Eukaryote-Dominated Biofilms in Extreme Environments: Overlooked Sources of Information in the Geologic Record

Abstract

Our quest to find the earliest forms of microbial life and understand their role in the mediation of surficial processes has focused primarily on bacteria, since they represent the most primitive forms of life and possess the ability to contribute to the precipitation and dissolution of minerals (e.g., Fortin et al., 1997; Little et al., 1997; Nealson and Stahl, 1997) and the formation of biolaminated structures such as stromatolites (e.g., Awramik et al., 1976; Golubic, 1976). Stromatolites are the mineralized counterparts of microbial mats and represent some of the most tangible sources of morphological, biological, and chemical evidence for life on early Earth (e.g., Grotzinger and Knoll, 1999; Hofmann, 2000; Schopf et al., 2007). Stromatolite microstructures preserve paleobiological and paleoenvironmental information that can provide insight on the early evolution of the biosphere, atmosphere, and geosphere. Today, as in the distant past, bacteria are found in a range of environments from oxic to anoxic, thermal to arctic, and acid to alkaline, and they are important in biofilms and in rare, modern stromatolites (Cavicchioli, 2002; Gupta et al., 2004; Edwards et al., 2005). Our understanding of bacteria is, therefore, important in our search for similar life forms in extraterrestrial locations. Our focus on the role of bacteria, however, has created a gap in our understanding of the importance of eukaryotic microorganisms in some of these same environments. Eukaryotes are often overlooked in the study of early Earth since their body fossils are rarely preserved in the geologic record (Knoll, 1985; Schopf, 1999). The origin of eukaryotic cells, however, is considered to be one of the most important evolutionary steps in the history of life (Schopf, 1999), as it led to the evolution of multicellular life (Awramik, 1981). Fossil evidence indicates that eukaryotic microorganisms evolved …