The Diversity and Coping Mechanisms of Life Inhabiting the Hypersaline Dead Sea

Open Access
Author:
Rhodes, Matthew E
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 08, 2011
Committee Members:
  • Christopher Howard House, Dissertation Advisor
  • Christopher Howard House, Committee Chair
  • Jennifer Macalady, Committee Chair
  • Mark E Patzkowsky, Committee Member
  • Ming Tien, Committee Member
Keywords:
  • Lateral Gene Transfer
  • Haloarchaea
  • Halophile
  • Dead Sea
  • Metagenomics
Abstract:
Salinity has been shown to be a highly important determinant in microbial community composition. Consequently the mechanisms and genes that allow microorganisms to cope with salinity are fundamental for understanding microbial biogeography and evolution. At a salinity approaching 350‰, or approximately ten times that of the Earth’s oceans, the Dead Sea represents one of the most saline naturally occurring bodies of water on our planet. When combined with a slightly acidic pH, near toxic magnesium levels, and the dominance of divalent cations, the Dead Sea becomes a truly inhospitable ecosystem. Nevertheless, the Dead Sea is home to a select few halophilic microbes and with a roughly 33% decrease in the salinity of its surface waters, the Dead Sea teems with microbial life. Here we employ “next generation” metagenomic and bioinformatic techniques to explore the limits and evolution of hyperhalophilic life inhabiting the Dead Sea and other hypersaline bodies. To that end we extracted and/or obtained DNA from the March 2007 Dead Sea, four artificial Dead Sea blooms, and cryopreserved samples from both June and September 1992. We amplified and sequenced portions of the 16S rRNA gene for all samples and sequenced metagenomes for both the March 2007 and September 1992 samples. The amplicons revealed a significant population shift between the 1992 samples. Most striking was the virtual disappearance of the major bacterial lineages that were present in the June sample. The amplicons also revealed major differences between all bloom samples and the residual 2007 population. This indicates that the archaeal taxa capable of surviving under the most extreme conditions are not the ones that flourish under relatively mild hypersaline conditions. The shift in populations between the bloom and non-bloom environments was also observable in the translated amino acid profiles of both environments. Many hyperhalophiles balance the osmotic gradient present in their native environments with multi-molar quantities of KCl necessitating radical protein alterations. These alterations were more prevalent in the 2007 metagenome. We analyzed the metagenomes of a number of hypersaline environments, including both Dead Sea metagenomes and found that the degree of protein alteration plots linearly with salinity, suggesting its use as a salinity proxy. We also utilized the protein alterations inherent in hyperhalophilic proteins in combination with the amplicon data sets to identify both lateral gene transfer events involving hyperhalophilic organisms and a number of bacterial lineages with putative hyperhalophilic members. From there we chose to delve deeper into lateral gene transfer amongst the Halobacteria. Halobacteria are notorious for lateral gene transfer and lateral gene transfer has played a significant role in their evolution. We analyzed over 1,000 genomes for instances of lateral gene transfer and discovered that the Halobacteria in contrast to other microbial lineages often participate in lateral gene transfer with non-halophiles. This paints the picture of the Halobacteria as the consummate opportunists, utilizing DNA from all sources they encounter.