Diatom Ecology and Microbial Mat Structure and Function in Antarctic Dry Valleys

Streams in the McMurdo Dry Valleys of Antarctica (MDV) are harsh habitats for life. Months of perpetual darkness and subzero temperatures are punctuated by months of perpetual sunlight, intense UV radiation, and temperatures that hover around freezing. The availability of liquid water regulates biological activity and limits the growing season to 6–10 weeks each year. These harsh conditions likely allow perennial microbial mats to persist in this freshwater environment (Fig. 1), as the rates of biomass accumulation exceed losses due to grazing metazoans and scouring (McKnight and Tate, 1997). Despite the extreme selective conditions, MDV microbial mats harbor a diverse diatom flora. Diatoms comprise a large lineage of eukaryotic algae that are broadly distributed in aquatic habitats and contribute significantly to global primary productivity (Tréguer et al., 1995; Field et al., 1998). Their cell walls, or frustules, are constructed out of biogenic silica (SiO2), which can persist as a biosignature in sediments and the rock record for millions of years (Kooistra et al., 2007). The morphology of the frustule is generally species specific and is, therefore, relied upon for taxonomic identification, although cryptic species do exist (Beszteri et al., 2005). In MDV streams, approximately 42 species belonging to 17 genera have been taxonomically described. Phylogenetically, all of these species belong to the relatively young (30– 50 Ma [Kooistra et al., 2007; Sorhannus, 2007]) group of bilaterally symmetric, raphid pennates that dominate in both modern marine and freshwater habitats. Diatom remains are frequently used as paleoecological indicators, and we confer ecological preferences of ancient diatoms based on the ecological preferences of their modern counterparts. Understanding modern diatom ecology is therefore essential for constraining diatombased paleoreconstructions. In the MDVs, for example, glaciologists have concluded that diatom assemblages from ancient stream-delta microbial mats indicate that the sea level during the late Quaternary Period was ,300 m higher than today (Miagkov et al., 1976). A later study revisited this analysis and found that the previous study incorrectly characterized the ecology of the diatom assemblages as marine when they were in fact inland species (Kellogg et al., 1980). This latter study concluded that the diatom assemblages reflect the occurrence of an ice dam that raised lake levels in the MDVs during the late Quaternary. Characterizing the ecological preferences of diatoms in MDV streams is valuable not only for constraining paleoclimate studies, but also as a proxy for the function of microbial mats in the current state of rapid environmental change in the Antarctic. Physical processes have long been suspected to exert strong influences on MDV diatoms (McKnight et al., 1999). Recent studies have corroborated this assertion, showing that the duration and intensity of stream flow drives variation in diatom communities (Esposito et al., 2006; Stanish et al., 2011, 2012). Diatoms that inhabit MDV streams break into two groups: one is more abundant in fast-moving water with a stable flow regime; the other is more abundant in slow-moving, hydrologically unstable streams. The underlying mechanism for such habitat preferences is not understood, although size may be important (Stanish et al., 2011), as smaller, faster growing species can outcompete larger, slower growing species in a more stable environment. Microbial mats typically contain consortia of bacteria, Archaea, and Eucarya that interact with both the external environment and their microbial neighbors (Spear et al., 2003; Ley et al., 2006; Feazel et al., 2008; Robertson et al., 2009). The first traces of cellular life on Earth are recorded in ancient stromatolites—laminated, lithified microbial mats—where oxygenic photosynthesis is thought to have evolved (Buick, 1992, 2008). Consideration of the relative importance of