A variety of biological materials including schwertmannite, jarosite, iron-sulfur cluster (ISC) and magnetosomes can be produced by Acidithiobacillus ferrooxidans (A. ferrooxidans). Their possible formation mechanisms involved in iron transformation, iron transport, and electron transfer were proposed. The schwertmannite formation usually occurs under the pH of 2.0-3.51, and a lower or higher pH will promote jarosite to be produced. Available Fe2+ in the environment and the carrier proteins that can transport Fe2+ to the intracellular membranes of A. ferrooxidans play a critical role in the synthesis of magnetosomes and ISC. The potential applications of these biological materials were reviewed, including removal of heavy metal by schwertmannite, detoxification of toxic species by jarosite, the transference of electron and ripening the iron sulfur protein by ISC, and biomedical application of magnetosomes. Additionally, some perspectives for the molecular mechanisms of synthesis and regulation of these biomaterials were briefly described.

Biological materials formed by Acidithiobacillus ferrooxidans and their potential applications.

Characteristics of magnetotactic bacteria and magnetosomes were summarized in this paper. Conditions and affecting factors for magnetosomes formation were reviewed. Problems for mass culture of magnetotactic bacteria and enhancement of magnetosomes yields were indicated. Methods to resolve these problems were proposed. A hypothesis involved in mechanism and physiological function of magnetosomes formation was suggested. Ferrous irons were transported into magnetotactic bacteria cells resulted in ferric iron transportation through membranes as electron acceptors under stresses of low oxygen levels. It is to avoid cytotoxicity of ferrous iron and further transform to Fe3O4 on surface of inner membrane the reason of magnetosome formation.

Characteristics of Magnetotactic Bacteria and Biosynthesis Conditions of Nano-magnetosomes

Magnetotactic bacteria magnetosomes are magnetic nanoparticles enclosed by biofilms and arrange in the form of a chain. Magnetic nanoparticles formed by the biomineralization of magnetosome usually have regular shape, uniform particle size and high crystallinity, which have attracted extensive attention of researchers. The magnetosome membrane is composed of phospholipids and fatty acids, and the magnetosome membrane lipid vesicle acts as a nanoreactor which controls the precise synthesis of magnetic nanoparticles. A series of biomineralization proteins in the magnetosome membrane control the iron transport, redox reaction, nucleation and growth of the magnetic nanoparticles. At present, the specific biomineralization process of magnetosome is still unclear and the large-scale production of magnetosome is difficult, so the biomimetic synthesis of magnetosome has been initiated. In vivo studies have shown that magnetosome proteins of magnetotactic bacteria including Mms6, MamC, MmsF, MamG and MamD play an important role in regulating the size and morphology of the magnetosome, and they have been identified as the best candidates for the biomimetic synthesis of magnetosome. Biomimetic synthesis of magnetic nanoparticles mediated by recombinant magnetosome proteins such as Mms6, MamC and MmsF has been studied. These studies can not only help us better understand the biomineralization process of magnetosome, but also help us prepare high-quality magnetosome-like magnetic nanoparticles without the use of organic solvents, surfactants and high reaction temperature. This article mainly reviews the research progress of the biomimetic synthesis of magnetic nanoparticle mediated by several key magnetosome proteins and prospects its future development. Mms6 seems to mainly control over the growth kinetics of magnetic crystal by acting as an iron reservoir. MamC seems to preferably control the nucleation kinetics of magnetic crystals due to the ionotropic and template effects. MamP as an iron oxidase can support to form ferrihydrite which required for the formation of magnetite crystals in vivo . Although Mms6 is the most abundant magnetosome protein and there are the most researches on Mms6 mediated biomimetic synthesis of magnetic nanoparticles, the specific regulation mechanism of Mms6 is still unclear. In addition, the production cost of Mms6 is relatively high. These are all the shortcomings of Mms6 mediated biomimetic synthesis of magnetic nanoparticles. Therefore, in order to realize large scale biomimetic preparation of high-quality magnetic nanoparticles with controllable size and morphology, it is necessary to further clarify the regulation mechanism of Mms6 and develop cheaper and simpler methods to prepare Mms6 based functional polypeptides and additives. The biomineralization of magnetosome is a process regulated by a variety of mineralization proteins. Therefore, if several kinds of magnetosome proteins with different functions were used together to regulate the formation process of magnetic nanocrystal in the biomimetic synthesis methods, magnetic nanocrystals with more abundant morphology, size and surface crystal structure would be synthesized. Particularly, if MamP, MamE and other magnetosome proteins which control the redox reaction were used to assist the biomimetic synthesis of magnetic nanocrystals, the synthesis conditions might be further optimized, and the yield and performance of magnetic nanoparticles might be improved. In addition, constructing a micro lipid vesicle reactor which like the magnetosome membrane through the surface assembly of key magnetosome proteins and phospholipid molecules to control the biomimetic synthesis process is expected to introduce novel ideas and methods for the biomimetic synthesis of magnetic nanoparticles.

