SEM micrograph of inner layer of rust from steel immersed in seawater for 2 years. Sulfate-reducing bacteria and/or iron-reducing bacteria attached on large and small green rust platelets. (From J. Duan et al. / Electrochimica Acta 54 (2008) 22-28)
Microorganisms, despite of their simple structure and tiny size, are often vital to humans and the environment. You might know about their contributions to the Earth's carbon cycle and amazing ability of decomposing waste products. But have you ever heard of microbes that eat away undersea metal, or those who protect iron from corrosion? Some may even wow you with the feat to generate electricity and clean up polluted water!
As the first forms of life to develop on Earth, these small creatures shall never be underestimated, according to marine chemist DUAN Jizhou with the Research and Development Center for Marine Corrosion and Protection, the CAS Institute of Oceanology. With strong supports from the National Natural Science Foundation of China, Duan, HOU Baorong and other co-workers have been working at the vast frontier of microbiologically influenced corrosion and their inhibition technologies for over a decade.
Microbial community dwelling in rust
In the oceanic environment which covers more than two thirds of the Earth's surface, iron is widely used for building bridges, ships and pipelines. Corroded steelwork under the sea not only brings about extra cost of construction materials, but may lead to disasters and loss of lives.
For a long time, people simply took the destruction process as the oxidation whereby iron reacts with oxygen. Later, scientists revealed the existence of microbes in rust samples, and came to realize a major role these tiny creatures have played.
"The rust presents an ideal habitat for some microbes," Duan notes.
In 2004, German experts obtained a novel species of bacteria which live on electrons provided by iron.
Later on, Duan and collaborators discovered several species of anaerobic bacteria and archaea from different rust layers. Through biological analyses, they even identified a microbial "community" in there.
Among the massive microorganisms living in the ocean, a considerable number of them tend to attach themselves on the surface of steelwork and thus form "biofilm" as scientists call it. At first, the biofilm is mainly composed of aerobic bacteria. Gradually, aerobic bacteria disperse and are largely replaced by anaerobic ones.
According to Duan, many experiments and observations have demonstrated anaerobic bacteria's power in iron corrosion, particularly for a group named sulfate-reducing bacteria (SRB). In seawater, SRB can use their enzymes to accelerate the reduction of sulfate compounds to hydrogen sulfide, which is highly corrosive to iron.
With stainless steel, Duan's group carried out on-the-spot experiments as well as laboratory simulations to study how SRB form biofilm on the metal's surface and eat away parts of it. They also investigated the transformation of the rusted steel from metallic oxide to metallic sulfide.
Bugs that breathe ion?
Rust layers are microbes' palace. At the same time, the little creatures need to find something to eat and breathe like we human beings do.
"There are plenty of nutrient substances in the ocean for their growth. So the question is what they respire."
A recent analysis by Duan's group shows there are at least two kinds of relevant microbes in their rust samples: SRB and iron-reducing bacteria (IRB). While SRB take in sulfate and releases corrosive hydrogen sulfide, IRB pass electrons onto iron during breathing.
"We speculate the respiration mechanism of these microorganisms is closely related to electron transfer. They give electrons to ferric ions, and reduce them to ferrous ones. It's just a way these bacteria live, and they don't have to know they've completed the mineralization of rust by just breathing in and out electrons," Duan so explains his new discovery.
This peculiar characteristic may be applied to the protection of iron. In industrial practices, people often coat less active metal with more active metal to protect the former from losing electrons and being eroded. In this sense, the ion-respiring bacteria, by passing electrons onto steel, just serves a barrier to prevent the oxidation of iron. Maybe scientists will be able to develop various bioactive film in the presence of such bacteria for cathodic protection in the near future.
Bacteria to clean water and generate power
State-of-the-art studies show that the interactions between microorganisms and undersea metals involve a very complex process of electron transfer. The interactions, experts say, might boost the development of technologies concerning corrosion prevention, water remediation and microbial fuel cells.
For example, a new species of SRB named Desulfovibrio dechloracetivorans, which was reported by Duan's group in 2008, can effectively degrade organic chlorine and similar pollutants in the ocean in the presence of iron.
Other bacteria (iron-reducing bacteria, D. R. Lovley) are able of decomposing compounds containing heavy metals like uranium in contaminated underground water.
"In the electron transfer process, the microorganism changes the metal from its dissolved form to a solid form," Duan says.
Scientists find that the biofilm on stainless steel is capable of accelerating oxygen reduction rate, which is now being applied to the research and development of marine sediment-related microbial fuel cells. As designed, the cell will have a bio-anode, where anaerobic microbes oxidize organic compounds and transport electrons to its bio-cathode, where electrons pick up reducing speed with the help of aerobic bugs. Once external load is added, the cell will be able to transform the organic compounds in the sediment and generate power. Researchers in the US, Belgium, France, Germany and Korea have carried out pioneering bio-anode studies, with an aim to develop bio-cathode aspect.
"However, we still need to overcome the limitations of current technologies, and to look for simpler and cheaper methods," the scientist adds.
In Duan's eye, there are still large areas of controversy for microbiologically influenced corrosion. For instance, its fundamental mechanism is not properly understood by far. Nevertheless, this is a very sophisticated but equally promising subject to work on.
