Some microbes used toxic gases to fight iron in Earth’s early oceans, geomicrobiologists say

Early in Earth’s development, the atmosphere did not contain oxygen. However, dissolved iron in the oceans was oxidized in gigantic quantities and deposited as rock. For example, it can be seen today in the form of banded iron ore in South Africa.

A new study examines how various bacteria excrete insoluble iron as part of their metabolic processes. Some, phototrophic iron oxidizers, gain energy by oxidizing iron using sunlight, and others by reacting iron with nitrate as an oxidizing agent.

An international research team including Dr Casey Bryce from the University of Bristol, Dr Verena Nikeleit and Professor Andreas Kappler, geomicrobiologists from the University of Tübingen, examined these processes and asked: Which microbes had the upper hand in the competition for iron? Rival bacteria also used nitrogen monoxide, a toxic gas.

The study was published in the journal Natural geosciences.

Two to three billion years ago, the composition of the Earth’s atmosphere was completely different.

“The oceans at that time contained large quantities of iron in its reduced form. Under today’s conditions, it would have been rapidly oxidized by atmospheric oxygen to form rusty iron minerals,” explains Kappler. Although there was no oxygen during this first phase on Earth, huge rock deposits of iron show that microbes were already effectively oxidizing it.

Laboratory experiments

“Before there was oxygen on Earth, phototrophic iron oxidizers formed huge deposits of iron oxide known today as banded iron ores,” explains Dr. Casey Bryce , project manager. Formerly of the University of Tübingen, Bryce is now a senior lecturer in the School of Earth Sciences at the University of Bristol.

“We wanted to know if these bacteria were in competition with other iron oxidizers using nitrate,” she adds. This led to the question of whether these competing microbes could actually coexist and, if so, which of them were primarily responsible for iron oxidation.

“In order to better understand the situation at the beginning of the Earth, we carried out laboratory experiments,” explains Verena Nikeleit, who since the study joined the Norwegian research center NORCE.

The research team used one bacterial strain from each of the different iron oxidizers and allowed them to grow under the conditions that prevailed two to three billion years ago, in light and with the same iron concentrations. , nitrate and carbon dioxide.

“To our surprise, the nitrate was quickly depleted and the iron oxidized. But we could not detect any iron oxidation by phototrophic iron oxidizers,” says Nikeleit.

Analyzes showed that nitrate-consuming iron oxidizers formed nitrogen monoxide as a toxic byproduct. “This completely shut down the activity of the phototrophic iron oxidants. In other words, these microbes killed the phototrophic iron oxidants by producing a toxic gas.”

A complex network of interactions

“One hypothesis is that phototrophic iron oxidizers probably contributed very little to the formation of banded iron ores during later phases of Earth’s history,” says Kappler. Indeed, the activity of other microbes caused Earth’s atmosphere to contain more and more oxygen – a sort of first major environmental pollution event.

“It may also have reached certain areas of the oceans where nitrates could then form. Our results provide the first experimental evidence for the hypothesis that phototrophic iron oxidizers in areas of high productivity may have been exposed to carbon dioxide. toxic nitrogen during this period They had to move away from nutrient-rich areas and therefore were not able to store as much iron.

According to Casey Bryce, based on the research team’s calculations, iron oxidation by nitrate-reducing bacteria may have initially compensated for the reduced contribution of phototrophic iron oxidants.

“The initial competition between the different bacteria would therefore not immediately stop the formation of banded iron formations,” she explains. Further measurements and investigations are needed to obtain a more accurate picture of the processes.

“Our study provides insight into how oxygen enrichment of Earth’s atmosphere might have affected other nutrient cycles in the oceans. This illustrates the complex network of biogeochemical interactions that controlled life in the early oceans. of the Earth,” Bryce explains.