Magnetic optimization in a multicellular magnetotactic
organism
Michael Winklhofer 1*, Leida G Abracado 2, Alfonso F Davila 3, Carolina N Keim 4 and Henrique GP Lins de Barros 2
1 University of Munich
2 CBPF Rio de Janeiro
3 Nasa Ames Research Center
4 Universidade Federal do Rio de Janeiro
* To whom correspondence should be addressed. E-mail: michaelw{at}lmu.de.
Submitted on July 21, 2006
Revised on August 24, 2006
Accepted on 3 October 2006
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Abstract |
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Unicellular magnetotactic prokaryotes, which typically carry a natural remanet magnetic moment equal to the saturation magnetic moment, are the prime example of magnetically optimized organisms. We here report magnetic measurements on a multicellular magnetotactic prokaryote (MMP) consisting of 17 undifferentiated cells (mean from 148 MMPs) with chains of ferrimagnetic particles in each cell. To test if the chain polarities of each cell contribute coherently to the total magnetic moment of the MMP, we used a highly sensitive magnetization measurement technique (1 fAm2) that enabled us to determine the degree of magnetic optimization (DMO) of individual MMPs in vivo. We obtained DMO values consistently above 80%. Numerical modelling shows that the probability of reaching a DMO>80% would be as low as 0.017 for 17 randomly oriented magnetic dipoles. We simulated different scenarios to test whether high DMOs are attainable by aggregation or self-organization of individual magnetic cells. None of the scenarios investigated is likely to yield consistently high DMOs in each generation of MMPs. The observed high DMO values require strong Darwinian selection and a sophisticated reproduction mechanism. We suggest a multicellular life-cycle as most plausible scenario for transmitting the high DMO from one generation to the next.
Key Words:
bacterial colonies, biogenic magnetic materials, greigite, magnetization measurement techniques;, self-organisation, swimming microorganisms
