**Abstract**

Biomineralization in the microbial realm usually gives origin to finely structured inorganic nanomaterials. Perhaps, one of the most elegant bioinorganic processes found in nature is the iron biomineralization into magnetosomes, which is performed by magnetotactic bacteria. A magnetosome gene cluster within the bacterial genome precisely regulates the mineral synthesis. The spread and evolution of this ability among bacteria are thought to be a 2,7-billion-year process mediated by horizontal gene transfers. The produced magnetite or greigite nanocrystals coated by a biological membrane have a narrow diameter dispersibility, a highly precise morphology, and a permanent magnetic dipole due to the molecular level control. Approaches inspired by this bacterial biomineralization mechanism can imitate some of the biogenic nanomagnets characteristics in the chemical synthesis of iron oxide nanoparticles. Thus, this chapter will give a concise overview of magnetosome synthesis's main steps, some hypotheses about the evolution of magnetosomes' biomineralization, and approaches used to mimic this biological phenomenon *in vitro*.

**Keywords:** magnetotactic bacteria, magnetosomes, magnetic nanoparticles, magnetite, magnetosome gene cluster, horizontal gene transfer, biomimetics

### **1. Introduction**

Among everything that is known in Microbiology, magnetotactic bacteria (MTB) are known to perform one of the finest examples of a controlled biomineralization process. MTB were first observed in the late 1950s, by the medical Doctor Salvatore Bellini in the Italian city of Pavia and later described in Massachusetts by Richard Blakemore in the 1970s [1, 2]. MTB are known to align its motility axis to the geomagnetic field and use it for orientation. When observed under the light microscope, MTB present unidirectional swimming to the North or South Magnetic Poles from an applied external magnetic field (a magnet); this behavior is called magnetotaxis [3]. This behavior occurs due to the presence of magnetic

#### **Figure 1.**

*Transmission electron microscopy of: (A) uncultured coccoid magnetotactic bacteria from Monsimet Cove, Antarctica. (B) Uncultured coccoid magnetotactic bacteria from Punta Ullman, Antarctica. (C) Magnetovibrio blakemorei strain MV-1T .*

nanocrystals—the magnetosomes—, usually aligned in single or multiple chains within the bacterial cytoplasm (**Figure 1**), and flagellar propulsion guided by chemotaxis [3]. In a simple way, chemotaxis in MTB is assisted by bacterial orientation along Earth's magnetic field (magnetotaxis). Therefore, magnetotaxis allows MTB to find the optimum position for survival and growth in a chemically stratified water column, seeking for an optimum environment where proton motive driving force reaches maximum potential. For MTB, which are frequently microaerophilic or anaerobic microrganisms, this environment is near the oxic/anoxic interface [4].

Magnetosomes are composed of a magnetic nanoparticle in most cases composed of magnetite (Fe3O4) and sometimes greigite (Fe3S4) with species specific shapes and sizes, and enveloped by a phospholipid bilayer with associated proteins, which constitutes the magnetosome membrane (MM) [3]. The gene regulation of magnetosome biomineralization (MB) and organization within the cell will be discussed in more detail in the sections ahead. Based on the total iron amount within a magnetotactic bacterium cell, MTB appear to play a major role in the biogeochemical cycling of iron [5]. MTB through magnetosome synthesis,

#### **Figure 2.**

*Map of the distribution of known cultured and non-cultured magnetotactic bacteria across de world by phylogenetic group (see the correspondence between taxa and colors on the bottom left corner of the image).*

**21**

gene function is not available.

vesicle docking in the bacterial cytoskeleton [3].

*Biomineralization of Magnetosomes: Billion-Year Evolution Shaping Modern Nanotools*

assimilate the iron solubilized in the environment to an inorganic crystal. After cell lysis, the magnetosome is deposited in the sediment, forming what is known as magnetofossils [6]. Besides, MTB can be ingested by protozoans, and the iron from magnetosomes is, then, incorporated in the food chain [7]. Apart from iron and based on their physiology, MTB seem to have relevant roles in other biogeochemical

MTB are an extremely diverse group of Gram-negative bacteria with a variety of morphotypes (i.e., rods, vibrios, spirilla, coccoid, and ovoid) and species affiliated to Proteobacteria (Alpha-, Beta-, Gamma-, Delta-, and *Ca.* Etaproteobacteria class), Omnitrophica and Nitrospirae phyla [9]*.* MTB affiliation to other taxa have been proposed based on metagenomics studies, but observation of the magnetosomes was not performed to confirm this matter. This great diversity is reflected in MTB ubiquity in almost all aquatic habitats across the Earth (**Figure 2**), including extreme environments such as thermal trenches and saline-alkaline lakes [6, 10]. More than being interesting species for their unique evolutionary process and ecological importance, MTB are also proving to be of interest for biotechnological applications. Their unique physiology makes MTB potential bioremediators of heavy metals and magnetosomes can be extracted and used as nanotools for magnetic controlled drug targeting, contrast agents for magnetic resonance imaging, enzyme immobilization and many more industrial and biomedical applications [11].

