Metagenomic insights into strategies of carbon conservation and unusual sulfur biogeochemistry in a hypersaline Antarctic lake.
Organic Lake in Antartica’s Vestfold Hills region has the highest reported dimethylsulfide (DMS) concentration of any natural water body. To try and better understand the microbial community, pathways and mineral cycles involved in this unusual occurrence, shotgun metagenomics were performed on size-fractioned samples along the lake’s depth profile. This was complimented by physiochemial profiles of major nutrients so that links between species, genes and chemical distribution could be made.
Data from the metagenome grouped into 983 operational taxanomic units (OTU); 76.2% Bacteria, 16.3% Eucarya and 7.5% could not be classified. Of the 3959 SSU rRNA gene reads, only two were related to Archaea implying that this domain, usually associated with extreme conditions, especially hypothermal and anoxic, have a minor role here.
Heterotrophic Psychroflexus, Marinobacter and Roseovarius genera dominated the Organic lake community. Evidence for this came in the form of OTU abundance relating to each genus. The Viridiplantae, primarily Dunaliella, performed carbon fixation as was shown by the ribulose-bisphosphate caboxylase oxygenase (RuBisCO) homolog relatedness. Some RuBisCO was found to relate to the Gammaproteobacteria indicating some autotrophy in the group. The other gene used to detect photosynthesis is prkB. In this case its detection was not affirmative of autotrophy as, within the Marinobacter the gene is associated with pentose phosphate metabolism, not carbon fixation.
Within the heterotrophic community strategies were identified that may help these organism to capitalize on the limited available resources and reduce the loss of important nutrients. In the upper mixed zone Psychroflexus was predicted to remineralise recalcitrant polymeric algal material and particulate matter, shown by the clustering of Psychroflexus OTUs with Dunaliella chloroplasts in the serration analysis and a previous study showing their abundance in correlation with hours of sunlight. In the deep zone this function is likely performed by Firmicutes. Both these organisms provide labile substrates that are required by other heterotrophs in the system. Marinobacter and Roseovarius were linked to rhodopsin-mediated and AAnP photoheterotrophy with the AAnP abundance being higher here than any other studied system, indicating that the use of light as an energy scource may be an important survival strategy in this low nutrient environment. Chemolithoheterotrophy, sulfur oxidation and CO oxidation may also be important sources of energy and the potential for each was high in this system.
Potential for nitrogen cycling in the lake was dominated by mineralization/assimilation pathways. It is suggested that ammonia assimilation occurs in the upper mixed zone but accumulates in the anaerobic deep zone.
Organic lake’s sulfur chemistry is unusual in that it has the highest DMS concentration of any natural water body. The primary source of DMS in this case seems to be the cleavage rather than the demythilisation of DMSP produced by eucaryal algae. Marinobacter and Roseaovarius seem to be the main organisms involved. The lake has low potential for dissimilatory sulfur cycling, represented by low abundance of dsrAB, aprAB and soxAB genes, hence the build up of DMS.
This study shows how important different trophic strategies are in overcoming varying levels of nutrient availabilities and may help to further our understanding of other uncommon ecosystem types. The utilization of photoheterotrophy and other mixotrophic strategies allows for carbon to be used in biosynthesis enabling growth to occur with very limited carbon sources.
Yau, S., Lauro, F. M., Williams, T. J., Demaere, M. Z., Brown, M. V, Rich, J., Gibson, J. A., et al. (2013). Metagenomic insights into strategies of carbon conservation and unusual sulfur biogeochemistry in a hypersaline Antarctic lake. The ISME journal, 7(10), 1944–1961.