Glacier Algae: A Dark Past and a Darker Future

Abstract

"Glacier algae" grow on melting glacier and ice sheet surfaces across the cryosphere, causing the ice to absorb more solar energy and consequently melt faster, while also turning over carbon and nutrients. This makes glacier algal assemblages, which are typically dominated by just three main species, a potentially important yet under-researched component of the global biosphere, carbon, and water cycles. This review synthesizes current knowledge on glacier algae phylogenetics, physiology, and ecology. We discuss their significance for the evolution of early land plants and highlight their impacts on the physical and chemical supraglacial environment including their role as drivers of positive feedbacks to climate warming, thereby demonstrating their influence on Earth's past and future. Four complementary research priorities are identified, which will facilitate broad advances in glacier algae research, including establishment of reliable culture collections, sequencing of glacier algae genomes, development of diagnostic biosignatures for remote sensing, and improved predictive modeling of glacier algae biological-albedo effects.

Keywords: Streptophytes; albedo; glacier algae; ice; terrestrialization.

Figure 1

Glacier algae and the supraglacial environment: (A) RGB composite image of the southwestern Greenland Ice Sheet (GrIS) margin near Kangerlussuaq, derived from European Space Agency Sentinel-2 data. Note the conspicuous “dark zone” running parallel to the ice sheet margin for which glacier algal blooms are thought responsible. (B) GrIS surface ice within the dark zone dominated by a glacier algal bloom during the 2016 ablation season. (C) Glacier algae assemblage sampled from the surface of the GrIS. (D) M. berggrenii; (E) A. nordenskiöldii; and (F) C. brebissonii (contributed by Nozomu Takeuchi). Scale bars are 20 km (A) and 10 μm (C–F).

Figure 2

Streptophyte phylogeny and congruent age estimates, highlighting the Zygnematophyceae as sister lineage to land plants, and the timing of the Chlorophyte/Streptophyte division during the Cryogenian period (adapted with permission from Morris et al., 2018). Blue density plots show 95% highest posterior density of age estimates.