MB is highly regulated at the genetic level [12]. Magnetosome gene clusters (MGCs) [13], structured as *operons*, are responsible for MB in MTB. MTB genomes contain: (i) conserved *mam* genes, encountered in all MTB; and (ii) restricted genes encountered in some phylogenetic groups of MTB [14]. Examples of genes restricted

As previously described, MTB are capable of biomineralizing magnetosomes, an organelle with a ferrimagnetic mineral core surrounded by a biological membrane [3]. A series of complex mechanisms occur in order to transform the environmental bioavailable iron into a complete and fully functional magnetic organelle. MB process involves different steps such as iron uptake, magnetosome vesicle formation, specific protein recruiting, crystal nucleation, redox balance, and pH control in magnetosome vesicle, size and crystalline morphology control and magnetosome

Mam and Mms proteins involved in MB belong to different protein families including: TPR proteins (from Tetratrico Peptide Repeat; MamA) [17], CDF transporters (Cation Diffusion Facilitators: MamB and MamM) [14, 18], serine proteases HtrA-like (MamE, MamP, and MamO) [14], actine-like proteins (MamK) [19], liposome tubulation protein (MamY) [20], generic transporters (MamH and MamN) [14, 21] and MTB specific proteins without prior homology in other non-magnetotactic microrganisms

to certain MTB are: (i) *mms* (from magnetosome membrane specific) genes found in magnetotactic Proteobacteria; (ii) *mad* (from magnetosome associated Deltaproteobacteria), which were first reported in magnetotactic deltaproteobacteria [15] and recently encountered in MTB affiliated to Omnitrophica and Nitrospirae phyla [9]; and (iii) *man* (from magnetosome genes in Nitrospirae), which are genes reported in MTB affiliated to Nitrospirae phylum [16]. Comprehension of MB were inferred by *mam* and *mms* genes deletion in the cultured magnetotactic alphaproteobacteria *Magnetospirillum magneticum* strain AMB-1 and *Magnetospirillum gryphiswaldense* strain MSR-1 [14]. Precise *man* and *mad* genes roles in MB remain unclear as they were studied in uncultured MTB [16], thus genetic systems to test

*DOI: http://dx.doi.org/10.5772/intechopen.94465*

cycles of sulfur, nitrogen, and carbon [8].

**2. Steps of magnetosome biomineralization in MTB**

*Biomineralization of Magnetosomes: Billion-Year Evolution Shaping Modern Nanotools DOI: http://dx.doi.org/10.5772/intechopen.94465*

assimilate the iron solubilized in the environment to an inorganic crystal. After cell lysis, the magnetosome is deposited in the sediment, forming what is known as magnetofossils [6]. Besides, MTB can be ingested by protozoans, and the iron from magnetosomes is, then, incorporated in the food chain [7]. Apart from iron and based on their physiology, MTB seem to have relevant roles in other biogeochemical cycles of sulfur, nitrogen, and carbon [8].

MTB are an extremely diverse group of Gram-negative bacteria with a variety of morphotypes (i.e., rods, vibrios, spirilla, coccoid, and ovoid) and species affiliated to Proteobacteria (Alpha-, Beta-, Gamma-, Delta-, and *Ca.* Etaproteobacteria class), Omnitrophica and Nitrospirae phyla [9]*.* MTB affiliation to other taxa have been proposed based on metagenomics studies, but observation of the magnetosomes was not performed to confirm this matter. This great diversity is reflected in MTB ubiquity in almost all aquatic habitats across the Earth (**Figure 2**), including extreme environments such as thermal trenches and saline-alkaline lakes [6, 10]. More than being interesting species for their unique evolutionary process and ecological importance, MTB are also proving to be of interest for biotechnological applications. Their unique physiology makes MTB potential bioremediators of heavy metals and magnetosomes can be extracted and used as nanotools for magnetic controlled drug targeting, contrast agents for magnetic resonance imaging, enzyme immobilization and many more industrial and biomedical applications [11].

