Les bactéries magnétotactiques représentent un groupe d’une grande diversité écologique et phylogénétique. Elles sont capables de biominéraliser des nanocristaux de magnétite [un oxyde de fer (Fe(II)Fe(III)2O4)] ou de greigite [un sulfure de fer (Fe(II)Fe(III)2S4)] dans leurs magnétosomes, organites alignés en chaînes permettant la navigation le long des lignes de champ magnétique terrestre. Jusqu'à récemment, seules des souches produisant de la magnétite étaient disponibles en culture pure, conduisant à des études sur les mécanismes de biominéralisation de cet oxyde de fer. En 2011, une nouvelle bactérie capable de former de la magnétite et de la greigite, Desulfamplus magnetovallimortis souche BW-1, a été cultivée avec succès en laboratoire. Dans cette thèse, nous proposons d'utiliser une approche intégrée et multidisciplinaire pour comprendre les mécanismes de biominéralisation de la greigite en utilisant comme modèle d’étude la souche BW-1. Nous avons donc cherché à déterminer les conditions environnementales et biologiques favorisant la formation de la magnétite et de la greigite. Ces travaux ont également conduit à la caractérisation physiologique et phylogénétique de BW-1. Puis, l’utilisation d’approches globales et ciblées de transcriptomique ont permis d'évaluer le taux d'expression des gènes impliqués dans la formation des magnétosomes (magnétite vs. greigite) dans diverses conditions de croissance. Une approche de protéomique a permis d’apporter des informations supplémentaires à cette étude. Ces résultats ont permis de progresser dans la compréhension fondamentale de la biominéralisation in vivo, en particulier pour des bactéries formant de la greigite. / Magnetotactic bacteria represent a phylogenetically and ecologically diverse group of prokaryotes able to biomineralize magnetic nanocrystals composed of magnetite [an iron oxide (Fe(II)Fe(III)2O4)] or greigite [an iron sulfide (Fe(II)Fe(III)2S4)] in their magnetosomes, a prokaryotic organelle whose cytoplasmic alignement in chain allows the cell to navigate along the Earth’s magnetic field lines. Until recently, only magnetite-producing strains were available in pure culture. Thus, only the magnetite biomineralization has been studied. In 2011, a new bacterium able to form both magnetite and greigite, Desulfamplus magnetovallimortis strain BW-1, was isolated from Death Valley, California and cultivated in pure culture. In this work, we propose to use an integrated and multidisciplinary approach to understand the mechanisms involved in greigite biomineralization in BW-1 strain. First, we determined the environmental and biological conditions in which magnetite and greigite are formed. This first part of my thesis also contributed to the physiologic and phylogenetic characterization of this bacterium. Secondly, we used global and targeted transcriptomic approaches to evaluate the transcription levels of genes putatively involved in magnetosomes formation (magnetite vs. greigite) under various growth conditions. A proteomic approach provided additional informations to this study.Results obtained during my thesis contribute to the understanding of in vivo biomineralization, particularly for greigite production in magnetotactic bacteria.

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Interaction between minerals and bacteria represents an abundant natural phenomenon depictingnature's complexity and how abiotic and biotic components are intertwined. This interaction is evidentin modern-day ecosystems, and it significantly shaped the early stage of life on Earth by influencinggeochemical processes. Evidence of this interaction includes microbialites. In the first part, this master's thesis explores the impact of iron-redox bacteria on the magneticproperties of synthetic and natural magnetic materials, with significant implications for understandingearly Earth conditions and paleoenvironments. In the second part, the magnetic signal recorded in thesediment core from the Baltic Sea is examined to identify the ferrimagnetic minerals responsible for itand their origin (biotic versus abiotic). A better understanding of the origin of ferrimagnetic mineralsallows for a more conclusive interpretation of palaeomagnetism and palaeoenvironmental history of theBaltic Sea. In both parts, the change in magnetic susceptibility was used as the main method to depictmechanisms of mineral-bacteria interaction. Experiment with iron-oxidising bacteria (Leptothrix mobilis) showed a decrease in magneticsusceptibility over time, consistent with the oxidation of solid iron/magnetic materials. However, asmall difference between bacteria culture and control samples points out that the decrease is caused byabiotic oxidation rather than bacterial. Supporting evidence is the absence of viable cells in all bacterialsamples, suggesting that L. mobilis did not grow in these experiments. In experiments with iron-reducing bacteria (Geobacter sulfurreducens), magnetic susceptibility increased by 7%. Controlsamples with the same reducing media did not show a change in magnetic susceptibility, indicating thatthe susceptibility change is caused by bacterial reduction of iron oxides. Magnetic susceptibility signal obtained in the sediment core from the Baltic Sea indicates rapidlyoxidising, ferrimagnetic nanoparticles in two organic-rich sapropels. The pattern of the signal isconsistent with the presence of bacterial greigite (magnetofossils). Contrary to previous research, nomagnetic enhancement is observed in these layers.  Magnetic susceptibility, as a non-destructive and relatively simple method, may serve as a significantindicator of mineral-bacterial interactions. Combining it with other techniques and methods can providedeeper insights into the mechanisms behind these interactions. This approach can reveal the importanceof these interactions on early Earth, enhance our understanding of palaeomagnetism, and unveil possibleconditions of ancient environments.

