Single cell organisms found in highly salty marine waters share DNA with a variety of organisms including viruses and bacteria.
In the latest edition of the journal Archaea, Australian researchers published a paper that uses DNA sequencing technology and bioinformatics (software-based genome analysis) to understand the evolution and life history of an archaeon known as Halococcus hamelinensis. They found that this archaeal species has several adaptations to its specific environment and, surprisingly, that its genome contains many transposases (enzymes that cause the movement of genes) as well as genes with significant positive matches to Bacteria, other Archaea, and viruses. These results show that horizontal gene transfer, or movement of functional genes between living microorganisms, was likely a key driver of the organism’s evolutionary history.
Archaea are single-celled organisms that were once thought only to dwell in Earth’s most extreme environments (think deep-sea hydrothermal vents, hot springs, and extremely saline lakes). In the last decade, however, the wide range of ambient habitats in which archaea are found has increased dramatically; as scientists begin to find archaea in “normal” ecosystems, many speculate that they may have a much larger role in biological processes than previously thought.
In the current paper, Prof. Brendan Burns and his students at the UNSF School of Biotechnology and Biomolecular Sciences focus on H. hamelinensis, which is the first ever archaea discovered in a stromatolite community. Stromatolites are dense colonies of bacteria (commonly cyanobacteria) that form rocky structures in shallow, highly saline marine waters. These structures were once ubiquitous during early life on Earth and are still found in some parts of the world today, such as Shark Bay in Australia. Stromatolites are increasingly studied because their profound success billions of years ago contributed to the oxidation of the atmosphere that allowed complex life to arise.
Burns and his team isolated and sequenced the genome of H. hamelinensis to understand its evolutionary patterns and hoped to learn about this organism’s role in stromatolite communities. Interestingly, the researchers found that the archaeon has adaptations specifically tailored for its environment, such as genes for high salt tolerance and high UV irradiance protection, and does not share many of the common characteristics of the archaeal lineage. This shows that the archaeon likely has lived in relative isolation within this habitat for millions of years. Similarly, the paper provides evidence that cyanobacteria that make up the stromatolite structures produce compounds essential to the survival of H. hamelinensis, and vice versa.
The final major finding of this research is quite remarkable. The team found that there was a 22.2% similarity between the genome of H. hamelinensis and the genome of a common marine bacterial lineage called Actinobacteria. They also discovered a 7.7% genome similarity with cyanobacteria, showing that over the course of their evolution, the physical proximity of these two organisms likely led to the transfer of genetic information. Of further significance, it was found that H. hamelinensis contains four genes with greater than 30% similarity to DNA viruses. Although these shared genetic elements make up a small proportion of the overall genome of the organism, it shows that in the world of marine archaea, sharing is caring!