## **2. Steps of magnetosome biomineralization in MTB**

MB is highly regulated at the genetic level [12]. Magnetosome gene clusters (MGCs) [13], structured as *operons*, are responsible for MB in MTB. MTB genomes contain: (i) conserved *mam* genes, encountered in all MTB; and (ii) restricted genes encountered in some phylogenetic groups of MTB [14]. Examples of genes restricted to certain MTB are: (i) *mms* (from magnetosome membrane specific) genes found in magnetotactic Proteobacteria; (ii) *mad* (from magnetosome associated Deltaproteobacteria), which were first reported in magnetotactic deltaproteobacteria [15] and recently encountered in MTB affiliated to Omnitrophica and Nitrospirae phyla [9]; and (iii) *man* (from magnetosome genes in Nitrospirae), which are genes reported in MTB affiliated to Nitrospirae phylum [16]. Comprehension of MB were inferred by *mam* and *mms* genes deletion in the cultured magnetotactic alphaproteobacteria *Magnetospirillum magneticum* strain AMB-1 and *Magnetospirillum gryphiswaldense* strain MSR-1 [14]. Precise *man* and *mad* genes roles in MB remain unclear as they were studied in uncultured MTB [16], thus genetic systems to test gene function is not available.

As previously described, MTB are capable of biomineralizing magnetosomes, an organelle with a ferrimagnetic mineral core surrounded by a biological membrane [3]. A series of complex mechanisms occur in order to transform the environmental bioavailable iron into a complete and fully functional magnetic organelle. MB process involves different steps such as iron uptake, magnetosome vesicle formation, specific protein recruiting, crystal nucleation, redox balance, and pH control in magnetosome vesicle, size and crystalline morphology control and magnetosome vesicle docking in the bacterial cytoskeleton [3].

Mam and Mms proteins involved in MB belong to different protein families including: TPR proteins (from Tetratrico Peptide Repeat; MamA) [17], CDF transporters (Cation Diffusion Facilitators: MamB and MamM) [14, 18], serine proteases HtrA-like (MamE, MamP, and MamO) [14], actine-like proteins (MamK) [19], liposome tubulation protein (MamY) [20], generic transporters (MamH and MamN) [14, 21] and MTB specific proteins without prior homology in other non-magnetotactic microrganisms

*Materials at the Nanoscale*

*(C) Magnetovibrio blakemorei strain MV-1T*

**Figure 1.**

nanocrystals—the magnetosomes—, usually aligned in single or multiple chains within the bacterial cytoplasm (**Figure 1**), and flagellar propulsion guided by chemotaxis [3]. In a simple way, chemotaxis in MTB is assisted by bacterial orientation along Earth's magnetic field (magnetotaxis). Therefore, magnetotaxis allows MTB to find the optimum position for survival and growth in a chemically stratified water column, seeking for an optimum environment where proton motive driving force reaches maximum potential. For MTB, which are frequently microaerophilic or anaerobic microrganisms, this environment is near the oxic/anoxic interface [4]. Magnetosomes are composed of a magnetic nanoparticle in most cases composed of magnetite (Fe3O4) and sometimes greigite (Fe3S4) with species specific shapes and sizes, and enveloped by a phospholipid bilayer with associated proteins, which constitutes the magnetosome membrane (MM) [3]. The gene regulation of magnetosome biomineralization (MB) and organization within the cell will be discussed in more detail in the sections ahead. Based on the total iron amount within a magnetotactic bacterium cell, MTB appear to play a major role in the biogeochemical cycling of iron [5]. MTB through magnetosome synthesis,

*Transmission electron microscopy of: (A) uncultured coccoid magnetotactic bacteria from Monsimet Cove, Antarctica. (B) Uncultured coccoid magnetotactic bacteria from Punta Ullman, Antarctica.* 

*.*

*Map of the distribution of known cultured and non-cultured magnetotactic bacteria across de world by phylogenetic group (see the correspondence between taxa and colors on the bottom left corner of the image).*

**20**

**Figure 2.**

(MamG, MamF, MamD, MamC, MamJ, MamW, MamX, MamY, Mms6, MtxA) [3]. MB involves four major steps as they are: (i) MM formation (participation of MamI, MamL and MamAB proteins) [3, 14]; (ii) crystal nucleation (which include MamE, Mms6, MamB and MamM) [3, 14]; (iii) crystal maturation (participation of MamE, MmsF, MamGFDC and Mam P, S, T) [3, 14]; and (iv) magnetosome chain alignment within cell body (participation of MamJ, MamK and MamY) [14, 20]. Mam and Mms protein functions involved in MB are described in **Table 1** and **Figure 3**.


