A Masterpiece of Evolution: One Species, a Few Drops of Seawater, Hundreds of Subpopulations Living in Coexistence

A study published in SCIENCE led by Dr. Nadav Kashtan who was a postdoctoral student at MIT discovered the enormous genetic diversity of microorganisms in the oceans

The smallest and most common creature in the oceans, Prochlorococcus, is a photosynthetic bacterium essential to the ecosystem. A billion billion billion such bacteria live in the oceans and form the basis of the food chain. These invisible forests of the oceans carry out about 50% of the total photosynthesis on Earth and are responsible for half of the amount of oxygen in the atmosphere. The climate changes we have experienced in recent years affect the living environment of these microorganisms in the oceans (for example by changing the temperature of sea water and their acidity level ) and therefore a better understanding of this ecosystem is of great importance. These bacteria live in the upper two hundred meters of the oceans and seas (including the Mediterranean Sea and the Gulf of Eilat) and inhabit a wide range of ecological niches that differ in temperature, light intensity, chemical environments and interactions with other creatures. But the extent and nature of the genetic diversity of these creatures that are considered to belong to a single species is a mystery.

To investigate this enigma, scientists from the Massachusetts Institute of Technology (MIT) led by Israeli researcher Dr. Nadav Kashtan and Professor Sally Chisholm, performed a genetic analysis of populations from ocean waters by genetic sequencing of thousands of individual cells sampled in a single milliliter of seawater, and found Hundreds of genetically distinct populations living in coexistence.

Each subpopulation is characterized by different alleles of the group of genes common to all strains (this group of genes is called the core group) that appear together with a small group of genes that distinguish each subpopulation - a combination that the researchers called the "genetic backbone" - which contributes to each subpopulation - Adaptation population to a unique ecological niche.

An article describing the research findings was published a few days ago in Science magazine (issue April 25, 2014). In the article, the researchers estimate that the sub-populations diverged more than several million years ago and therefore the genetic backbone is an ancient component that undergoes slower evolution than other areas of the genome that contain sets of alternating genes.

The study also found that the relative number of bacteria from each subpopulation varies between samples taken at different times of the year (in the Atlantic Ocean near Bermuda) - a finding that strengthens the claim that the subpopulations differ not only genetically but also ecologically.

"The enormous biological diversity of Prochlorococcus cells in the oceans is almost unimaginable, and apparently constitutes an important pillar in the stability of the ecosystem in the oceans," says Professor Chisholm.

Ocean currents and turbulence play an important part in the evolution and ecology of the bacteria, as they are passively carried with the seawater. Using a fluid mechanics model, the researchers concluded that two daughter cells that had just divided would be centimeters apart within minutes, tens of meters within an hour, and kilometers apart within a week.

"The interesting question is, why are there so many subpopulations?" says Kashtan. "The ocean currents and turbulence, together with a high density of 10 million cells per liter of sea water, create a situation where the size of the population undergoing selection by natural selection is huge. This indicates that the tremendous genetic diversity and the way it is organized is not random, but a masterpiece of evolution."

Cashton and Chisholm say that such a large number of coexisting subpopulations is probably a common phenomenon in microorganisms with very large populations in environments characterized by frequent mixing.

"The above study sets a new record in progress in mycobiological research - and this by analyzing only three drops of sea water," says Ramons Stephanouskes, a member of the research team, from the Bigelow Research Institute in the state of Maine.

Dr. Kashtan, recently returned to Israel after a postdoctoral position at MIT, and who completed his doctorate and master's degree at the Weizmann Institute of Science under the guidance of Prof. Uri Alon and previously completed his bachelor's degree in computer science at Tel Aviv University. In an interview with the Hidan website, he explains: "The genetic distance between The subpopulations in the study (which all belong to one Prochlorococcus ecotype called Light II (High- is of the order of magnitude of the genetic distance (between the coded gene segments) between all apes). The genetic diversity of all Prochlorococcus strains is much greater, like this More than all mammals.

On the same topic on the science website:

• Semi-artificial living creature: Scientists have created the first bacteria that translates a pair of artificial DNA letters
• Lateral transfer of genes in archaea can indicate a period of rapid evolution

Where are the bacteria in periods when their strain is uncommon?
"All the species are found in the upper two hundred meters of the oceans, their frequency varies geographically with the seasons and with the depth. The populations are enormous in size and so on. The species we identify as less common (in a 1 milliliter sample) still represent very large populations (to illustrate - a species that is Only one tenth of a percent of a sample exhibited at least 100 trillion bacteria in 10 KM of ocean water). The massive mixing of the ocean water in this way contributes to the fact that the environment makes a selection and "chooses" which species will be more common and which will be less common at different points in the ocean (referring to location, depth and time).

And a general question regarding the cyanobacteria, is there a fear of increasing the phenomenon of marine deserts, and how will this affect the production of oxygen and the absorption of CO2?
"This is one of the reasons for the great importance of research on the hidden forests of the oceans - a significant part of which is cyanobacteria. We do not know enough today to predict how the populations react to the warming of the ocean water and its oxidation, and how this will affect the production of oxygen and the absorption of C02."

to the MIT announcement