Algal photophysiology drives darkening and melt of the Greenland Ice Sheet

Christopher J. Williamson*, Joseph Cook, Andrew Tedstone, Marian Yallop, Jenine McCutcheon, Ewa Poniecka, Douglas Campbell, Tristram Irvine-Fynn, James McQuaid, Martyn Tranter, Rupert Perkins, Alexandre Anesio

*Corresponding author for this work

Research output: Contribution to journal/Conference contribution in journal/Contribution to newspaperJournal articleResearchpeer-review

80 Citations (Scopus)
52 Downloads (Pure)

Abstract

Blooms of Zygnematophycean “glacier algae” lower the bare ice albedo of the Greenland Ice Sheet (GrIS), amplifying summer en- ergy 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 phe- nolic pigmentation (11 times the cellular content of chlorophyll a) enables glacier algae to tolerate extreme irradiance (up to ∼4,000 μmol photons·m−2·s−1) while simultaneously repurposing captured ultraviolet and short-wave radiation for melt generation. Total cellular energy absorption is increased 50-fold by pheno- lic pigmentation, while glacier algal chloroplasts positioned be- neath shading pigments remain low-light–adapted (Ek ∼46 μmol photons·m−2·s−1) and dependent upon typical nonphotochemical quenching mechanisms for photoregulation. On the GrIS, glacier algae direct only ∼1 to 2.4% of incident energy to photochemistry versus 48 to 65% to ice surface melting, contributing an additional ∼1.86 cm water equivalent surface melt per day in patches of high algal abundance (∼104 cells·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 (R2 = 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.
Original languageEnglish
JournalProceedings of the National Academy of Sciences (PNAS)
Volume117
Issue11
Pages (from-to)5694-5705
Number of pages12
ISSN0027-8424
DOIs
Publication statusPublished - 17 Mar 2020

Keywords

  • Cryosphere
  • Glacier algae
  • Greenland Ice Sheet
  • Melt
  • Photophysiology

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