Quantification of the Evolutionary Forces Shaping Biogenic Magnetite and Criteria for Detecting it in the Fossil Record

Abstract

One of the goals of the Exobiology Program is to learn more about the early evolution of life from the geological record in rocks. The detection of fossil microbes is difficult, and existing methods have faced a lot of controversy in the last few years. To get beyond the controversy we need clear quality criteria for biomarkers. I propose to develop quantitative criteria for detecting fossil magnetotactic bacteria.

A good biomarker should satisfy at least four criteria:
1. It should be difficult to synthesize inorganically
2. It should show the fingerprint of natural selection.
3. It should be well preserved.
4. It should be easy to detect in situ.
(The fourth criterion is desirable on Earth and necessary for planetary exploration). Using theoretical methods I will attempt to determine how well magnetosomes (chains of magnetite particles created by magnetotactic bacteria) meet the latter three criteria.

The proposal has three parts. First, I will develop an improved model for the motion of magnetotactic bacteria and compare magnetotaxis to other methods for searching for an optimal environment. This work will address another goal of the Exobiology Program, to improve our understanding of the evolutionary forces working on a simple life form.

In the second part I will test the hypothesis that magnetosomes are designed to maximize their magnetic moment. To do so I will calculate the limits on particle size, length-width ratio, crystallographic orientation and chain length if the magnetosomes are to be uniformly magnetized. I will also calculate the effect of defects on the magnetic moment.

Finally, in the third part I will develop models of the magnetic properties of magnetosomes to look for reliable methods of detecting biogenic magnetofossils. Experiments that I will simulate include low temperature cycles and ferromagnetic resonance. The models will include the effects of random orientation, size distributions, chain collapse and maghemitization. I will attempt to determine whether fossil magnetosomes can be detected when they are mixed with inorganic magnetite.