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Paris, 13 October 2009

When glass develops into a shell : new findings in diatoms

Diatoms are microalgae that are responsible for nearly a quarter of the oxygen we breathe, but how does their glass-like skeleton develop? Researchers from CNRS and ENS Paris (1) have solved part of the mystery concerning these organisms, so abundant in our oceans, by discovering several genes that are involved in the storage and transport of silica, the principal constituent of glass. Published today in the journal PLoS One, their study suggests a reorganization of certain genes that optimizes their response in the presence of silica. Above all, they confirm the important silicon requirements of diatoms. Elucidation of these mechanisms will enable a clearer understanding of glass chemistry and the anticipation of certain environmental modifications linked to the silicon and carbon cycles.

Silicon (2), the most abundant element on Earth after oxygen, has long been used by architecture and industry, notably as a component in glass (in the form of silica).  This substance is essential to the growth of certain species of microalgae called diatoms.  These astonishingly diverse, microscopic algae prosper in most of the oceans, rivers and lakes of the world. Endowed with a glass-like shell, they are one of the most abundant types of phytoplankton and are of considerable interest to scientists because of their numerous applications (as a model in the field of nanotechnologies (3), for their role in climate regulation (4), etc.).

A team of scientists led by Pascal Jean Lopez from CNRS has tried to understand the mechanisms that control the formation of their glass-like extracellular skeleton.  Indeed, the processes involved in their assimilation, storage and transport of silicon have so far remained poorly understood.  Clarification of these factors would improve our overall understanding of diatoms.  And the stakes are high: these algae produce nearly a quarter of the oxygen we breathe, which is almost as much as tropical forests!

This study focused on one of the rare diatom species in which the synthesis of a silicon skeleton is not obligatory, called Phaeodactylum tricornutum.  The scientists thus revealed that even if this particular species can survive without silicon, it still seeks to assimilate it. Above all, they discovered that a grouping of certain genes must have been favored during its evolution.  This spatial rearrangement enabled a better coordination of the genome response in the presence of silicic acid (the dissolved form of silicon).  The scientists also managed to identify genes likely to be implicated in the storage and metabolism of this compound, as well as demonstrating certain types of gene regulation responsible for silicon transport, both at the level of their expression and their cellular localization.

"Elucidation at the molecular level of silicon biomineralization is essential if we are to predict the effects of anthropogenic environmental changes on the biogeochemical cycle of silicon", explained Lopez.

Synthesis of the glass-like skeleton (in green) during division in the diatom Coscinodiscus

© Pascal Jean Lopez - CNRS

Synthesis of the glass-like skeleton (in green) during division in the diatom Coscinodiscus. The chloroplast is shown in red, and the nucleus in blue.


 
Silicic acid transporters (in green) located at the cell surface during division (P. tricornutum)

© Pascal Jean Lopez - CNRS

Silicic acid transporters (in green) located at the cell surface during division (P. tricornutum). In red, the chloroplast of the cell.


Notes:

1 - "Biologie moléculaire des organismes photosynthétiques" Joint Research Unit (ENS Paris/CNRS), in collaboration with the Plate-forme Transcriptome de la Montagne Sainte Geneviève.
2 - It does not exist in a free form but as compounds: silica (or silicon dioxide), silicates, silicic acid.
3 - At present, no being is able to reproduce a glass-like skeleton that could serve as a model for the manufacture of microchips.
4 - Because they are able to photosynthesize, these algae trap CO2.

References:

Genome-wide transcriptome analyses of silicon metabolism in Phaeodactylum tricornutum reveal the multilevel regulation of silicic acid transporters. Sapriel G., Quinet M., Heijde M., Jourdren L., Tanty V., Luo G.Z., Le Crom S., Lopez P.J. PLoS One. 13 October 2009.

Contact information:

CNRS researcher
Pascal Jean Lopez l T 01 44 32 35 35 l pjlopez@biologie.ens.fr

CNRS Press Office
Priscilla Dacher l T 01 44 96 46 06 l priscilla.dacher@cnrs-dir.fr


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