Bacterial Activity and Preservation of Sedimentary Organic Matter: The Role of Exopolymeric Substances

Abstract

Although exopolymeric substances (EPS) are associated with the microorganisms contributing to the production/degradation of sedimentary organic matter, their role in theses processes have so far never been mentioned. Using high-resolution 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., respectively sulphuroxidizing bacteria and cyanobacterial biofilms. This comparison shows that, particularly in the case of Kimmeridgian cyanobacterial mats deposited in a shallow back-reef environment, organic matter preservation is conditioned by exopolymeric substances secreted by bacteria. A model is proposed for the evolution through time of exopolymeric 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 from degradation and remineralization. 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 contributing 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.

Keywords:

• amorphous organic matter
• cyanobacterial biofilm
• microscopy
• Miocene black shales
• Kimmeridgian bituminous shales
• sulphuroxidizing bacteria

ACKNOWLEDGMENTS

This study is supported by the Swiss National Science Foundation (grants no. 2000.68091 and 2000.112320) and by the UMR-CNRS 8148 I.D.E.S (France), Paris-Sud. The authors thank Jeril Degrouard and Danielle Jaillard (Service laboratory for electron microscopy, department of cellular biology, Paris-Sud University, France) for their assistance with TEM imaging and ultramicrotom. Philippe Pradel is kindly acknowledged for his assistance with AFM imaging (UMR-CNRS 8148 I.D.E.S, Paris-Sud, France). They also thank Rosanna Martini for her help with SEM imaging and Crisogono Vasconcelos for providing the recent bacteria samples. The authors are indebted to two anonymous reviewers for their highly constructive comments.