The unicellular alga Chlamydomonas nivalis leaves its mark on snowfields with a telltale pinkish hue. The red algae typically live at altitudes higher than 2,500 meters and temperatures below freezing. Despite such harsh conditions and correspondingly high radiation levels, C. nivalis can remove carbon dioxide from its surroundings through photosynthesis. Indeed, scientists report this week in the online edition of the Proceedings of the National Academy of Sciences that "gas-exchange from snow-algae-covered areas might represent a small but significant global carbon sink not previously noted, because summer snowfields cover significant areas of the Earth."

William E. Williams, a professor of biology at St. Marys College of Maryland, and his colleagues studied patches of snow containing C. nivalis in the Snowy Range of the Rocky Mountains. The scientists assembled a shallow Plexiglass chamber containing sensors for tracking photosynthesis and took measurements in early July three years in a row (see image). They found that the bacteria absorbed carbon dioxide from both the soil and the air when exposed to the ambient light of their surroundings. By filtering the natural light, the team also determined that C. nivalis absorbs the greenhouse gas more efficiently in red light than in white, blue or green light because of its own coloring. The organisms react to varying light levels in a manner similar to that of the leaves of higher plants. Although the findings are preliminary, they suggest that photosynthesis in a square meter of snow could soak up five grams of carbon each year, and that "summer snowfields can be surprisingly productive."