There are rod-shaped bacteria that make their own built-in compass to help in navigating the layers of mud where they make their home. Similar to the needle on a compass that always points North, these creatures make their own internal compass by taking in iron and keeping it in storage sacks arranged in a chain. The magnetite crystals that form are strongly magnetic, and a chain of them orients itself with the magnetic field of the earth, just like the compass needle. The bacteria use that magnetic sense to navigate in the layers of muddy sediment to find the best mix of oxygen and sulfur ions.
Force fields of earth
Brodzik, M. J. and K. W. Knowles. 2002. EASE-Grid: A Versatile Set of Equal-Area Projections and Grids in M. Goodchild (Ed.) Discrete Global Grids. Santa Barbara, California USA: National Center for Geographic Information & Analysis.
Arash Komeili, Hojatollah Vali, Terrance J. Beveridge, and Dianne K. Newman, PNAS March 16, 2004 101 (11) 3839-3844; https://doi.org/10.1073/pnas.0400391101
The chain of black dots in A are the crystals. The organism is a Magnetospirillium sp.
(Cool aside - magnetite in bird beaks gives them magnetic senses as well and aids them in migration.)
Now, why would the bacteria want to find sulfur ions? Living things make energy by using a chain of reactions to produce energy. These bacteria can strip electrons from their “food”, the sulfur ions, send them through a series of reactions involving helper enzymes, and give those electrons to the oxygen molecules waiting at the end. In the process, energy for the bacteria to live and grow is generated. As long as there are BOTH sulfur ions and oxygen around, the process can keep going. It is as if a chain of electrons are “pulled” through the series of reactions involving enzymes and emerge at the end to be taken up by oxygen and hydrogen and become water. This whole process of “pulling” the electrons from the sulfur ions and giving them to the oxygen is what we call a REDOX reaction, and it produces energy the bacteria need to live and grow. Personally, I would rather eat a hamburger, but…
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