Team:Hong Kong-CUHK/Project/electricity

Controlled Bacterial Magnetic-movement

Rapid uni-directional movement of living cells in a controlled manner continuously provides concentration gradient of the solution. The gradient is important for electricity generation. Once we could control the site where bacteria absorb Cl- and the site where bacteria release Cl-, a concentration graient of Cl- could be maintained which is critical for a constant electricity generation.

The strategy for controlling cell movement is through providing a steady attractive force which directs the cell movement. Micro-magnetic particles which contains anti-E.coli antibody allow E.coli to attach to the particle. Once the bacteria attach to the particle, we can attract the bacteria by a magnetic field.

In our proof-of-concept design, we first mixed and incubated the antibody-tagged micro-magnetic particles and the bacteria with RFP Coding Device (BBa_J04450). After the incubation, bacteria would attach on the particles to form bacterial complexes. The complexes could be isolated by magnetic field. The complexes were then added to a salt solution for illustrating our biobrick BBa_K559010 functioning under NaCl solution. A magnetic field was applied again to remove the bacterial complexes from the solution.

Figure 1. This is the microscopic view of the bacteria with RFP Coding Device (BBa_J04450) coated on antibody-tagged micromagnetic particles

The results show magnetic field controlled the movement of the bacteria being captured and moved by our control. In our method, we use 5ug of antibodies tagged on magnetic particles to capture the bacteria. Without magnetic complex capturing bacterial cell, the O.D. 600 after 1 hour incubation did not change much. While, with magnetic complex capturing bacterial cells, the O.D. 600 after 1 hour incubation change around 0.2, that is 4x107 cells/ug antibodies used.

Figure 2. Under magnetic attraction (magnet was located on the right side), the bacterial complex started to move, shown by the red arrow.

The below video shows the formation of bacterial-magnetic complex.

Movie 1: Excess bacteria with Biobrick BBa_J04450 transformed were placed in container. Fixed amount of antibody-tagged micro-magnetic particles were added and incubated for 1 hour at room temperature. The bacterial-magnetic complex were isolated by a fixed magnetic field.

Movie 2: The bacterial magnetic complexes were placed in NaCl solution for. After its function, the complexes were removed from the salt solution by a fixed magnetic field applied for 30s.

Movie 3: The microscopic view on the movement of bacterial complexes under magnetic field. The micromagnetic particle with bacteria loaded on it showsed a spherical shape. In the presence of a magnetic field, the complexes moved along the magnetic field towards the magnet, and they might conjugate together to form a large bacterial complex.

From proof-of-concept to large scale design Although we can successfully use the mixing-entropy battery to generate electricity, it does have some limitations. First of all, as we use seawater as the source of saline water, the use of this battery is restricted to the regions near the estuary. Secondly, the efficiency of this battery is constricted by the chloride concentration of seawater which is around 0.4-0.6 mM.

In order to convert solar energy to electricity in a larger scale and extend its application to more districts, we improved this prototype to a light-driven electricity power generation plant, (Fig. 1). We separate the whole process into three parts: salinity separation, power generation and material recycling, aiming at maximizing the efficiency of power generation at each part.

In salinity separation, there are two reservoirs containing high salinity sodium chloride solution and low salinity sodium chloride solution respectively. They are connected by a pipeline, which allows material exchange and bacteria movement. Halorhodopsin-transformed bacteria move uni-directionally from low salinity reservoir towards high salinity reservoir. There is illumination to the low salinity reservoir, the pipeline and part of the high salinity reservoir. Bacteria in these regions can uptake chloride ion, in order to carry chloride ions from low salinity reservoir to high salinity reservoir against chloride ion diffusion.

By this means the salinity potential between low salinity reservoir and high salinity reservoir can be maintained. We propose to employ magnetic field to control the movement of bacteria.

Although using antibody-tagged micro-magnetic particles to allocate bacteria and control its movement is workable, as shown in our result part, it is not suitable to use in large scale. It is because when antibody tagging is used in a large scale, a large number of antibodies are needed and it will severely increase the construction cost of the system. Also, as the stability of the tagged antibodies is not high, when it is used in such a complex and large system, there is high likelihood that the antibodies will detach from the bacteria and therefore reduce the efficiency of the system.

To overcome this obstacle, we proposed an alternative approach for bacteria movement. In the nature, a group of bacteria called magnetotatic bacteria, with inborn special organelle magnetosomes, have magnetotaxis to move along magnetic field as weak as the earth’s magnetic field. In 2003, D. Schultheiss et al. successfully developed transformation platform for Magnetospirillum gryphiswaldense, one model organism of magnetotatic bacteria1. It is possible to accomplish salinity separation with halorhodopsin-transformed magnetotatic bacteria under the control of magnetic field, even the earth’s magnetic field.

Similar to the design of mixing-entropy battery, in power generation, high salinity sodium solution and low salinity solution from reservoir respectively are alternatively flushed to power generation compartment cycle by cycle. After power generation, solutions are collected in high salinity collection tank and low salinity collection tank respectively. These two tanks are also connected by a pipeline, which allows bacteria to move from high salinity solution towards low salinity solution while releasing chloride ions in dark. All bacteria, solution and materials are returned back to reservoirs respectively to repeat the recycling process.