Introduction

Magnetotactic bacteria (Fig. 5.1A–E) are micro-organisms which feel the torque of the Earth's magnetic field because they produce strongly magnetic particles of magnetite or greigite within their cells. They have been identified in diverse sedimentary environments including lacustrine, brackish, and marine sediments, rivers, saltmarshes, ponds, terrestrial soils and also within stratified ocean waters (Table 5.1). The magnetic particles formed by magnetotactic bacteria may contribute significantly to the magnetic properties of sediments. If, upon death and lysis of the bacteria, the magnetic particles survive burial and become magnetofossils, they may play a significant role in the magnetic record of Quaternary and older sediments. They may thus encode information about the Earth's magnetic field direction and intensity, and the environment at the time of their existence.

Early studies on the physiology of magnetotactic species such as Magnetospirillum magnetotacticum (Blakemore et al., 1979, 1984) and Bifidococcus magnetotacticum (Moench, 1989) suggested that all magnetotactic bacteria are microaerobic (i.e. that they require small amounts of molecular oxygen for their metabolism). This prompted some workers to suggest that their occurrence in extant sediments, and their magnetofossils in ancient sediments, could be used as an indicator of microaerobic conditions (Kirschvink & Chang, 1984; Rhoads et al., 1991). However, we now know that there are several different physiological types of magnetotactic bacteria, including strict anaerobes (species which can only grow in the absence of oxygen). Thus, bacterial magnetite may contribute to the magnetic properties of both oxic and anoxic sediments.