Microbial Communities and Their Predicted Metabolic Functions in Growth Laminae of a Unique Large Conical Mat from Lake Untersee, East Antarctica

Hyunmin Koo,Yukiko Tanabe,Joseph A Hakim,Dale T Andersen,Ian Hawes,Asim K Bej,Nazia Mojib

doi:10.3389/fmicb.2017.01347

Abstract

In this study, we report the distribution of microbial taxa and their predicted metabolic functions observed in the top (U1), middle (U2), and inner (U3) decadal growth laminae of a unique large conical microbial mat from perennially ice-covered Lake Untersee of East Antarctica, using NextGen sequencing of the 16S rRNA gene and bioinformatics tools. The results showed that the U1 lamina was dominated by cyanobacteria, specifically Phormidium sp., Leptolyngbya sp., and Pseudanabaena sp. The U2 and U3 laminae had high abundances of Actinobacteria, Verrucomicrobia, Proteobacteria, and Bacteroidetes. Closely related taxa within each abundant bacterial taxon found in each lamina were further differentiated at the highest taxonomic resolution using the oligotyping method. PICRUSt analysis, which determines predicted KEGG functional categories from the gene contents and abundances among microbial communities, revealed a high number of sequences belonging to carbon fixation, energy metabolism, cyanophycin, chlorophyll, and photosynthesis proteins in the U1 lamina. The functional predictions of the microbial communities in U2 and U3 represented signal transduction, membrane transport, zinc transport and amino acid-, carbohydrate-, and arsenic- metabolisms. The Nearest Sequenced Taxon Index (NSTI) values processed through PICRUSt were 0.10, 0.13, and 0.11 for U1, U2, and U3 laminae, respectively. These values indicated a close correspondence with the reference microbial genome database, implying high confidence in the predicted metabolic functions of the microbial communities in each lamina. The distribution of microbial taxa observed in each lamina and their predicted metabolic functions provides additional insight into the complex microbial ecosystem at Lake Untersee, and lays the foundation for studies that will enhance our understanding of the mechanisms responsible for the formation of these unique mat structures and their evolutionary significance.

Highlights

Microbial mats are often considered to be modern day analogs of ancient organo-sedimentary structures such as microbialites and stromatolites, and the geological record indicates their origin as early as ∼3.5 Ga BP (Gerdes, 2010; Seckbach and Oren, 2010)
Phormidium (65.8%) was found to be a dominant taxon followed by Leptolyngbya (11.1%), and Pseudanabaena (1.8%) (Figure 2)
The results showed that cyanobacteria dominated the U1 lamina (Figures 2, 4, 5)

Summary

Introduction

Microbial mats are often considered to be modern day analogs of ancient organo-sedimentary structures such as microbialites and stromatolites, and the geological record indicates their origin as early as ∼3.5 Ga BP (Gerdes, 2010; Seckbach and Oren, 2010). In the deeper segments in the mats, reduced light and depleted oxygen enables the growth of organisms capable of a cascade of anaerobic metabolisms (Kühl and Fenchel, 2000). Such layering is often visible as color changes, with an orange surface from abundant lightprotective carotenoid pigments, transitioning to green lightharvesting phycobilin and chlorophyll pigments and pink layers corresponding to photosynthetic sulfur bacteria in anoxic zones. Some argue that the organized complexity and physiological cooperativity of a microbial mat is analogous to that of eukaryotic tissues (Webb et al, 2003; Guerrero and Berlanga, 2010)

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Conclusion