Polymeric substances and biofilms as biomarkers in terrestrial materials: Implications for extraterrestrial samples

Organic polymeric substances are a fundamental component of microbial biofi lms. Microorganisms, especially bacteria, secrete extracellular polymeric s ubstances (EPS) to form slime layers in which they reproduce. In the sedime ntary environment, biofilms commonly contain the products of degraded bacte ria as well as allochthonous and autochthonous mineral components. They are complex structures which serve as protection for the colonies of microorga nisms living in them and also act as nutrient traps. Biofilms are almost ub iquitous wherever there is an interface and moisture (liquid/liquid, liquid /solid, liquid/gas, solid/gas). In sedimentary rocks they are commonly reco gnized as stromatolites. We also discuss the distinction between bacterial biofilms and prebiotic films. The EPS and cell components of the microbial biofilms contain many cation chelation sites which are implicated in the mi neralization of the films. EPS, biofilms, and their related components thus have strong preservation potential in the rock record. Fossilized microbia l polymeric substances (FPS) and biofilms appear to retain the same morphol ogical characteristics as the unfossilized material and have been recognize d in rock formations dating back to the Early Archaean (3.5 b.y.). We descr ibe FPS and biofilms from hot spring, deep-sea, volcanic lake, and shallow marine/littoral environments ranging up to 3.5 b.y. in age. FPS and biofilm s are more commonly observed than fossil bacteria themselves, especially in the older part of the terrestrial record. The widespread distribution of m icrobial biofilms and their great survival potential makes their fossilized remains a useful biomarker as a proxy for life with obvious application to the search for life in extraterrestrial materials.