to 'fix' the microbial cells in the sediment. The cores were then divided at 5 cm intervals in the direction of depth and three 25 cc syringes were inserted from the top surface of each and aliquoted into 50 ml tubes. The samples were then promptly 'fixed' by adding 10% neutral buffered formalin and measuring up to about 40 ml. Cells were collected from two samples at a depth of 15-20 cm by density gradient centrifugation (density separation) and each was concentrated to 5 ml. To increase the concentration of magnetic bacteria, magnetic separation was carried out using magnetic separation columns. 2 µl of the magnetically separated solution was dropped onto a Cu circular grid and dried spontaneously to prepare samples for transmission electron microscopy (TEM) observation.
TEM observations revealed numerous cells measuring 1-3 µm long by 0.5-1 µm wide. A total of 15 magnetic bacteria of two types were identified, 14 of which formed 3-12 teardrop-shaped magnetic particles 100-150 nm long by 20-30 nm wide in the cells, and one formed six cuboidal magnetic particles 50-80 nm in diameter. As most of the magnetic bacteria identified were teardrop-shaped magnetic particles, it is possible that this species is the dominant species. Elemental analysis and electron diffraction image analysis of the teardrop-shaped magnetic particles showed that they were magnetite.
In this study, sediment cores were collected from the bottom of the Kokubu River near Takasu, Kochi, in order to investigate a method for quantitatively evaluating the proportion of magnetic particles of magnetic bacterial origin contributing to the NRM acquired by sediments during the present era. To avoid sandy sediments, a push core with a diameter of approximately 5 cm was collected from the river bottom where muddy sediments were exposed, at a time and date selected when the tide level was negative.
The sediment cores were promptly permeated with a mixture of 1.5% NaCl and 10% formalin to 'fix' the microbial cells in the sediment. The cores were then divided at 5 cm intervals in the direction of depth and three 25 cc syringes were inserted from the top surface of each and aliquoted into 50 ml tubes. The samples were then promptly 'fixed' by adding 10% neutral buffered formalin and measuring up to about 40 ml. Cells were collected from two samples at a depth of 15-20 cm by density gradient centrifugation (density separation) and each was concentrated to 5 ml. To increase the concentration of magnetic bacteria, magnetic separation was carried out using magnetic separation columns. 2 µl of the magnetically separated solution was dropped onto a Cu circular grid and dried spontaneously to prepare samples for transmission electron microscopy (TEM) observation.
TEM observations revealed numerous cells measuring 1-3 µm long by 0.5-1 µm wide. A total of 15 magnetic bacteria of two types were identified, 14 of which formed 3-12 teardrop-shaped magnetic particles 100-150 nm long by 20-30 nm wide in the cells, and one formed six cuboidal magnetic particles 50-80 nm in diameter. As most of the magnetic bacteria identified were teardrop-shaped magnetic particles, it is possible that this species is the dominant species. Elemental analysis and electron diffraction image analysis of the teardrop-shaped magnetic particles showed that they were magnetite.