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.
Highlights
Microbial mat communities dominated by cyanobacteria are important ecosystem components across a diverse range of marine and freshwater environments and form the oldest identifiable fossil assemblages from the Earth’s early biosphere (Allwood et al, 2006)
The evolutionary mapping analysis showed that several Amplicon Sequence Variant (ASV) were related to environmental sequences and cyanobacteria strains previously identified from microbial mats in Antarctica including Pyramid Trough and the McMurdo Dry Valley Lake Hoare and Lake Vanda (Southern Victorialand)
The benthic microbial mats of the southern basin of Lake Untersee were dominated by Cyanobacteria, and harbored a diverse community including Proteobacteria, Verrucomicrobia, Planctomycetes, and Actinobacteria
Summary
Microbial mat communities dominated by cyanobacteria are important ecosystem components across a diverse range of marine and freshwater environments and form the oldest identifiable fossil assemblages from the Earth’s early biosphere (Allwood et al, 2006). Wellstudied examples come from the McMurdo Dry Valleys lake systems, an array of lakes with unique physical and chemical properties where thick microbial mats cover the floors of the lakes within the photic zone These structures encompass a diverse range of morphologies from flat prostrate and pinnacle mats in Lake Hoare (Sutherland and Hawes, 2009; Hawes et al, 2016), branched stromatolites in Lake Joyce (Mackey et al, 2015) and cuspate pinnacles in Lake Vanda (Sumner et al, 2016), and their microbial assemblages have been studied with both microscopy and molecular methods. Models for the initiation of pinnacle structure morphogenesis include the initiation of growth from random irregularities in prostrate flat mats followed by biomass accrual over time and the colonization by a more diverse community of cyanobacteria (Sumner et al, 2016), and the growth of cyanobacteria around trapped photosynthetic oxygen bubbles within mats and subsequent lithification (Bosak et al, 2009, 2010)
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