Microbial Diversity of Pinnacle and Conical Microbial Mats in the Perennially Ice-Covered Lake Untersee, East Antarctica

Abstract

Antarctic perennially ice-covered lakes provide a stable low-disturbance environment where complex microbially mediated structures can grow. Lake Untersee, an ultra-oligotrophic lake in East Antarctica, has the lake floor covered in benthic microbial mat communities, where laminated organo-sedimentary structures form with three distinct, sympatric morphologies: small, elongated cuspate pinnacles, large complex cones and flat mats. We examined the diversity of prokaryotes and eukaryotes in pinnacles, cones and flat microbial mats using high-throughput sequencing of 16S and 18S rRNA genes and assessed how microbial composition may underpin the formation of these distinct macroscopic mat morphologies under the same environmental conditions. Our analysis identified distinct clustering of microbial communities according to mat morphology. The prokaryotic communities were dominated by Cyanobacteria, Proteobacteria, Verrucomicrobia, Planctomycetes, and Actinobacteria. While filamentous Tychonema cyanobacteria were common in all mat types, Leptolyngbya showed an increased relative abundance in the pinnacle structures only. Our study provides the first report of the eukaryotic community structure of Lake Untersee benthic mats, which was dominated by Ciliophora, Chlorophyta, Fungi, Cercozoa, and Discicristata. The eukaryote richness was lower than for prokaryote assemblages and no distinct clustering was observed between mat morphologies. These findings suggest that cyanobacterial assemblages and potentially other bacteria and eukaryotes may influence structure morphogenesis, allowing distinct structures to form across a small spatial scale.

Lipid Biomarkers From Microbial Mats on the McMurdo Ice Shelf, Antarctica: Signatures for Life in the Cryosphere

“…This agrees with a previous study that has detected these phyla in the microbial mats from the McMurdo Ice Shelf (Varin et al, 2012). While similar 16S rRNA gene communities were identified for the microbial mats on the McMurdo Ice Shelf and Lake Untersee, we are also aware that PCR can lead to amplification biases (Greco et al, 2020;Jackson et al, 2021). Further evaluation of the composition will benefit from metagenomic approaches that cover all DNA, including ribosomal and protein-coding genes, without PCR.…”

16S rRNA gene and 18S rRNA gene diversity in microbial mat communities in meltwater ponds on the McMurdo Ice Shelf, Antarctica

“…Our study is the first detailed 18S rRNA gene highthroughput sequencing analysis of microbial mats on the McMurdo Ice Shelf. The 18S rRNA gene analysis suggested that microbial Eukaryotes had a lower richness than the Bacteria and Archaea, which was also found for microbial mats in the perennially ice-covered Lake Untersee and Lake Fryxell, Antarctica (Greco et al 2020), and been described from other ecosystems such as cryoconite holes in the McMurdo Dry Valleys (Sommers et al 2018). The communities were dominated by Ochrophyta, Chlorophyta and Ciliophora, and previous work on 18S rRNA gene clone library and morphological surveys had demonstrated that microalgae, especially diatoms, are present in meltwater ponds (Sutherland 2009;Jungblut et al 2012b).…”

“…The communities were dominated by Ochrophyta, Chlorophyta and Ciliophora, and previous work on 18S rRNA gene clone library and morphological surveys had demonstrated that microalgae, especially diatoms, are present in meltwater ponds (Sutherland 2009;Jungblut et al 2012b). Sommers et al (2018) and Greco et al (2020) also found abundance of ciliates and similar groups of ciliates in Lake Untersee and cryoconite holes from Taylor Valley glaciers; however, it appears that more ciliate 18S rRNA gene sequences were recovered from microbial mats on the McMurdo Ice shelf. In contrast, fungi had a lower relative abundance on the McMurdo Ice Shelf and cryoconite holes than in the microbial mats in the ice-covered Lake Untersee.…”

Glacial lake outburst floods enhance benthic microbial productivity in perennially ice-covered Lake Untersee (East Antarctica)

“…The total inorganic carbon (TIC) and total organic carbon (TOC) concentrations in the lake are very low (0.3-0.4 mg C L −1 ) 22 , 1-3 orders of magnitude lower than in the MDV lakes and Antarctic subglacial lakes [23][24][25][26] . Despite the ultra-oligotrophic, low-light conditions, and lack of a seasonal moat, the lake hosts a benthic microbial ecosystem composed of photosynthetic microbial mats, small cuspate pinnacles, and large conical stromatolitic structures to depths of at least 130 m [27][28][29][30] (Fig. 2).…”

“…2). The top layer of the microbial mats comprises a carbonfixing cyanobacteria community (Tychonema sp., Phormidium sp., Leptolyngbya sp., and Pseudanabaena sp) that shift to a heterotrophic community in the underlying layers (Actinobacteria, Verrucomicrobia, Proteobacteria, and Bacteroidetes) 28,30,31 . The phototrophic mats are growing in very low light conditions: c. 5% of incident photosynthetically active radiation (PAR) irradiance is transmitted through the lake ice and decreases to c. 0.1% at a 135 m depth 27 .…”