#### **Table 1.**

*Mam and Mms protein functions inferred by mutant construction in the cultured magnetotactic alphapreoteobacteria Ms. magneticum strain AMB-1 and Ms. gryphiswaldense strain MSR-1.*

**23**

**Figure 3.**

*Biomineralization of Magnetosomes: Billion-Year Evolution Shaping Modern Nanotools*

The advances of molecular biology techniques provided a much greater understanding of the MB mechanism over the last years as cultured and environmental MTB had their genomes sequenced. Magnetite MGCs and magnetite magnetosomes

*Three major steps of MB in MTB. 1st step: protein recruitment initiating the biomineralization process while forming the invagination of the magnetosome membrane (MM) and iron uptake. 2nd step: Crystal nucleation, characterized by the incorporation of iron and oxygen for magnetite biomineralization. Interestingly, oxygen for the synthesis of magnetite is derived from water [31]. So far, the sulfur source for the synthesis of greigite has not been clarified. Magnetosome begins to grow in size while morphology, pH and redox balance are strictly regulated. Magnetosomes are aligned in chains within the cell's cytoskeleton. 3rd step: Magnetosomes continue to grow under strict regulation until de crystal maturation is complete. OM: outer membrane; IM: inner* 

MGC origin and evolution within the Bacteria domain is a constantly discussed topic in the literature. The scattering of MGCs and the magnetotactic behavior raises questions as MTB encompasses high diversity regarding their ecology, metabolism, and phylogeny. The first proposed hypothesis was the polyphyletic origin of magnetite and greigite MB [53]. According to this hypothesis, biomineralization of greigite and magnetite magnetosomes would have evolved without sharing a last universal common ancestor of magnetotactic bacteria (LUCA MTB). At that time MGCs were not discovered. Thus, this assumption relied on the information that the biochemical and nutritional parameters for greigite and magnetite biomineralization are different. Likewise, all known MTB affiliated to Alphaproteobacteria synthesized magnetite magnetosomes, while the ones affiliated to Deltaproteobacteria

were studied in magnetotactic proteobacteria affiliated to the classes Alpha- [32–36], Beta- [37], Gamma- [38, 39], Delta- [40–43], *Ca.* Eta- [9, 32, 44, 45], *Ca.* Lambda- [9] and Zetaproteobacteria [9] and MTB affiliated to Nitrospirae [13, 16, 46–50] and Omnitrophica [49] phyla. Greigite MGC and greigite magnetosomes were characterized in magnetotactic deltaproteobacteria [51, 52] and MTB affiliated to *Ca.* Latescibacteria [8] and Planctomycetes [9] phyla. Culturing environmental MTB and mutant constructs different from the already known magnetotactic alphaproteobacteria *Ms. magneticum* strain AMB-1 and *Ms. gryphiswaldense* strain

MSR-1 may provide a greater comprehension of the MB mechanism.

**3. Evolutionary history of MGCs within Bacteria domain**

*DOI: http://dx.doi.org/10.5772/intechopen.94465*

*membrane, meaning the cytoplasmic membrane.*

*Biomineralization of Magnetosomes: Billion-Year Evolution Shaping Modern Nanotools DOI: http://dx.doi.org/10.5772/intechopen.94465*

#### **Figure 3.**

*Materials at the Nanoscale*

(MamG, MamF, MamD, MamC, MamJ, MamW, MamX, MamY, Mms6, MtxA) [3]. MB involves four major steps as they are: (i) MM formation (participation of MamI, MamL and MamAB proteins) [3, 14]; (ii) crystal nucleation (which include MamE, Mms6, MamB and MamM) [3, 14]; (iii) crystal maturation (participation of MamE, MmsF, MamGFDC and Mam P, S, T) [3, 14]; and (iv) magnetosome chain alignment within cell body (participation of MamJ, MamK and MamY) [14, 20]. Mam and Mms protein

**Protein** *Operon* **Function MTB strain Reference** MamA *mam*AB Protein recruitment AMB-1 [22]

MamC *mam*GFDC Size and morphology control AMB-1 [23] MamD *mam*GFDC Size and morphology control AMB-1 [23] MamE *mam*AB Protein targeting and redox control AMB-1 [14] MamF *mam*GFDC Size control AMB-1 [23] MamG *mam*GFDC Size and morphology control AMB-1 [23]

