Algal photophysiology drives darkening and melt of the Greenland Ice Sheet

• DOI: 10.1073/pnas.1918412117
• Publication Date: 2020-02-25T01:26:17Z
• Authors: Christopher J. Wi...; Joseph Cook; Andrew Tedstone; Marian Yallop; Jenine McCutcheon; Ewa Poniecka; Douglas Campbell; Tristram Irvine-Fynn; James McQuaid; Martyn Tranter; Rupert Perkins; Alexandre Anesio
• Keywords: Meltwater; Greenland ice sheet; Melt pond; Ice-albedo feedback

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