The Salt in the Soup of Evolution

Microbiologists at the Institute of Biology II of the University of Freiburg have discovered a previously unknown central metabolic pathway in microorganisms. The life forms use this pathway to survive under extremely salty conditions, such as in the Dead Sea.

Contrary to popular belief, the Dead Sea is not dead. It is populated by microorganisms, most of which belong to the group of salt-tolerant archaea. Archaea (from the Greek archaĩos, „archaic“) are among the most primordial life forms on earth and have managed to survive particularly in extreme environments. A research group in Freiburg led by Dr. Ivan Berg has studied the metabolic processes in these microorganisms, which have previously eluded evolutionary biologists.

Scientists have long known that salt-tolerant archaea make use of various organic compounds as a source of nourishment in order to synthesize the necessary cell building blocks and vitamins with activated acetic acid (acetyl coenzyme A). Using the microorganism Haloarcula marismortui as a model, Dr. Ivan Berg and his colleagues in Freiburg Dr. Maria Khomyakova, Özlem Bükmez, Lorenz Thomas, and Dr. Tobias Erb have succeeded in unraveling the details of this metabolic pathway. In the current issue of the journal Science, the researchers describe how they were able to shed light on the entire reaction cycle, including all of its intermediate steps, with the help of a variety of biochemical and microbiological methods. The team named the complete metabolic pathway the „methylaspartate cycle“ after its characteristic key intermediate.

The Freiburg research group also addressed the question as to how this new metabolic pathway originated, as the ancestors of salt-tolerant archaea changed their lifestyle in the course of evolution and thus had to „discover“ the metabolic pathway in order to acclimatize themselves to their salty habitat. The researchers discovered to their surprise that the genes for this metabolic pathway of the ancestors of salt-tolerant archaea were „gathered together“ from other microorganisms. The phenomenon of chance transfer of genes between organisms is already known under the name of „lateral gene transfer.“ However, scientists have not yet observed a methylaspartate cycle composed entirely of old genes of various functions and completely different metabolic pathways. The chance (re-)combination of all of these genes in an ancestor of salt-tolerant archaea led to this new metabolic pathway. The researchers explain this by stating that it is more difficult and slower to „invent“ new genes than it is to create them by combining existing genes according to the „evolutionary tinkering” principle.

The concept of „evolutionary tinkering“ is already a fixed phrase in the biosciences. It refers to the idea that evolution is not a perfect engineer who plans everything and knows exactly what he wants to create from the outset. Rather, biologists understand evolution more as a tinker, who finds improvised solutions to solve pressing problems in any way he can. Tinkers do not necessarily use new parts (or, biologically speaking, genes), but rather whatever happens to be available and seems to fit the bill. This principle is also at work in the emergence of new metabolic pathways under extreme conditions, as Berg and his colleagues demonstrate in their article.

Title of the original publication:

“A Methylaspartate Cycle in Haloarchaea”

Maria Khomyakova, Özlem Bükmez, Lorenz K. Thomas, Tobias J. Erb,

 Ivan A. Berg, Science, Volume 331, 21 January 2011