Every living organism uses small amounts of metals to perform biological functions, including breathing, transcribing DNA, turning food into energy, or a number of essential life processes.
Life has used metals in this way since single-celled organisms floated in Earth’s earliest oceans. Nearly half of the enzymes (proteins that carry out chemical reactions in cells) in organisms require metals, many of which are transition metals, named for the space they occupy in the periodic table.
Now a team of scientists from the University of Michigan, the California Institute of Technology and the University of California, Los Angeles claims that iron was the first and only transition metal in life. Their research has been published in the Proceedings of the National Academy of Sciences.
“We make a radical proposal: Iron was the original and only transition metal of life,” said Jena Johnson, assistant professor in UM’s Department of Earth and Environmental Sciences. “We argue that life depended only on metals with which it could interact, and that the iron-rich early ocean would make other transition metals essentially invisible.”
To explore this idea, Johnson joined UCLA professor Joan Valentine and Caltech researcher Ted Present.
Valentine, a bioinorganic chemist, became interested in how earliest life evolved from microscopic to the proliferation of complicated organisms that exist today. Specifically, she wondered which metals were incorporated into enzymes during early life so that organisms could carry out the necessary life processes. Repeatedly she heard other researchers say that the oceans were full of iron for the first half of Earth’s history.
“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 only present in small amounts,” Valentine said. “When these guys told me that iron wasn’t a trace element, I was amazed.”
Johnson, whose group studies iron formations and the biogeochemistry of the early oceans, and Present were familiar with geological evidence suggesting that early oceans were rich in iron — specifically an iron ion called Fe(II). Fe(II) is easily dissolved in water and is thought to have been the primary metal found in the oceans during the Archean Eon, a geological period that began about 4 billion years ago and ended about 2.5 billion years ago.
The end of the Archean Eon was marked by something called the Great Oxygen Event. At this time, life developed the ability to perform oxygen-producing photosynthesis. According to the researchers, over the next billion years, Earth’s ocean transformed from an iron-rich, anoxic sea to today’s oxygen-rich water. This also oxidized Fe(II) to Fe(III), making it insoluble.
Although Johnson and Present said that geologists at the time were aware of iron’s ubiquity on Earth, it wasn’t until they started talking to Valentine that they realized how great an impact iron might have been on early life.
To investigate the potential impact, Present designed a model that updated predictions of the concentrations of certain metals, including iron, manganese, cobalt, nickel, copper and zinc, that could have been available in Earth’s oceans when life began . The group was able to estimate the maximum concentration and availability of these elements for earliest life, he said.
“What changed most dramatically when the Great Oxygen Event happened wasn’t really the concentration of these other trace elements,” Present said. “What changed most dramatically was a decrease in dissolved iron concentrations. The implications for what that meant for life and how it ‘sees’ elements in water had not really been grappled with.”
After the group determined which metals were available in the early oceans, they investigated which metals simple biomolecules would bind to in these iron-rich solutions.
“We realized that iron would have to do almost everything,” Johnson said. “Biomolecules can capture magnesium and iron, but zinc can’t get in – maybe nickel can get into some biomolecules under the right conditions, but zinc is not competitive. Cobalt is invisible. Manganese is quite invisible. This order of magnitude difference in the concentration of iron in the oceans had a very tangible effect on what biomolecules can ‘see’ and bind to the environment.’
To determine whether iron would work in metalloenzymes that currently rely on other metals, Valentine and Johnson delved into the scientific literature to find out how life uses certain metals today. In each case, they found examples of how iron or magnesium could be substituted instead. Although a metalloenzyme can use a certain type of metal, such as zinc, they found that this does not mean it is the only metal the enzyme can use.
“Zinc and iron are a very dramatic example, because zinc is absolutely essential for life today,” Valentine said. “I found the idea of life without zinc very difficult to think about until we started looking into it and realized that as long as you don’t have oxygen around to oxidize your iron from Fe(II) to Fe(III), iron is often better than zinc in these enzymes.”
Present said that once iron oxidized and was no longer as bioavailable as it was before the Great Oxygen Event, life had to find other metals to plug into its enzymes.
“Life, in the face of orders of magnitude more iron than other metals, could not have evolved to such a sophisticated way of managing them,” Present said. “The fall of iron’s abundance forced life to manage these other metals to survive, but that also made possible new functions and the diversity of life we have today.”
