Observations of Magnetosome Organization, Surface Structure, and Iron Biomineralization of Undescribed Magnetic Bacteria: Evolutionary Speculations

Magnetotactic bacteria display one of the clearest behavioral responses to the geomagnetic field of any living organism, and are one of the few known prokaryotes which have the ability to produce intracellular biominerals. In the 15 years since they were discovered (10), these organisms have been found in environments ranging from freshwater to hypersaline, aerobic to anoxic, but usually in microaerophilic wnes (6, 11, 21, 31, 50, 53, 71, 76). They are also interesting from an evolutionary aspect because structures similar to their magnetosome chains (their 'biological bar magnets') have been discovered in eukaryotic algae (20, 72), as well as in vertebrates (46), where they may serve as part of a specialized geomagnetic sensory organelle (35, 37). The fossil record of these bacteria, based on the fossilized magnetosomes, or magnetofossils (36) now extends back nearly 2 billion years into Precambrian time, prior to the earliest known eukaryotes (17, 77). Hence, the presence of virtually identical magnetosome chains in the eukaryotes is consistent with an inheritance through the process of serial endosymbiosis (47). For the geosciences, the magnetic bacteria provide an important supply of fine-grained magnetite to sediments, where they are often preserved after the bacteria die (17, 34, 59, 71). The fossil bacterial magnetosomes, termed magnetofossils by Kirschvink & Chang (36), have the same morphology (17, 51, 71, 76) and crystal structure (75) as the crystals in the living bacteria (41-45). As the sediments solidify, the magnetofossils usually align themselves with the local geomagnetic field, and thereby preserve a record of its direction. Thus, these sediments often can be used to investigate the past history of the geomagnetic field. The accuracy of this recording process is influenced by the interaction between the bacteria and the sediment matrix, and hence may be influenced by the binding properties of the bacterial cell surface