Why are diatoms so successful? Researchers from the Alfred Wegener Institute find hints to this question in the algae's evolution.

Bremerhaven, June 25th. Diatoms play a key role in the photosynthesis of the oceans and are therefore intensively studied. Researchers from the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association together with international collaborators have made a new discovery regarding the diatoms' photosynthesis. It has so far been believed that diatoms have inherited their photosynthetic capabilities exclusively from red algae. The molecular biologists have now shown that a significant part of the diatoms' genes originates from green algae. The photosynthetic cellular structure of the diatoms, the plastids, therefore combine features both from their green and red algae predecessors which could explain their enormous success in the oceans.

The results are presented in the current issue of the periodical SCIENCE.

Photosynthesis is the basis of all life on earth and it is an almost equal capacity on land and in oceans. Complex green plants are mainly responsible on land, whereas algae are responsible in the oceans - particularly unicellular species. Diatoms are the most important group contributing about 40 % of marine photosynthesis.  Land plants, red alga, and green algae fundamentally differ in their evolutionary history from diatoms: they descend from a symbiosis of photosynthetic cyanobacterium within a eukaryotic cell. The process is called endosymbiosis because one cell lives as a symbiont in another cell. As a result, the photosynthetic organelles in plant and algal cells, the plastids, developed.

Diatoms possess plastids, too, but these developed through the merging of two higher cells: a eukaryotic host cell incorporated a photosynthetic unicellular red alga. So-called secondary plastids developed in this secondary endosymbiosis. It has so far been believed that the absorbing host cell was colourless and not photosynthetic. “It was possible for us to show, within an international collaboration, that the incorporating host cell already had chloroplasts similar to those of the green algae,” explains Klaus Valentin, researcher from the Alfred Wegener Institute for Polar and Marine Research. Actually, more traces of green than red algal descent were found in the diatom genomic material. It could therefore be postulated that the plastids of current diatoms are a hybrid of two types of plastids, those from red and green algae. “Diatoms therefore possess probably more metabolic potential than each of the two predecessors alone which could explain the huge success of the diatoms in the oceans,” continues Valentin. Their plastids could virtually unite 'the best of both worlds'.

In the meantime, Valentin and his colleague Bànk Beszteri have also discovered traces of green endosymbiosis in other marine algae which originated similar to diatoms from secondary endosymbiosis. Among these are for instance brown algae. Our next goal is to find out what kind of advantage has been gained by the marine algae through this form of symbiosis. We intend to quantify this advantage and to identify metabolic pathways which exist additionally in diatoms or which function better than in red or green algae alone. We then perhaps can understand why plants with secondary plastids are so successful in the oceans while plants with primary plastids won the race on land.

Notes for Editors:
Name of the Science-Paper: Genomic footprints of a cryptic plastid endosymbiosis in diatoms. During their evolution the dominant phytoplankters in the world's oceans sampled genes from both red and green algae.“ (Authors: Ahmed Moustafa, Bánk Beszteri, Uwe G. Maier, Chris Bowler, Klaus Valentin, Debashish Bhattacharya)
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