Linking extreme light availability to cellular function in algae-dominated communities on the Greenland Ice Sheet.

Glacier ice algae of the streptophyte genus Ancylonema bloom on glaciers globally, including the Greenland Ice Sheet. These algae survive under extreme high light conditions in the summer, as well as under very low light or total darkness during (polar) winters and winter burial under snow. However, little is known about the cellular mechanisms underpinning glacier ice algae ecophysiological plasticity in response to extreme light availability. To address this knowledge gap, we evaluated the response of Ancylonema-dominated taxa in samples from the Greenland Ice Sheet to light and dark conditions during a 12-day period using combined multi-omics analyses. The microbial community was not substantially altered during the 12 days of dark incubation, however transcriptomic analysis demonstrated that the algae-associated heterotrophs became more active in the dark. In contrast, we identified a striking algal transcriptome stability in light conditions, in addition to high oxidative stress responses and evidence for high photosystem protein turnover. We also identified transcriptional reprogramming linked to sugar uptake and phytohormone signalling during dark incubation. These results provide crucial clues into the ability of glacier ice algae to adapt and survive in a harsh and extremely variable light environment.