The most abundant record of the first ~3 billion years of life on Earth consists of fossilized microbial communities, e.g. stromatolites and microbialites. Modern microbial communities make analogous structures that vary depending on biological and environmental factors (e.g. Abed et al. 2003; Jones et al. 2002; Wharton 1994; Hofmann 1969; Bertrand-Sarfati & Moussine-Pouchkine 1985; Sumner 1997 2000; Ben-Jacob 2003). Understanding the early evolution of life depends on inferring biological properties from the remnants of fossilized microbial communities (e.g. Buick 1992; Farmer & DesMarais 1994; Sumner 1997, 2000; Knoll & Semikhatov 1998; Hofmann et al. 1999; Batchelor et al. 2005). Extracting biological information from these ancient structures has been extremely challenging because we do not yet understand how microbial behaviors interact with environmental parameters to produce distinctive mat morphologies. In addition, most of our understanding of microbialite formation comes from tropical or subtropical locations (e.g. Pringault 2004) even though microbial communities must have dominated all environments for most of earthÕs history.
We propose to add to our understanding of microbial community behaviros and microbialite formation by investigating modern calcifying microbial communities in Lake Joyce, Antarctica. Previous work on uncalcified mats in Lake Hoare, Antarctica, demonstrates that microscale gradients in water chemistry are directly related to both mat morphology and metabolic activity (Vopel and Hawes 2006). Because cyanobacteria are the major primary mat producers in both lakes (Wharton et al. 1983) and cyanobacteria create intricate, calcifying mats in diverse other settings (e.g. Walter 1976; Brock 1978; Jones et al. 2002; Abed et al. 2003), we hypothesize that the patterns of carbonate precipitation in Lake Joyce record cyanobacterial activity, both morphologically and geochemically, and that microbial influences will be identifiable and applicable to interpreting the origins of ancient microbialites.
There are several reasons why studies of mats in Lake Joyce will be particularly productive. Recent results from field and lab studies suggest that motile, filamentous bacteria form peaked and ridged mats irrespective of their chemical environment (Shepard et al. 2006; Sumner 2006). These lab results are consistent with the observation that modern mats with these morphologies are composed of filamentous bacteria even though their environments vary dramatically (e.g. Love et al. 1983; Wharton 1994; Jones et al. 2002). Some of the Lake Joyce mats have a similar morphology even though they are metabolically limited by low light levels, and chemical transport in the lake is mostly diffusive. Thus, if Lake Joyce peaked and ridged mats are dominated by motile, filamentous bacteria, we will be able attribute at least some of the morphological characteristics to specific microbial behaviors irrespective of environmental influences. Similar structures in ancient stromatolites and microbialites can then be evaluated within this context, providing substantially more rigorous constraints on interpretation of morphology. If the Lake Joyce peaked mats are NOT dominated by motile, filamentous bacteria, they will provide important new models for the growth of such structures, requiring a revision of interpretations of at least some ancient stromatolites.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616