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.