Phytoplankton consists of microscopic single-celled organisms, invisible to the naked eye that at sea have the same function that plants have on earth. Thanks to their ability to perform photosynthesis, they form the basis of the food chain in the oceans: they are the pastures and forests of the sea. Among the multitude of unicellular microalgae, diatoms play a key role. Wherever there is water, there are diatoms: obviously in the oceans, lakes and rivers, but also in the soil and on the surface of buildings.
A new study, published in the journal Nature, has identified genetic variations that allowed diatoms of the Southern Ocean to adapt to extreme and very variable climates. This study represents an important step toward understanding of how polar organisms are influenced by climate change.
The team, led by prof. Thomas Mock of the University of East Anglia in Norwich, UK, studied the evolution of the genome of the polar diatom Fragilariopsis cylindrus, adapted to thrive in the Southern Ocean. The Zoological Station Anton Dohrn in Naples contributed to the study with the work of Remo Sanges and Mariella Ferrante.
Until now we had no idea of the mechanisms that allowed the polar marine phytoplankton to adapt to conditions characterized by strong seasonality, sub-zero temperatures and long periods of darkness. The genome of Fragilariopsis cylindrus represents the first complete eukaryotic genome of a polar eukaryote, a higher organism whose cell is remarkably complex. Fragilariopsis cylindrus is a key species in the Southern Ocean as it is able to thrive in the sea ice and is at the basis of one of the most unique food chains on Earth, being food for krill, penguins, seals and whales. Most of the phytoplankton in the Southern Ocean is included in the ice that forms each winter only to be released in the summer when much of the sea ice melts.
Species such as Fragilariopsis cylindrus have therefore evolved adaptations to cope with these drastic environmental changes and be able to live both in water and in ice. The researchers found that they do it by changing their alleles, a feature never seen before in a marine organism. Alleles are different forms of the same gene, which are in the same position, or genetic locus, on a given chromosome. Fragilariopsis cylindrus is a diploid organism because it has two alleles for each genetic locus, as it is for humans. The study found that nearly a quarter of the genome of this diatom contains allelic variants surprisingly very different. These alleles are expressed in a different way in relation to different environmental conditions in which the organism is located. Probably, it was because of the extreme conditions that the allelic differentiation was so high and was maintained. The actual size of the population of this polar diatom is very large, therefore it is as if, in the entire population, there was an allele for each occasion, which makes this species genetically ready to respond to changing environmental conditions.
The study greatly enhances our understanding of how polar organisms have evolved to cope with extreme and extremely variable environmental conditions, as well as of the opportunities for adaptation of organisms to environmental changes induced by human activities. The work also has value for the biotechnology industry, which has great interest in extremophiles - organisms that thrive in extreme conditions - as they represent a valuable source of new potentially important enzymes in the production processes.
The contribution of scientists of the Zoological Station was important for data analysis and interpretation, and because it allowed the comparison of Fragilariopsis cylindrus alleles with those of Pseudo-nitzschia multistriata, a diatom living in temperate climates that thrives in the Gulf of Naples, and on which the SZN worked for several years. The comparison between alleles of these two species has corroborated the discovery described in the work: the polar species, although it is a fairly close relative of Mediterranean species, differs in number of alleles and for the mechanisms by which it uses them.
Mock et al. (2017). Evolutionary genomics of the cold-adapted diatom Fragilariopsis cylindrus. Nature 2017, 541: 536-540.
