Light absorption and albedo reduction by pigmented microalgae on snow and ice

Abstract

Pigmented microalgae inhabiting snow and ice environments lower the albedo of glacier and ice-sheet surfaces, significantly enhancing surface melt. Our ability to accurately predict their role in glacier and ice-sheet surface mass balance is limited by the current lack of empirical data to constrain their representation in predictive models. Here we present new empirical optical properties for snow and ice algae and incorporate them in a radiative transfer model to investigate their impact on snow and ice surface albedo. We found ice algal cells to be more efficient absorbers than snow algal cells, but their blooms had comparable impact on surface albedo due to the different photic conditions of their habitats. We then used the model to reconstruct the effect of ice algae on bare ice albedo spectra collected at our field site in southern Greenland, where blooms dropped the albedo locally by between 3 and 43%, equivalent to 1–10 L m $^{-2}$ d $^{-1}$ of melted ice. Using the newly parametrized model, future studies could investigate biological albedo reduction and algal quantification from remote hyperspectral and multispectral imagery.

Pigment signatures of algal communities and their implications for glacier surface darkening

“…This is at least partly due to a “pigment packaging effect” that is particularly pronounced in snow algae (Fig. 6 c), which is in line with the study by Chevrollier et al 16 from south-west Greenland. Depending on cell size, internal structure and presence of pigment-protein complexes, the absorption of “packaged” pigments (i.e., inside the cell) can be lower compared to the potential absorption of the same amount of pigments in solution 71 – 73 .…”

“…They contribute to regional nutrient and carbon cycling 3 – 5 and significantly reduce surface albedo, accelerating the melting of ice 6 – 9 and snow 10 – 13 . Their most striking adaptation is their dark pigmentation, making them effective absorbers of solar irradiance 6 , 10 , 14 – 16 . They account on average for 9–13% of surface ice melt on the south-western margin of the Greenland Ice Sheet (GrIS) 6 and 13% of snow albedo reduction across the Arctic 10 .…”

“…Glacier ice algae accumulate large amounts of secondary, brown-coloured phenolic pigments, mainly purpurogallin-derivatives, in vacuoles in their cells 17 , 18 . Purpurogallin-derivatives absorb light broadly across the near-UV and visible wavelength range (λ max 338 nm 18 , 19 ) and influence the absorption properties of the glacier ice algal cells 15 , 16 . The accumulation of secondary pigments protects the internal photosynthetic apparatus from potential damage by excess irradiance 15 , 18 , 20 and can also be observed in the unicellular snow algae (mostly Chlamydomonadales, Chlorophyta) 1 .…”

“…Uncertainties on the light absorption efficiencies of glacier ice 15 and snow algae 24 , 31 have been highlighted as one of the major factors limiting our ability to assess algal-driven surface darkening 31 . Since the pigmentation of algae influences their light absorption and, hence, their darkening potential 15 , 16 , it is crucial to improve our understanding of the extent of pigment accumulation in glacier ice and snow algae for deciphering their impact on surface darkening. For glacier ice algae, in particular, data on their pigmentation are scarce and only available from the south-western margin of the GrIS 15 , 19 and it remains unclear to what extent their pigmentation could vary regionally.…”

Dark ice in a warming world: advances and challenges in the study of Greenland Ice Sheet's biological darkening

“…1a, b). The optical properties of glacier ice algae have recently been measured (Chevrollier and others, 2022;Fig. 1d) and incorporated into the radiative transfer model, BioSNICAR (Flanner and others 2007;Cook and others, 2020), along with an adding-doubling solver (Briegleb and Light, 2007) that was shown to accurately predict glacier ice albedo (Whicker and others, 2022).…”