Marked Seasonal Changes in the Microbial Production, Community Composition, and Biogeochemistry of Glacial Snowpack Ecosystems in the Maritime Antarctic

We describe seasonal changes in the biogeochemistry, microbial community and ecosystem production of two glacial snowpacks in the maritime Antarctic during a cold summer. Frequent snowfall and low, intermittent melt on the glaciers suppressed surface photosynthesis and promoted net heterotrophy. Concentrations of autotrophic cells (algae and cyanobacteria) were therefore low (average: 150-500 cells mL(-1)), and short-term estimates of primary production were almost negligible in early summer (<0.1 mu g C L-1 d(-1)). However, order of magnitude increases in Chlorophyll a concentrations occurred later, especially within the mid-snowpack and ice layers below. Short-term primary production increased to ca. 1 mu g C L-1 d(-1) in mid-summer, and reached 53.1 mu g C L-1 d(-1) in a mid-snow layer close to an active penguin colony. However, there were significantly more bacteria than autotrophs in the snow (typically 10(3) cells mL(-1), but >10(4) cells mL(-1) in basal ice near the penguin colony). The ratio of bacteria to autotrophs also increased throughout the summer, and short-term bacterial production rates (0.2-2000 mu g C L-1 d(-1)) usually exceeded primary production, especially in basal ice (10-1400 mu g C L-1 d(-1)). The basal ice represented the least diverse but most productive habitat, and a striking feature was its low pH (down to 3.3). Furthermore, all of the overlying snow cover became increasingly acidic as the summer season progressed, which is attributed to enhanced emissions from wet guano in the penguin colony. The study demonstrates that active microbial communities can be expected, even when snowmelt is intermittent in the Antarctic summer.