Williamson, CJ; Cook, J; Tedstone, A; Yallop, M; McCutcheon, J; Poniecka, E; Campbell, D; Irvine-Fynn, T; McQuaid, J; Tranter, M; Perkins, R; Anesio, A (2020). Algal photophysiology drives darkening and melt of the Greenland Ice Sheet. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 117(11), 5694-5705.

Blooms of Zygnematophycean "glacier algae" lower the bare ice albedo of the Greenland Ice Sheet (GrIS), amplifying summer energy absorption at the ice surface and enhancing meltwater runoff from the largest cryospheric contributor to contemporary sea-level rise. Here, we provide a step change in current understanding of algal-driven ice sheet darkening through quantification of the photophysiological mechanisms that allow glacier algae to thrive on and darken the bare ice surface. Significant secondary phenolic pigmentation (11 times the cellular content of chlorophyll a) enables glacier algae to tolerate extreme irradiance (up to similar to 4,000 mu mol photons center dot m(-2)center dot s(-1)) while simultaneously repurposing captured ultraviolet and short-wave radiation for melt generation. Total cellular energy absorption is increased 50-fold by phenolic pigmentation, while glacier algal chloroplasts positioned beneath shading pigments remain low-light-adapted (E-k similar to 46 mu mol photons center dot m(-2)center dot s(-1)) and dependent upon typical nonphotochemical quenching mechanisms for photoregulation. On the GrIS, glacier algae direct only similar to 1 to 2.4% of incident energy to photochemistry versus 48 to 65% to ice surface melting, contributing an additional similar to 1.86 cm water equivalent surface melt per day in patches of high algal abundance (similar to 10(4) cells center dot mL(-1)). At the regional scale, surface darkening is driven by the direct and indirect impacts of glacier algae on ice albedo, with a significant negative relationship between broadband albedo (Moderate Resolution Imaging Spectroradiometer [MODIS]) and glacier algal biomass (R-2 = 0.75, n = 149), indicating that up to 75% of the variability in albedo across the southwestern GrIS may be attributable to the presence of glacier algae.