For the very first time, the genetic make-up of a planktonic marine alga has been sequenced. During this process, a team of international scientists found unexpected metabolic pathways in the diatom Thalassiosira pseudonana. The results will be published in the scientific journal ‘Science’ this week.
The fact that Thalassiosira pseudonana operates a urea cycle, has been a special discovery. Up to now, this metabolic pathway for ammonia detoxification was known only from the liver cells of animals and humans. It remains unclear how the cycle works in the alga. In addition, the diatom has two separate means for digesting fat, which is also unusual. One digestive mechanism is carried out as in animals, within mitochondria, the cell’s ‘power stations’. In contrast, fatty acids are broken down in regular plant-like fashion inside peroxysomes used for detoxification. Hence, the boundary between animals and plants appears blurred in this species of diatom.
The genome sequencing of Thalassiosira pseudonana is also of great interest for evolutionary biologists. Scientists came across genes which originate from the nucleus of a red alga. Gene transfer of this kind supports the theory of secondary endosymbiosis. Eukaryotes, such as diatoms, are complex cells with membrane bound nucleus and cell organelles. All living organisms other than bacteria are comprised of eukaryotic cells. Almost all eukaryotic cells, including human ones, have mitochondria. Plant and algal cells also contain plastids for photosynthesis. Originally, both types of organelles were bacteria that were incorporated by eukaryotic cells. For this reason, they are often termed ‘primary endosymbionts’. In several cases, secondary endosymbiosis took place in that one eukaryotic cell was incorporated by another and subsequently reduced to a – now secondary – organelle. Diatoms appear to have engulfed a unicellular species of red alga and transformed it into a secondary plastid. “The diatom is some kind of a chimera of several organisms”, says Dr Klaus Valentin of the Alfred Wegener Institute for Polar and Marine Research. This explains the presence of red algal genes in T. pseudonana according to Klaus Valentin, who participated in this project, among other ways, through identification of genes.
Ingrid Zondervan | alfa
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