DIATOM ECOLOGY AND MICROBIAL MAT STRUCTURE AND FUNCTION IN ANTARCTIC DRY VALLEYS

Abstract

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). FIGURE 1— Harnish Creek Tributary in the McMurdo Dry Valleys of Antarctica. A) Glacial meltwater flows into stream channels that drain into closed-basin lakes on the valley floors. The unconsolidated sediments in this image represent much of the ground surface, while a stable stone pavement is found in numerous stream channels, which results from freeze-thaw action over many years. Large seepage areas and a wide hyporheic zone develop as meltwater infiltrates the unconsolidated sediment. B) Close-up of area. C) Vibrant black and orange colors are UV-protective pigments produced by filamentous cyanobacteria, which form the matrix. Gas bubbles within the microbial mat demonstrate the high photosynthetic rates that can be achieved. Photo credit: Breana Simmons. 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 (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 …