Tübingen and Bristol Universities Uncover Microbial Warfare Over Iron in Early Earth Oceans
On the early Earth, the atmosphere did not yet contain oxygen; nevertheless, the iron dissolved in the oceans was oxidized in gigantic quantities and deposited as rock, for example as banded iron ores in South Africa. Different bacteria excrete insoluble iron through their own metabolic reactions: some, the phototrophic iron oxidizers, gain energy by oxidizing the iron with the help of sunlight, and others by converting the iron using nitrate as an oxidizing agent. An international research team including Dr. Casey Bryce from the University of Bristol and Dr. Verena Nikeleit and Professor Andreas Kappler from Geomicrobiology at the University of Tübingen is investigating who had the upper hand in the competition for iron. The bacterial opponents also used toxic nitrogen monoxide. The study was published in the journal Nature Geoscience.
Two to three billion years ago, the Earth’s atmosphere had a completely different composition than it does today. “Today, the reduced iron present in the oceans at that time would quickly be oxidized by the oxygen in the atmosphere to form rusty iron minerals,” explains Andreas Kappler. Although there was no oxygen on the early Earth, huge rock deposits of iron show that microbes were already effectively oxidizing it at that time.
experiments in the laboratory
“Before oxygen existed on Earth, phototrophic iron oxidizers formed the huge iron oxide deposits known today as banded iron ores,” says Casey Bryce, leader of the project, formerly at the University of Tübingen, now at the University of Bristol. “We wanted to know whether these bacteria were in competition with other iron oxidizers that used nitrate.” This raised the questions of whether these competing microbes could actually coexist, and if so, which of them were primarily responsible for iron oxidation.
“To better understand the situation on the early Earth, we conducted laboratory experiments,” says Verena Nikeleit from the University of Tübingen, who has since moved to the Norwegian research center NORCE. The research team used one bacterial strain of each of the different iron oxidizers and let them grow under the conditions that prevailed two to three billion years ago, in the light and with the same concentrations of iron, nitrate and carbon dioxide. “To our surprise, the nitrate was quickly used up and the iron was oxidized. But we could not detect any iron oxidation by the phototrophic iron oxidizers,” says Nikeleit. The analyses showed that the nitrate-consuming iron oxidizers produced nitric oxide as a toxic byproduct. “This completely stopped the activity of the phototrophic iron oxidizers. In other words, these microbes killed the phototrophic iron oxidizers by producing a toxic gas.”
Complex network of interactions
“One hypothesis is that the phototrophic iron oxidizers probably contributed little to the formation of banded iron ores in later phases of Earth’s history,” says Andreas Kappler. This is because the activity of other microbes meant that the Earth’s atmosphere contained more and more oxygen – in what was essentially the first major environmental pollution event. “This may also have reached some areas of the oceans, where nitrate could then be formed. Our results provide experimental evidence for the first time for the hypothesis that phototrophic iron oxidizers in areas of high productivity may have been exposed to toxic nitrogen monoxide during this time. They must have moved further away from the nutrient-rich areas and were therefore able to deposit less iron.”
Casey Bryce reports that, according to the research team’s calculations, iron oxidation by nitrate-reducing bacteria may have initially compensated for the reduced contribution of phototrophic iron oxidizers. “The initial competition between the different bacteria would not immediately stop the formation of the banded iron formations,” she says. To get a more precise picture of the processes, further measurements and investigations are needed. “Our study provides an insight into how the oxygen enrichment of the Earth’s atmosphere may have affected other nutrient cycles in the oceans. This illustrates the complex network of biogeochemical interactions that controlled life in the Earth’s early oceans.”