Submitted by Biblioteca de Pós-Graduação em Geoquímica BGQ (bgq@ndc.uff.br) on 2017-07-03T17:14:01Z No. of bitstreams: 1 Dissertação Rodrigo Sobrinho.pdf: 3591957 bytes, checksum: 2927de76042f83d833c2294256be9c4a (MD5) / Made available in DSpace on 2017-07-03T17:14:01Z (GMT). No. of bitstreams: 1 Dissertação Rodrigo Sobrinho.pdf: 3591957 bytes, checksum: 2927de76042f83d833c2294256be9c4a (MD5) / Fundação de Amparo a Pesquisa do Rio de Janeiro / Universidade Federal Fluminense. Instituto de Química. Programa de Pós-Graduação em Geoquímica, Niterói, RJ / Candidatus Magnetoglobus Multicelularis são organizações multi celulares, classificada como pertencentes ao grupo de sulfato redutoras e que possuem em seu citoplasma cristais de greigita, que dá a eles a característica magnetotática. São encontrados em ambientes aquáticos na região de tra nsição oxica-anóxica onde há uma estratificação química vertical. Entretanto, po uco conhecimento se tem sobre as características ambientais que favorecem seu cresci mento e sua sobrevivência, o que impede seu cultivo em laboratório e a compreens ão sobre sua influência nos ciclos biogeoquímicos dos ambientes onde são encont rados. O presente trabalho tem, neste sentido, o objetivo de caracterizar as c ondições geoquímicas que exercem influência sobre a população de Candidatus Magnetoglobus Multicelularis através de uma avaliação das variáveis mais relevan tes para seu metabolismo. Foram realizadas, para isso, duas coletas nos meses de março e junho de 2007 em três pontos da Lagoa de Araruama, quantificando a d istribuição vertical da população de Ca . M. multicellularis em extratos de sedimento e as variáveis abióticas nos mesmos extratos. Os resultados mostra ram que o ferro exerce influência sobre o crescimento populacional na maio ria dos pontos. Entretanto, no ponto da Praia da baleia, onde foram encontrados 89 % de todos os indivíduos quantificados, o ferro encontra-se em concentrações maiores e outros fatores exercem maior influência sobre o crescimento popula cional, como a disponibilidade de enxofre e nitrogênio. O trabalho conclui que Ca . M. multicellularis são encontrados em maior abundância em sedimentos que f avorecem a formação de sulfetos de ferro e a população concentra-se em ext ratos onde as condições ambientais favorecem a formação e a manutenção dos cristais encontrados no citoplasma ou onde há maior disponibilidade de nutr ientes, de acordo com a necessidade metabólica das células. / andidatus Magnetoglobus Multicelularis consist of multicellul ar aggregate that are classified as sulfate reductors, with grei gite crystals in their cytoplasm, which give them magnetotactic characteristics. They are f ound in aquatic environments, on the oxi-anoxi transition zone, where a vertical che mical stratification. However, little is known about the environmental characteristics that favor their survival and growth. This fact prevents their controlled breed and cultu re in a laboratorial environment and also impairs the understanding of their influence o n biogeochemical cycles of the environments where they are found. Therefore, the o bjective of the present work is to characterize the environmental conditions that favo r the Ca . M. multicellularis survival through an in situ evaluation of the variables that are more importan t to the cellular metabolism. In order to obtain this data, two sampl e collections were conducted in three spots of the Araruama lagoon, in the months m arch and in june of 2007 quantifying the vertical distribution of the Ca . M. multicellularis population in sediment extracts and their abiotic variables. The results s howed that the quantified iron acts as a limiting factor in most of the collection spot s. However, in Praia da Baleia, where 89% of the total quantified individuals were found, a higher concentration of iron was also found and it does not act as a limiting factor . At the latter collection spot, other factors such as sulfur and nitrogen availability re gulate the population growth. As a conclusion, this work demonstrates that Ca . M. multicellularis are found in more abundance in sediments that enable iron sulfide for mations. Moreover, they use their magnetotactic characteristics to adapt themselves t o environmental variations that occur frequently in the referred sediment region an d they try to find a region where the environmental conditions may allow the formatio n and maintenance of the crystals found in the cytoplasm or where there are more nutrients available.