Chemical signature of magnetotactic bacteria

Abstract

There are longstanding and ongoing controversies about the abiotic or biological origin of nanocrystals of magnetite. On Earth, magnetotactic bacteria perform biomineralization of intracellular magnetite nanoparticles under a controlled pathway. These bacteria are ubiquitous in modern natural environments. However, their identification in ancient geological material remains challenging. Together with physical and mineralogical properties, the chemical composition of magnetite was proposed as a promising tracer for bacterial magnetofossil identification, but this had never been explored quantitatively and systematically for many trace elements. Here, we determine the incorporation of 34 trace elements in magnetite in both cases of abiotic aqueous precipitation and of production by the magnetotactic bacterium Magnetospirillum magneticum strain AMB-1. We show that, in biomagnetite, most elements are at least 100 times less concentrated than in abiotic magnetite and we provide a quantitative pattern of this depletion. Furthermore, we propose a previously unidentified method based on strontium and calcium incorporation to identify magnetite produced by magnetotactic bacteria in the geological record.

Keywords: biomineralization; magnetite; magnetotactic bacteria; trace element incorporation.

Fig. 1.

Transmission electron microscopy images of (A) bacteria cultivated in bottles and (B) in fermentor, (C) extracted chain of magnetosomes, (D) magnetosomes treated with SDS-Triton-phenol preparation, (E) bacterial magnetite leached with EDTA, and (F) untreated magnetosome. Extracted magnetosomes display chain structures assembled by magnetosome membranes and proteins (arrows in C). In contrast with magnetite treated with SDS-Triton-phenol (E), untreated magnetosomes show traces of organic matter (arrow in F). Once treated, magnetite from AMB-1 agglomerated, as shown in D. We also observed such agglomeration in abiotic magnetite. Magnetite did not seem to be affected by EDTA leaching.

Fig. 2.

Element partition coefficients between magnetite and solution for biotic versus abiotic experiments. The partition coefficients are normalized to iron. K = 1 indicates a similar behavior of an element relative to iron; K <1 and K >1 correspond to a depletion and an enrichment in the magnetite relative to the exchange of iron between solution and magnetite, respectively. Most elements are depleted in the biogenic magnetite compared with the abiotic magnetite (below the 1:1 thick line). This illustrates the high degree of chemical purity in this biologically controlled mineralization but also shows that the presence of these trace elements in the biomineralization medium is not zero.