"Hydrocarbon pollution has been recognized to be a major environmental and human health problem that require accurate exposure assessment and remediation. Oil and oily products are extremely complex mixtures, containing hundreds (even thousands) of different compounds, among which polycyclic aromatic hydrocarbons (PAHs) are of greatest regulatory concern due to their potential toxic, mutagenic and carcinogenic properties. 2- and 3-ring PAHs are water soluble and can be transported over significant distances.Natural attenuation is a low-cost bioremediation option widely accepted for the clean-up of hydrocarbon polluted sites. Many efforts have been made to study and enhance aerobic biodegradation of hydrocarbons. However, anaerobic degradation of oily products is practically unknown, although in many environments, such as aquifers, marshes or intertidal zones oxigen is often a limiting factor.Some studies have proven the ability of microorganisms to degrade aromatic hydrocarbons in different conditions, but there is a significant gap of knowledge regarding in situ anaerobic biodegradation of these compounds (metabolism, key microorganisms involved, etc.).Stable isotope techniques (compound specific stable isotope analysis, CSIA, and stable isotope probing, SIP) are novel techniques which can help overcoming this situation, providing valuable information on biodegradation and coping suitably with linking biodegradation processes to microbial taxa. Despite their clear advantatges these techniques have seldom been applied to field studies.In the light of this situation, the main goals of this proposed project are to assess in situ biodegradation of PAHs under anaerobic environments in marine and fresh water systems, to describe microbial activities and to identify microbial key players.The project will be carried out in the Isotope Biogeochemistry Department at the UFZ (Leipzig), which provide outstanding facilities for the achievement of these objectives."