Methyl mercury is highly toxic and is mainly produced by sulfate reducing bacteria (SRB) in anoxic aquatic ecosystems. Understanding this Hg biomethylation process is important. In this study, we examine the role of bHgS nanoparticles in Hg methylation by the model SRB strain Pseudodesulfovibrio hydrargyri (BerOc1). Our objective is to identify and quantify the various Hg species in the cells and different fractions. For that, we will use Hg L3-edge High-Energy Resolution Fluorescence Detected -X-ray Absorption Near Edge Structure spectroscopy (HERFD-XANES) from FAME-UHD beamline that proved powerful to determine Hg species.