SIGNATURE OF MAGNETITE PRODUCED BY MAGNETOTACTIC BACTERIA: CHEMICAL AND ISOTOPIC PERSPECTIVES – SIGMAG

The two main tasks of SIGMAG have been started.

First, laboratory cultures of the bacteria Magnetovibrio blakemorei, strain MV1, were performed under various conditions for examining the effect of various redox conditions and iron concentrations in the growth medium. Two types of iron sources were tested, i.e. Fe(II)-ascorbate and Fe(III)-citrate, for a large range of Fe concentrations (from 30 to 150 micromol/L). For each condition, triplicates were done and growth medium before and after culture, magnetite (with and without organic membrane) and bacterial lysate (after magnetite extraction) were separated from each other and collected. The experimental samples are now ready for chemical analyses of trace elements and Fe isotope compositions.

The second research axis of SIGMAG focuses on the study of MTB of Lake Pavin, an anoxic and ferruginous crater lake located in the French Massif Central. Three field trips have been organized in 2019 in order to test different types of magnetic traps for concentrating MTB from the redox interface of the water column. One of the field trip lasted two weeks and allowed to optimize the technique for collecting and concentrating MTB of the water samples. We have been able to count MTB on the field using a microscope and carried out several vertical profiles in lake Pavin water column.

Preliminary results on cultures of the bacteria strain MV-1 show an enrichment of some trace elements (e.g. molybdenum, tin) in biomagnetite compared to abiotically-precipitated magnetite. Iron isotope compositions vary over 1‰ in contrast to a previous study where an absence of iron isotope fractionation was suggested (Mandernack et al., Science 1999). Mineralogical characterizations of experimental samples by transmission electron microscopy have been started. We are now studying systematically the size and shape distribution of magnetite nanocrystals for the various conditions of culture. All mineralogical and geochemical data will help in understanding biomineralization mechanisms of MV-1 magnetite and in determining the criteria to be used as a biosignature of magnetofossils for all MTB.

During our field trips at Pavin Lake, we have identified a strong seasonal variability of the distribution of MTB in the water column, as well as vertical heterogeneity of various species in the water column (for a single season). Lake Pavin MTB have been collected for physiological and phylogenetic characterization. In parallel, MTB have been collected for analysis of trace elements and iron isotope compositions. Finally, preliminary tests have been performed for collecting MTB from lake Pavin sediments. The diversity of MTB in the sediments is particularly high. New protocols were developed for a magnetic concentration and allowed to determine an MTB abundance > 100 000 MTB/mL.

To conclude, samples from laboratory culture have been produced and are now being analyzed for their mineralogy, iron isotope and trace element compositions. Samples from lake Pavin have been successfully collected and are also analyzed (phylogeny, biodiversity, mineralogy, iron isotopes, trace elements). Three manuscripts focused on these intermediate results should be published in 2020.

1. Mathon F., Lefèvre C.T., Guyot F., Amor M., Busigny V. (2019) Iron isotope fractionation by magnetotactic bacteria. Goldschmidt Conference, Barcelona, Spain.
2. Monteil C.L., Lefevre C.T. (2019) Magnetoreception in microorganisms. Trends Microbiol. pii: S0966 doi: 10.1016/j.tim.2019.10.012
3. Monteil C.L., Grouzdev D.S., Perrière G., Alonso B., Rouy Z., Cruveiller S., Ginet N., Pignol D., Lefevre C.T. (2020) Repeated horizontal gene transfers triggered parallel evolution of magnetotaxis in two evolutionary divergent lineages of magnetotactic bacteria. ISME J. Accepted with minor revisions.

Magnetotactic bacteria (MTB) synthesize magnetite crystals within their cell. Although MTB may represent some of the oldest biomineralizing microbial organisms on Earth, their identification in the rock record has remained elusive. In the present proposal, we aim at testing and improving new chemical (trace elements) and isotopic (Fe) proxies for identification of MTB. Two main tasks will be achieved: (1) laboratory experiments to explore chemical and isotopic signatures of MTB strains under various culture conditions, (2) analysis of modern MTB in their environment to determine if the conclusions derived from laboratory experiments can be translated to natural systems. This multidisciplinary project, which gathers experts in biology, mineralogy, and elemental and isotope geochemistry, will pave the way for future geochemical studies of MTB search in the sedimentary record.