Abstract
Modern stromatolites thrive only in selected locations in the world. Socompa Lake, located in the Andean plateau at 3570 masl, is one of the numerous extreme Andean microbial ecosystems described over recent years. Extreme environmental conditions include hypersalinity, high UV incidence, and high arsenic content, among others. After Socompa’s stromatolite microbial communities were analysed by metagenomic DNA sequencing, taxonomic classification showed dominance of Proteobacteria, Bacteroidetes and Firmicutes, and a remarkably high number of unclassified sequences. A functional analysis indicated that carbon fixation might occur not only by the Calvin-Benson cycle, but also through alternative pathways such as the reverse TCA cycle, and the reductive acetyl-CoA pathway. Deltaproteobacteria were involved both in sulfate reduction and nitrogen fixation. Significant differences were found when comparing the Socompa stromatolite metagenome to the Shark Bay (Australia) smooth mat metagenome: namely, those involving stress related processes, particularly, arsenic resistance. An in-depth analysis revealed a surprisingly diverse metabolism comprising all known types of As resistance and energy generating pathways. While the ars operon was the main mechanism, an important abundance of arsM genes was observed in selected phyla. The data resulting from this work will prove a cornerstone for further studies on this rare microbial community.
Highlights
Arsenic is a natural component in the Earth crust and in various minerals, but it is a major contaminant of aquatic ecosystems worldwide[1]
Among environments with high arsenic content, High Altitude Andean Lakes (HAAL) comprise a system of shallow waters that are distributed across the Puna at altitudes that vary between 3,000 m and 6,000 m above sea level[3]
High As content was detected at Socompa Lake which is located in the desert region of the province of Salta, Argentina, at the base of the Socompa Volcano at 3,570 masl, where modern stromatolites developed under the pressure of extreme environmental factors similar to the ones present in Early Earth’s atmosphere[19]
Summary
The number and diversity of the identified proteins suggest that many microorganisms in this environment are capable of using arsenic as an energy source This mechanism could be widespread in high altitude Andean arsenic systems, as shown in findings at Diamante Lake. Partial SSU RNA genes found in the bins were compared to the NCBI 16S database by BLAST, which yielded a different result from the whole genome closest neighbour analysis by RAST (Table 1). A small subset of incomplete arrA sequences are affiliated to Deltaproteobacteria and Firmicutes (Fig. 7), suggesting that some members of the microbial community might be able to precipitate arsenic under controlled conditions, like Desulfosporosinus auripigmentum[82, 83], Desulfovibrio strain Ben-RA84, or enrichment cultures from high arsenic content sites in northern Chile[16]. It cannot be stated whether the different concentrations of elements ameliorates arsenic’s toxic effects
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