Surface darkening by abundant and diverse algae on an Antarctic ice cap

Abstract

Algal blooms play important roles in physical and biological processes on glacial surfaces. Despite this, their occurrence and impacts within an Antarctic context remain understudied. Here, we present evidence of the large-scale presence, diversity and bioalbedo effects of algal blooms on Antarctic ice cap systems based on fieldwork conducted on Robert Island (South Shetland Islands, Antarctica). Algal blooms are observed covering up to 2.7 km² (~20%) of the measured area of the Robert Island ice cap, with cell densities of up to 1.4 × 10⁶ cells ml^-1. Spectral characterisation reveal that these blooms increase melting of the ice cap surface, contributing up to 2.4% of total melt under the observed conditions. Blooms are composed of typical cryoflora taxa, dominated by co-occurring Chlorophyceae, Trebouxiophyceae, and Ancylonema. However, morphological variation and genetic diversity in Ancylonema highlight the influence of regional endemism and point to a large and under-characterised diversity in Antarctic cryoflora.

Fig. 1. The Robert Island ice cap.

Imagery of bloom extent and in-situ community composition from the austral summer 2022/23 field sampling campaign. a Location of Robert Island (South Shetland Islands, north-west of the Antarctic Peninsula in the maritime Antarctic). b Drone photograph showing red snow algae and purple glacier algae blooming on the Robert Island ice cap on 8 February 2023. c Dino-Lite micrography of the glacial ice surface algal community taken on weathering crust near the ice cap margin on 18 February 2023. Scale bar ~50 µm, based on the average large dark-purple Ancylonema cell dimensions.

Fig. 2. WorldView 2 satellite image of the Robert Island ice cap and study area.

a Sampling locations on the Robert Island ice cap for algae cell counts (pink circles), field-spectra and paired cell density samples (green squares), community metabarcode samples (yellow diamonds), and location of the drone photograph in Fig. 1b. (orange circle). b Remote sensed algal extent on the Robert Island ice cap based on the WorldView satellite image from 6 February 2023 (pink pixel size 1.6 × 1.6 m). Contours are 10 m apart.

Fig. 3. Representative diversity of Ancylonema spp. morphotypes observed from Robert Island (South Shetland Islands, maritime Antarctic).

Micrographs of fresh samples imaged at the field station 1–5 h after collection. Scale bars 50 µm. a Ancylonema nordenskioeldii morphotype RI-N; b Ancylonema morph. RI-E; c, d cells of Ancylonema morph. RI-T, showing tufts or secretions at cell ends (arrows); e–g A. alaskanum morph. RI-A of various sizes; h the wider cell type of Ancylonema morph. RI-C. Representative of the images taken from 37 samples across the 198 ice cap sample locations. Higher-resolution micrographs are in Fig. S3.

Fig. 4. Biometric comparison of Ancylonema spp. cells showing variation within and between Robert Island morphotypes and existing studies.

a Average cell length comparisons between large Ancylonema morphotypes observed from Robert Island and in the wider literature. b Cell length versus width distributions across all measured Robert Island Ancylonema morphotypes. A randomised jitter (offset) of ≤0.2 µm has been applied to each point along the cell width axis to aid visualisation. c Average cell width comparisons between smaller A. alaskanum type cells from Robert Island (including variant RI-C) and in the wider literature. Colours represent the different Robert Island morphotypes. Solid lines in (a) and (c) represent measurement mean (RI-N N = 121, RI-T = 54, RI-E = 55, RI-C = 45, RI-A = 219) and standard deviations, and shaded areas represent cell measurement ranges, where available. Average and SD measurements from the following locations and studies: Svalbard, Greenland, Alaska, Switzerland, Austria, Chile, Windmill Is. (S) (small cells), Windmill Is. (B) (big cells).

Fig. 5. Biological community overview of the Robert Island ice cap (South Shetland Islands, maritime Antarctic) from 18S metabarcoding.

a 18S V4 metabarcode community composition from the Robert Island ice cap sequencing pool. Circled numbers refer to the top ten ASVs as outlined below. b The top 10 ASVs and their proportion within the 18S V4 dataset and the nearest NCBI blast-n accession (>99.6% similarity).

Fig. 6. Ancylonema spp. ITS2 haplotype diversity and phylogenetic placement from Robert Island and wider studies.

Sequence-structure phylogenetic maximum-likelihood tree of Ancylonema ITS2 haplotypes from Robert Island and elsewhere, including reference and environmental sequences for A. nordenskioeldii, A. alaskanum, and Ancylonema sp.^,, environmental sequence haplotypes from Greenland, Svalbard, Alaska, Sweden, and the European Alps^,,, and an outgroup reference, C. cushlekae. Sequence data for “Remias_2023 H2” are in supplementary data for Remias et al. (2023). Haplotype occurrence by region is indicated in the ‘Region’ column. Maximum CBC positions between members of assigned clades are indicated in the inset table. Read distributions for ITS haplotypes are in Table S2.

Fig. 7. Field spectral data from the Robert Island ice cap (South Shetland Islands, maritime Antarctic).

a Hemispherical directional reflectance factors (HDRF) for cryoflora on the Robert Island ice cap. Spectra with Ancylonema present are shown with solid red line, red cell dominated spectra shown with red dashed line, clean snow with no visible algae or dust with solid blue line, weathering snow crust with solid black line. Colour scale shows cell density (cells ml⁻¹). b Instantaneous radiative forcing (IRF W/m²) compared with cell density (cells/ml) for the samples with Ancylonema present (diamond) and red pigment-producing cryoflora (circle) observed on the Robert Island ice cap.