Diatom ecology and microbial mat structure and function in antarctic dry valleys

摘要

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 (Treguer et al., 1995; Field et al., 1998). Their cell walls, or frustules, are constructed out of biogenic silica (SiO_2), 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.