Bacterial activity and preservation of sedimentary organic matter: The role of exopolymeric substances

Although exopolymeric substances (EPS) are associated with the microorganisms contributing to the production/degradation of sedimentary organic matter, thier role in these process have so far never been mentioned. Using high-resoultion microscopical tools (scanning and transmission electron microscopy, atomic force microscopy), fossil organic matter in the Miocene monterey Formation (California) and Kimmeridgian laminites (France) has been compared with its present-day analogs, i.e., repectively sulphuroxidizing bacteria and cyanobacterial biofilms. This comparision shows that, particularly in the case of Kimmeridgian cyanobacterial mats deposited in a shallow back-reef enviornment, organic matter preservation is conditioned by exoplolymeric substances secreted by bacteria. A model is proposed for the evolution through time of exopolumeric substances in relation to the mechanical constrains they have been exposed to, during lithification and diagenesis. This model is based on the microscopical observation of sulphuroxidizing bacteria and could explain the morphology of fossil organic matter usually referred to as amorphous in standard light microscopy. The highly hydrated nature of exopolymeric substances helps to protect organic matter form degradation and rem-ineralization. These substances can be observed only in microscopy and are undetectable through organic geochemical methods, hence the need to combine these two methods in organic matter studies. Consequently, exopolymeric substances must be considered as an important contirbuting agent to organic matter preservation. These results confirm the complexity of the bacterial role in geoenvironments and add a new parameter in the productivity-vs-preservation debate.