7.18: Other Unique Bacteria

Magnetic bacteria exhibit a directed movement called magnetotaxis, driven by structures called magnetosomes. These magnetosomes consist of chains of magnetic particles made of either magnetite (Fe₃O₄) or greigite (Fe₃S₄) and are organized in a linear conformation by a protein scaffold within invaginations of the cell membrane. The bacteria align along the north–south magnetic field lines, much like a compass needle. They are typically microaerophilic or anaerobic and are commonly found near the oxic–anoxic interface in sediments or stratified lakes. Aerobic species generally contain magnetite in their magnetosomes, while anaerobic species contain greigite.

The ecological role of bacterial magnetosomes remains unclear, but they may provide a selective advantage by maintaining the bacteria in low-oxygen zones. Oxygen levels typically decrease with depth in sediments and stratified lakes. Since Earth’s magnetic field has a strong vertical component in both hemispheres, magnetotactic bacteria align with these field lines to swim downward, away from oxygen. The magnetosome functions like a compass, while flagellar movement is controlled by a chemotactic response to oxygen.

Magnetic bacteria display different magnetic polarities based on the orientation of their magnetosomes. In the Northern Hemisphere, north-seeking bacteria move downward, while in the Southern Hemisphere, south-seeking bacteria do the same.

Most magnetic bacteria belong to the Alphaproteobacteria, but species have also been found in the Gammaproteobacteria, Deltaproteobacteria, and Nitrospira group. One well-characterized species, Magnetospirillum magnetotacticum, is a chemoorganotrophic microaerophile that can also grow anaerobically by reducing nitrate (NO₃⁻) or nitrous oxide (N₂O). Another species, Desulfovibrio magneticus, is a sulfate-reducing obligate anaerobe. Magnetosomes have also been observed in sulfur oxidizers and purple nonsulfur bacteria.

Additionally, multicellular magnetotactic bacteria exist. These Deltaproteobacteria form aggregates of 10–20 cells arranged in a hollow sphere. While they are obligate anaerobes, their exact metabolic pathways remain unknown.

Deinococcus radiodurans, a member of Deinococcales, is highly resistant to radiation and desiccation, surviving doses of 15,000 Gy. It is often red or pink due to carotenoids and is isolated from soil, meat, dust, and air. While resistant to most mutagens, it is vulnerable to DNA deletions.

Its unique DNA repair system includes multiple enzymes, allowing efficient recovery from radiation damage. D. radiodurans arranges its DNA in a toroidal structure, which facilitates homologous recombination, enabling fragmented chromosomes to be reassembled. This exceptional repair capability allows the cell to resume growth and division after extreme DNA damage.