Living organisms depend on tiny quantities of metals to carry out essential biological processes such as breathing, DNA transcription, and energy production. These processes have relied on metals since life first appeared in Earth's early oceans. Many of the enzymes within organisms require metals, particularly transition metals, which occupy a specific space on the periodic table.
A research team from the University of Michigan, California Institute of Technology, and University of California, Los Angeles, has put forth the argument that iron was the earliest and possibly the only transition metal used by life at that time. Their findings are published in the 'Proceedings of the National Academy of Sciences'.
"We make a radical proposal: Iron was life's original and only transition metal," said Jena Johnson, assistant professor in the U-M Department of Earth and Environmental Sciences. "We argue that life only relied on metals that it could interact with, and the iron-rich early ocean would make other transition metals essentially invisible."
Johnson collaborated with UCLA professor Joan Valentine and Caltech researcher Ted Present to explore this idea. Valentine, a bioinorganic chemist, became intrigued by the metals used by early life forms to support biological processes. She was particularly fascinated by the fact that Earth's early oceans were full of iron.
"You have to understand that in my field of biochemistry and bioinorganic chemistry, in medicine and in life, iron is a trace element. These are elements that are present only in small amounts," Valentine said. "When these guys told me that iron wasn't a trace element, that blew my mind."
Johnson's research into early ocean biogeochemistry supported the idea that Fe(II), an ion of iron, was abundant in the oceans during the Archean Eon, a time period that began about 4 billion years ago and ended 2.5 billion years ago. During this time, Fe(II) dissolved readily in water and was the primary metal present. This era ended with the Great Oxygenation Event, a time when organisms evolved the ability to perform oxygen-producing photosynthesis. As a result, iron was oxidized from Fe(II) to Fe(III), becoming insoluble in the oceans.
The researchers realized how this shift in iron availability could have influenced the development of life. To further investigate, Present developed a model predicting the concentrations of various metals, including iron, manganese, cobalt, nickel, copper, and zinc, that would have been present in early oceans. He estimated the availability of these metals to the earliest life forms.
"The thing that changed most dramatically as the Great Oxygenation Event occurred was not really the concentration of these other trace elements," Present explained. "The thing that changed the most dramatically was a decrease in dissolved iron concentrations. The implications for what that meant for life and how it 'sees' elements in water hadn't really been wrestled with."
After determining which metals were present in the early oceans, the team examined how simple biomolecules would have interacted with these metals in the iron-rich environment.
"We realized iron would have to do almost everything," Johnson said. "Biomolecules could capture magnesium and iron, but zinc's not getting in-maybe nickel can get into some biomolecules in the right circumstances, but zinc's not competitive. Cobalt is invisible. Manganese is pretty invisible. This order of magnitude difference in the concentration of iron in oceans had this really tangible effect on what biomolecules can 'see' and bind from the environment."
Valentine and Johnson also looked at current metalloenzymes to see how metals are used today. They discovered that in some cases, iron or magnesium could substitute for other metals such as zinc.
"Zinc and iron is a really dramatic example because zinc is absolutely essential for life now," Valentine noted. "The idea of life without zinc was really hard for me to think about until we dug into this and realized that as long as you have no oxygen around to oxidize your iron from Fe(II) to Fe(III), iron is often better than zinc in these enzymes."
Present added that after the oxidation of iron during the Great Oxygenation Event, life had to adapt by finding other metals to support its enzymes.
"Life, in the face of orders of magnitude more iron than other metals, couldn't know to evolve toward such a sophisticated way of managing them," Present said. "The fall of the abundance of iron forced life to manage these other metals to survive, but that also enabled new functions and the diversity of life we have today."
Research Report:Iron: Life's primeval transition metal