MamI *mam*AB Membrane invagination AMB-1 [14] MamJ *mam*AB Magnetosome alignment MSR-1 [24] MamK *mam*AB Magnetosome alignment MSR-1 [19] MamL *mam*AB Membrane invagination AMB-1 [14]

MamN *mam*AB pH control AMB-1 [14]

MamP *mam*AB Redox control AMB-1 [14] MamQ *mam*AB Membrane invagination AMB-1 [14] MamR *mam*AB Size and morphology control AMB-1 [14] MamS *mam*AB Size and morphology control AMB-1 [14]

MamU *mam*AB Not defined AMB-1 [14] MamV *mam*AB Not defined MSR-1 [18] MamW *mam*AB Magnetosome alignment MSR-1 [27] MamX *mam*XY Redox control MSR-1 [21]

MamZ *mam*XY Iron uptake and redox control MSR-1 [21] Mms6 *mms6* Size and morphology control AMB-1 [29] MmsF *mms6* Size and morphology control AMB-1 [30]

magnetosome alignment

*Mam and Mms protein functions inferred by mutant construction in the cultured magnetotactic alphapreoteobacteria Ms. magneticum strain AMB-1 and Ms. gryphiswaldense strain MSR-1.*

MSR-1

MSR-1

MSR-1

MSR-1

AMB-1/ MSR-1

AMB-1 [26]

[14, 18]

[14, 21]

[14, 18]

[14, 25]

[20, 28]

functions involved in MB are described in **Table 1** and **Figure 3**.

MamB *mam*AB Membrane invagination and iron uptake AMB-1/

MamH *mam*AB Iron uptake AMB-1/

MamM *mam*AB Iron uptake AMB-1/

MamO *mam*AB Crystal nucleation AMB-1/

MamT *mam*AB Size e morphology control and redox control

MamY *mam*XY Membrane invagination and

**22**

**Table 1.**

*Three major steps of MB in MTB. 1st step: protein recruitment initiating the biomineralization process while forming the invagination of the magnetosome membrane (MM) and iron uptake. 2nd step: Crystal nucleation, characterized by the incorporation of iron and oxygen for magnetite biomineralization. Interestingly, oxygen for the synthesis of magnetite is derived from water [31]. So far, the sulfur source for the synthesis of greigite has not been clarified. Magnetosome begins to grow in size while morphology, pH and redox balance are strictly regulated. Magnetosomes are aligned in chains within the cell's cytoskeleton. 3rd step: Magnetosomes continue to grow under strict regulation until de crystal maturation is complete. OM: outer membrane; IM: inner membrane, meaning the cytoplasmic membrane.*

The advances of molecular biology techniques provided a much greater understanding of the MB mechanism over the last years as cultured and environmental MTB had their genomes sequenced. Magnetite MGCs and magnetite magnetosomes were studied in magnetotactic proteobacteria affiliated to the classes Alpha- [32–36], Beta- [37], Gamma- [38, 39], Delta- [40–43], *Ca.* Eta- [9, 32, 44, 45], *Ca.* Lambda- [9] and Zetaproteobacteria [9] and MTB affiliated to Nitrospirae [13, 16, 46–50] and Omnitrophica [49] phyla. Greigite MGC and greigite magnetosomes were characterized in magnetotactic deltaproteobacteria [51, 52] and MTB affiliated to *Ca.* Latescibacteria [8] and Planctomycetes [9] phyla. Culturing environmental MTB and mutant constructs different from the already known magnetotactic alphaproteobacteria *Ms. magneticum* strain AMB-1 and *Ms. gryphiswaldense* strain MSR-1 may provide a greater comprehension of the MB mechanism.

### **3. Evolutionary history of MGCs within Bacteria domain**

MGC origin and evolution within the Bacteria domain is a constantly discussed topic in the literature. The scattering of MGCs and the magnetotactic behavior raises questions as MTB encompasses high diversity regarding their ecology, metabolism, and phylogeny. The first proposed hypothesis was the polyphyletic origin of magnetite and greigite MB [53]. According to this hypothesis, biomineralization of greigite and magnetite magnetosomes would have evolved without sharing a last universal common ancestor of magnetotactic bacteria (LUCA MTB). At that time MGCs were not discovered. Thus, this assumption relied on the information that the biochemical and nutritional parameters for greigite and magnetite biomineralization are different. Likewise, all known MTB affiliated to Alphaproteobacteria synthesized magnetite magnetosomes, while the ones affiliated to Deltaproteobacteria

synthesized greigite magnetosomes, thus permitting the inference the polyphyletic hypothesis. Years later, after the discovery of MGCs, similarities between *mam* genes of magnetite and greigite MTB showed a common ancestor for both minerals synthesis in MTB [54]. It is speculated that greigite MGCs originated after events of duplication and divergence from magnetite MGCs in sulfate-reducing bacteria like the multicellular magnetotactic prokaryote (MMP) *Ca.* Magnetoglobus multicellularis strain Araruama affiliated to Deltaproteobacteria [54].

