New method provides first evidence of fine-scale in situ heterogeneity in glacier algal photophysiology

Streptophyte “glacier algae” thrive on the melting surfaces of glaciers and ice sheets, producing conspicuous blooms during summer ablation seasons. To-date, the direct application of pulse amplitude modulation (PAM) fluorometry to measure the in situ responses of glacier algae to their environment has been prevented by the high ambient light environment, causing over-saturation of fibre- and imaging-PAM systems. The use of cuvette-based systems has also relied upon bulk sampling and prolonged melting of surface ice, reducing or removing the short-term light history of glacier algae prior to measurements. Here, we outline a simple method that allows the rapid liberation of glacier algal cells into suspension for the measurement of their photophysiology using the sensitive Walz Water-PAM cuvette system, moving from ice to measurement with only a 5-min dark-adaptation period. We illustrate the effectiveness of our method by performing rapid light curves (RLCs) on glacier algae from naturally shaded versus fully exposed ice surfaces on the Greenland Ice Sheet and contrast this with typical 24 h melt/dark-adapt approaches employed to-date. Results highlight that fine-scale heterogeneity is apparent in glacier algal in situ photophysiology relative to dominant environmental forcings, with our method effective at constraining the near actual photochemistry of glacier algae as opposed to their more optimal photochemistry measured previously. Easy to implement on the ice, our method should allow future mapping of the fine-scale patterning in glacier algal responses to surface ice stressors.