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

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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
Pages (from-to)5694-5705
Number of pages12
JournalProceedings of the National Academy of Sciences of the United States of America
Volume117
Issue number11
Early online date24 Feb 2020
DOIs
Publication statusPublished - 17 Mar 2020

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Keywords

  • Greenland Ice Sheet
  • glacier algae
  • photophysiology
  • melt
  • cryosphere

Cite this

Williamson, C. J., 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. https://doi.org/10.1073/pnas.1918412117