On behalf of that, Lefèvre and colleges [55] hypothesized a monophyletic origin of MGCs concerning magnetotactic proteobacteria. The comparison of 16S rRNA gene and conserved Mam proteins evolution showed a convergence of both phylogenetic inferences. It was suggested that MTB affiliated to Proteobacteria phyla shared a LUCA MTB and over time, some proteobacteria would have lost the MGC, resulting in the inability of biomineralizing magnetosomes [55].

#### **Figure 4.**

*Geologic time and evolution model proposed for MGC and magnetotaxis evolution. (A) Geologic rule in million years ago (Mya). LUCA MTB origin (gray arrowhead) is estimated 2.7 billion years ago during the Archean eon. The first single-celled form of life originated ~4 billion years ago and the origin of phototrophs, that permitted great oxygenation in earth, only happened ~2.4 billion years ago. (B and C) Two models for MGC and magnetotaxis evolution adapted from [9]. (B) LUCA MTB containing magnetite MGC branched two MTB lineages: (i) MTB affiliated to Proteobacteria (without Delta-), Nitrospirae and Omnitrophica phyla with recent HGT events responsible for MGC scattering; and (ii) MTB affiliated to Deltaproteobacteria class that after events of duplication and divergence hosted microbes with magnetite, greigite or both MGCs. Ancient HGT events would have been responsible for greigite MGC acquaintance in Plantomycetes and Ca. Lastescibacteria phyla. Adapted from [9]. (C) LUCA MTB containing an unknown MGC after events of duplication and divergence gave origin for both magnetite and greigite MGC. A monophyletic origin is proposed for MTB affiliated to Proteobacteria (without Delta- class), Nitrospirae, Omnitrophica, Planctomycetes and Ca. Latescibacteria phyla and Deltaproteobacteria class. Recent HGT events originating from MTB affiliated to Proteobacteria (without Delta- class), Nitrospirae, Omnitrophica could have been responsible for the scattering of MGC and magnetotactic behavior. Adapted from [9].*

**25**

*Biomineralization of Magnetosomes: Billion-Year Evolution Shaping Modern Nanotools*

Opposing all previous statements, a considerable number of authors proposed the importance and influence of horizontal gene transfer (HGT) events on the evolution and scatter of MGC in Bacteria domain [9, 13, 56–59]. In light of these events, different non-MTB would have received MGCs by HGT, granting them the

The origin of MB was dated, by molecular Bayesian clock, before the divergence

presence of primary electron acceptors of Earth early ecosystems such as CO2, CO,

of MTB [13]. Known examples of such conditions that are in accordance with available resources of primitive Earth are: (i) microaerophilic or anaerobic respiration in all known MTB; (ii) chemolithoautotrophy as MTB are capable of CO2 fixation by Calvin–Benson–Bassham cycle, the reverse tricarboxylic acid cycle, or the reductive

[16, 49]; (iv) capacity of oxidizing H2S via sulfur oxidation pathway [16, 49]; (v) water temperature ranging from 26 to 85°C [64, 65] compatible with MTB growth as there are psychrophilic [66], mesophilic [8] and moderately thermophilic MTB [47]. Alongside these conditions, Earth's magnetic field originated 4.2 billion years ago enduring several inversions until the present time [67]. Considering this panorama, it is plausible that MTB and the geomagnetic fields have coevolved selecting the ones capable of undergoing all the continuous biotic and abiotic variations [13]. Large scale metagenome approach of MTB diversity demonstrated two possible routes concerning MGC evolution over time [9]. It is hypothesized that a LUCA MTB contained magnetite or an unknown MGC followed by events of MGC duplication, divergence, and loss combined with ancient and recent HGT events could explain the scattering of the magnetotactic behavior in the Bacteria domain [9] (**Figure 4**). The unending studies regarding MTB diversity and ecology are indispensable for an accurate decipherment of MGC evolution in the Bacteria domain.

