Evolutionary history reconstructed
2 billion years of microbial diversity. The latest technologies have enabled scientists to sequence the DNA of microbes that live in the samples taken. Boreal lakes, forest floors, the Indian Ocean and the Mariana Trench, among many other environments. A study recently published in the journal Nature Ecology and Evolution revealed some of the key processes of marine microbial evolution using these data.
The Barcelona Institute of Marine Sciences (ICM-CSIC) provided marine samples collected during the Malaspina expedition in different oceans and depths of the water column.
According to the work, carried out by the University of Uppsala (Sweden), the transitions of habitat – from the sea to the land and vice versa – which have occurred over the last million years help to explain the great diversity current.
Crossing the salinity barrier
It’s not at all easy for organisms, and when it happens, the resulting transitions are key evolutionary events that can trigger bursts of diversity,” the researchers explain.
Until now, however, it was unknown how common these transitions had been in the eukaryotic tree of life, which includes animals, plants, and a wide variety of eukaryotic microorganisms.
Small but very versatile
Specifically, work published today showed that microbial eukaryotes made hundreds of great leaps from sea to land. And also to freshwater habitats, and vice versa, during their evolution. This, in turn, made it possible to deduce where the ancestors of each of the microbial eukaryotic groups were located.
Through sequencing, it has been possible to construct large evolutionary trees of the organisms found in these environments and it has even been possible to observe a range of patterns in the evolution of habitat preference.
eukaryotic tree of life
“We found that organisms in the eukaryotic tree of life are generally grouped by whether they live in the oceans or in non-marine habitats.” Explains Mahwash Jamy, researcher at Uppsala University and lead author of this study.
In this regard, Jamy adds that this discovery confirms that adapting to a different salinity – or crossing the salt barrier – “is difficult, even for microbes”.
However, the study proves that microbial eukaryotes have successfully established themselves in new habitats several hundred times during their evolution. Therefore, the researchers suggest. That it is precisely these difficult transitions that would have allowed colonizing organisms to occupy vacant ecological niches, at the origin of the great diversity of eukaryotes today.
More clues about the first eukaryotes
On the other hand, evolutionary trees constructed from DNA sequences have also allowed researchers to look into the most distant past and deduce what the habitats of the ancestors of each microbial group might have looked like.
“It’s likely that two of the largest groups of eukaryotes, the SARs and the Obozoa, each of which is larger than, say, animals or plants, arose in completely different habitats.” Explains Fabien Burki, also a researcher at Uppsala University and another of the main authors of the study.
They would have emerged for the first time in the oceans of the Precambrian
According to Burki, “the SAR lineage—which includes groups such as diatoms, ciliates, dinoflagellates, or radiolarians—. It would have appeared for the first time in the Precambrian oceans. While the ancestor of the Obazoa group – which diversified into fungi, animals, choanoflagellates and amoebas – could have lived in non-marine habitats.
This shows, once again, that crossing the salinity barrier played an important role in shaping eukaryotic evolution. For this reason, for future research, they will turn to genomics to uncover what genetic mechanisms underlie these key evolutionary events. 2 billion years of microbial diversity.