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Biomimetic synthesis of magnetic nanoparticles mediated by magnetosome proteins

Magnetosomes are natural magnetic nanoparticles with special properties that are synthesized by magnetotactic bacteria. Bacterial magnetosomes have become increasingly attractive for researchers in biology, medicine, geology and other fields of scientific researches. It is significant to explore a promising nanomaterial for multiple applications in science and industry. Magnetosomes contains magnetic particles that are enclosed by intracellular membrane. These particles can be either ferrimagnetic crystal of magnetite (Fe3O4) or the iron sulfide greigite (Fe3S4). Three crystal morphologies including roughly cuboidal, roughly rectangular, bullet-shaped have been found in magnetotactic bacteria. Magnetosomes are organized as well-ordered chains which orient magnetotactic bacteria in geomagnetic fields. The question ‘what and how is genetic material responsible for magnetosome genesis and following diversity’ was raised when biologists tried to understand the formation mechanism of magnetosome in the last century. Now almost two decades have passed since 2000, magnetosome biogenesis still remains one of the hotspots in scientific researches. Both bioinformatic analyses and gene deletion experiments are extensively carried out in the study of magnetosome biogenesis. Consequently, multiple operons and genes are found to be involved in the process of magnetosome formation. This article introduces important findings in the field of operons and genes as controllers of magnetosome biogenesis. The mamAB operon is found to be more critical than other operons to magnetosome biogenesis and it is necessary for rudimentary biomineralization in bacteria. Less than 10 genes within the mamAB operon are essential for the magnetosome formation. Moreover, other four small operons ( mamGFDC , mamXY , mms6 and feoAB1 ) play non-critical roles for magnetosome biogenesis. Non-essential genes can be found in both the mamAB operon and non-critical operons. In addition, several genes and genomic regions outside of magnetosome-related operons such as the mms16 gene in Magnetospirillum magneticum AMB1 or the mad gene clusters in Deltaproteobacteria are also related to magnetosome synthesis. These discoveries provide insights into the molecular mechanisms of magnetosome biogenesis. Some researchers have separated the whole process of magnetosome formation into three or five key steps, they proposed several hypothesized mechanisms of magnetosome formation in magnetotactic bacteria by summarizing functional genes and proteins in each of key steps. The Arakaki’s model in 2018 is a relatively comprehensive model of magnetosome formation and can offer detail information about molecular and cellular mechanisms. With rapid development of research on magnetotactic bacteria emerging in China in recent years, Chinese scientists have made various types of research achievements such as discoveries of novel magnetotactic bacteria, calculations of diversity of magnetotactic bacteria in specific water areas and evolutionary mechanism of microbial magnetoreception. It is expected that more valuable achievements would be obtained from bacterial research than ever before. Natural magnetic nanoparticles have potentials to bring many advancements in science and they can help overcome technical challenges in various scientific fields. To release the power within magnetosomes, biologists are required to reveal the genetic foundation of magnetosome biogenesis and help material scientists invent new technology using biological knowledge. Magnetosome researches are attracting more and more scientists from all areas of science. Interdisciplinary research has become an increasingly common method of study in magnetosome. How well the material scientists can cooperate with other scientists will be crucial for important discoveries in magnetosomes in the future.

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Shiying He

Papers

The operons and genes related to magnetosome biogenesis in magnetotactic bacteria

Biomimetic synthesis of magnetic nanoparticles mediated by magnetosome proteins