The fact that related magnetotactic strains synthesize magnetosomes with significant differences in sizes and elongation is a clue that, despite a rigorous genetic control, environmental factors may influence the characteristics of the biomineralized nanocrystals [68]. Extensive experiments performed in cultures of MTB have pointed out temperature, pH, iron concentration, oxygen concentration, external magnetic fields, and nutrient concentrations as important factors driving physical

Ferric iron concentrations exert an important influence on the magnetic properties of *Magnetospirillum magnetotacticum* strain MS-1 cells due to alterations within biogenic magnetite [70]. The coercive force (HC), probably the most important criterion in the selection of magnetic nanoparticles for technological applications, is significantly affected [70]. The HC was increased from 216 Oe when cells were

In another study, it was shown that reducing conditions leads to an increase in magnetosomes crystals of *Ms. magneticum* strain AMB-1 in culture [71]. An oxidoreduction

acetyl-CoA pathway [63]; (iii) capacity of denitrification of NO, NO2

, Fe2+, CH4, NH4

+

[62]. These conditions favored the survival and growth

and CH2O [62]; and (iv)

−

and NO3

−

of the Nitrospirae and Proteobacteria phyla during the Archean eon [13]. The divergence happened 2.7 billion years ago before the appearance of phototrophs and Great Oxygenation at the time of Paleoproterozoic on the Proterozoic eon (**Figure 4**). This hypothesis is supported by: (i) low pressure or absence of O2 in the atmosphere and anoxic oceans in Archean [60]; (ii) abundant dissolved Fe2+ as concentrations of 40 to 120 μmol/L [61]; (iii) presence of primary electron donors of

*DOI: http://dx.doi.org/10.5772/intechopen.94465*

capacity of biomineralizing magnetosomes [9].

Earth early ecosystems such as H2, H2S, S0

and NO3

−

**4. Influence of the medium on biomineralization**

changes in magnetosomes [69].

cultured at 12 μM Fe3+ to 238 Oe at 68 μM [70].

−

SO4

2−, NO, NO2

*Biomineralization of Magnetosomes: Billion-Year Evolution Shaping Modern Nanotools DOI: http://dx.doi.org/10.5772/intechopen.94465*

Opposing all previous statements, a considerable number of authors proposed the importance and influence of horizontal gene transfer (HGT) events on the evolution and scatter of MGC in Bacteria domain [9, 13, 56–59]. In light of these events, different non-MTB would have received MGCs by HGT, granting them the capacity of biomineralizing magnetosomes [9].

The origin of MB was dated, by molecular Bayesian clock, before the divergence of the Nitrospirae and Proteobacteria phyla during the Archean eon [13]. The divergence happened 2.7 billion years ago before the appearance of phototrophs and Great Oxygenation at the time of Paleoproterozoic on the Proterozoic eon (**Figure 4**). This hypothesis is supported by: (i) low pressure or absence of O2 in the atmosphere and anoxic oceans in Archean [60]; (ii) abundant dissolved Fe2+ as concentrations of 40 to 120 μmol/L [61]; (iii) presence of primary electron donors of Earth early ecosystems such as H2, H2S, S0 , Fe2+, CH4, NH4 + and CH2O [62]; and (iv) presence of primary electron acceptors of Earth early ecosystems such as CO2, CO, SO4 2−, NO, NO2 − and NO3 − [62]. These conditions favored the survival and growth of MTB [13]. Known examples of such conditions that are in accordance with available resources of primitive Earth are: (i) microaerophilic or anaerobic respiration in all known MTB; (ii) chemolithoautotrophy as MTB are capable of CO2 fixation by Calvin–Benson–Bassham cycle, the reverse tricarboxylic acid cycle, or the reductive acetyl-CoA pathway [63]; (iii) capacity of denitrification of NO, NO2 − and NO3 − [16, 49]; (iv) capacity of oxidizing H2S via sulfur oxidation pathway [16, 49]; (v) water temperature ranging from 26 to 85°C [64, 65] compatible with MTB growth as there are psychrophilic [66], mesophilic [8] and moderately thermophilic MTB [47]. Alongside these conditions, Earth's magnetic field originated 4.2 billion years ago enduring several inversions until the present time [67]. Considering this panorama, it is plausible that MTB and the geomagnetic fields have coevolved selecting the ones capable of undergoing all the continuous biotic and abiotic variations [13].

Large scale metagenome approach of MTB diversity demonstrated two possible routes concerning MGC evolution over time [9]. It is hypothesized that a LUCA MTB contained magnetite or an unknown MGC followed by events of MGC duplication, divergence, and loss combined with ancient and recent HGT events could explain the scattering of the magnetotactic behavior in the Bacteria domain [9] (**Figure 4**). The unending studies regarding MTB diversity and ecology are indispensable for an accurate decipherment of MGC evolution in the Bacteria domain.

