Investigating Algal Communities in Lacustrine and Hydro-Terrestrial Environments of East Antarctica Using Deep Amplicon Sequencing

Abstract

Antarctica has one of the most extreme environments on Earth, with low temperatures and low nutrient levels. Antarctica's organisms live primarily in the coastal, ice-free areas which cover approximately 0.18% of the continent's surface. Members of Cyanobacteria and eukaryotic algae are important primary producers in Antarctica since they can synthesize organic compounds from carbon dioxide and water using solar energy. However, community structures of photosynthetic algae in Antarctica have not yet been fully explored at molecular level. In this study, we collected diverse algal samples in lacustrine and hydro-terrestrial environments of Langhovde and Skarvsnes, which are two ice-free regions in East Antarctica. We performed deep amplicon sequencing of both 16S ribosomal ribonucleic acid (rRNA) and 18S rRNA genes, and we explored the distribution of sequence variants (SVs) of these genes at single nucleotide difference resolution. SVs of filamentous Cyanobacteria genera, including Leptolyngbya, Pseudanabaena, Phormidium, Nodosilinea, Geitlerinama, and Tychonema, were identified in most of the samples, whereas Phormidesmis SVs were distributed in fewer samples. We also detected unicellular, multicellular or heterocyst forming Cyanobacteria strains, but in relatively small abundance. For SVs of eukaryotic algae, Chlorophyta, Cryptophyta, and Ochrophyta were widely distributed among the collected samples. In addition, there was a red colored bloom of eukaryotic alga, Geminigera cryophile (Cryptophyta), in the Langhovde coastal area. Eukaryotic SVs of Acutuncus antarcticus and/or Diphascon pingue of Tardigrada were dominant among most of the samples. Our data revealed the detailed structures of the algal communities in Langhovde and Skarvsnes. This will contribute to our understanding of Antarctic ecosystems and support further research into this subject.

Keywords: Algae; Antarctica; Cyanobacteria; Hydro-Terrestrial; Lacustrine; Microbiome; Tardigrade.

Figure 1

Collection of algal samples. (A) Location of sampling sites in Langhovde and Skarvsnes. Sampling sites of S1-S13 were shown as yellow stars. Names of the lakes of the sampling points were shown accordingly. Map data were obtained from Quantarctica package (http://quantarctica.npolar.no/) or Geospatial Information Authority of Japan (https://www.gsi.go.jp/antarctic/) with modifications. (B) Photographs of sampling positions of S1-S13 samples. The inlet photographs show the enlargement of the sampling positions indicated by yellow triangles.

Figure 2

Overall community structures of collected samples. Bar plots show the composition of sequence variants (SVs) that were agglomerated to the phylum level classification of the SILVA ver. 132 database for 16S rRNA (A) or the Rank 3 classification of the Protist Ribosomal Reference (PR2) database for 18S rRNA (B). Taxa below an average frequency of 2% are not shown. The NMDS plots of the Bray–Curtis distance matrix of each sample are shown for 16S rRNA (C) and 18S rRNA (D).

Figure 3

Relative abundance and assigned taxonomy of the major SVs (16S rRNA gene) retrieved from 13 lacustrine and hydro-terrestrial samples collected in Langhovde and Skarvsnes, East Antarctica. SVs had been represented if they reached a relative abundance over 0.2% of the overall dataset. IDs of SVs with high prevalence (>6/13 samples) are shown in red.

Figure 4

Relative abundance and assigned taxonomy of the major SVs (18S rRNA gene) retrieved from 13 lacustrine and hydro-terrestrial samples collected in Langhovde and Skarvsnes, East Antarctica. SVs had been represented if they reached a relative abundance over 0.2% of the overall dataset. IDs of SVs with high prevalence (>6/13 samples) are shown in red. Unidentified ranks were shown as ‘_X’ in as described in the PR2 database [29].

Figure 5

Phylogenetic tree, relative abundance, and genus level taxonomy of all 92 SVs of cyanobacteria other than chloroplast detected. Branches with a confidence value >80% in the Shimodaira-Hasegawa (SH) test using the FastTree program are shown as black circles [33]. Morphological classifications of sections I-V in Cyanobacteria were shown accordingly.