Photosynthetic Organisms in Subarctic Lake Ice

While sampling No-name lake, which is located in the Tanana Valley, Alaska, at 63°48'N., 144°35'W., we observed numerous bubbles containing mixed populations of photosynthetic bacteria and algae. Most of the material consisted of purple, pink and green colonies of bacteria. Few algae were present, and those were primarily life-cycle stages of Chlamydomonas , but the population was dominated by photosynthetic bacteria. This occurrence of large numbers of organisms in complex populations differs substantially from previous reports of organisms in lake ice. The ice environment was somewhat unusual in that numerous overflows had occurred during the winter. In these circumstances the rate of accretion at the bottom of the ice sheet is much reduced, temperatures remain close to the freezing point and free water can occur within the ice. The water below the ice was anoxic and this, in combination with the favourable physical factors and presence of bubbles, may have been responsible for the unusual occurrence of organisms in the lake ice.