## **4. Influence of the medium on biomineralization**

The fact that related magnetotactic strains synthesize magnetosomes with significant differences in sizes and elongation is a clue that, despite a rigorous genetic control, environmental factors may influence the characteristics of the biomineralized nanocrystals [68]. Extensive experiments performed in cultures of MTB have pointed out temperature, pH, iron concentration, oxygen concentration, external magnetic fields, and nutrient concentrations as important factors driving physical changes in magnetosomes [69].

Ferric iron concentrations exert an important influence on the magnetic properties of *Magnetospirillum magnetotacticum* strain MS-1 cells due to alterations within biogenic magnetite [70]. The coercive force (HC), probably the most important criterion in the selection of magnetic nanoparticles for technological applications, is significantly affected [70]. The HC was increased from 216 Oe when cells were cultured at 12 μM Fe3+ to 238 Oe at 68 μM [70].

In another study, it was shown that reducing conditions leads to an increase in magnetosomes crystals of *Ms. magneticum* strain AMB-1 in culture [71]. An oxidoreduction

*Materials at the Nanoscale*

synthesized greigite magnetosomes, thus permitting the inference the polyphyletic hypothesis. Years later, after the discovery of MGCs, similarities between *mam* genes of magnetite and greigite MTB showed a common ancestor for both minerals synthesis in MTB [54]. It is speculated that greigite MGCs originated after events of duplication and divergence from magnetite MGCs in sulfate-reducing bacteria like the multicellular magnetotactic prokaryote (MMP) *Ca.* Magnetoglobus multicel-

On behalf of that, Lefèvre and colleges [55] hypothesized a monophyletic origin of MGCs concerning magnetotactic proteobacteria. The comparison of 16S rRNA gene and conserved Mam proteins evolution showed a convergence of both phylogenetic inferences. It was suggested that MTB affiliated to Proteobacteria phyla shared a LUCA MTB and over time, some proteobacteria would have lost the MGC,

*Geologic time and evolution model proposed for MGC and magnetotaxis evolution. (A) Geologic rule in million years ago (Mya). LUCA MTB origin (gray arrowhead) is estimated 2.7 billion years ago during the Archean eon. The first single-celled form of life originated ~4 billion years ago and the origin of phototrophs, that permitted great oxygenation in earth, only happened ~2.4 billion years ago. (B and C) Two models for MGC and magnetotaxis evolution adapted from [9]. (B) LUCA MTB containing magnetite MGC branched two MTB lineages: (i) MTB affiliated to Proteobacteria (without Delta-), Nitrospirae and Omnitrophica phyla with recent HGT events responsible for MGC scattering; and (ii) MTB affiliated to Deltaproteobacteria class that after events of duplication and divergence hosted microbes with magnetite, greigite or both MGCs. Ancient HGT events would have been responsible for greigite MGC acquaintance in Plantomycetes and Ca. Lastescibacteria phyla. Adapted from [9]. (C) LUCA MTB containing an unknown MGC after events of duplication and divergence gave origin for both magnetite and greigite MGC. A monophyletic origin is proposed for MTB affiliated to Proteobacteria (without Delta- class), Nitrospirae, Omnitrophica, Planctomycetes and Ca. Latescibacteria phyla and Deltaproteobacteria class. Recent HGT events originating from MTB affiliated to Proteobacteria (without Delta- class), Nitrospirae, Omnitrophica could have been* 

*responsible for the scattering of MGC and magnetotactic behavior. Adapted from [9].*

lularis strain Araruama affiliated to Deltaproteobacteria [54].

resulting in the inability of biomineralizing magnetosomes [55].

**24**

**Figure 4.**

potential of 0 mV (neutral condition) led to a crystal diameter of 31.5 ± 1.3 nm, which augmented to 37.2 ± 0.6 nm when the culture was carried out at -500 mV (reducing condition) [71]. The reducing condition also caused an increase in the total magnetite mass per cell as 9.1 ± 1.9 magnetosomes were observed per μm (cell length), in contrast to 5.48 ± 1.3 in neutral condition.

The evidence that characteristics of biogenic magnetite can be modified is of great interest for practical applications because certain purposes may require specific particle properties. Therefore, the knowledge of the interplay between environmental conditions and process regulation by biomolecules in biomineralization can help develop methods for the *in vitro* biomimetic preparation of magnetic nanoparticles with tunable properties.
