Hydrothermal alteration of ultramafic rocks has a major impact on the global biogeochemical cycles. During peridotite alteration hydrogen is produced inducing the formation of divers sulfide and metal mineral assemblages. In addition, reduced gases such as H₂ and CH₄ provide an energy source for microbial activity that live in peridotite-hosted hydrothermal systems. In this project we study the links between mineral reactions and biogenic activity and how these chemical interactions impact the global geochemical cycles. Using mineralogical and petrological techniques (e.g., EMP, SEM, m-XANES) combined with bulk rock and in-situ (isotope) geochemistry we study mafic und ultramafic-rocks from ophiolite sequences and ocean drill cores to reveal the complex interplay between magmatic, hydrothermal and biogenic processes taking place in the oceanic lithosphere.