The oceans of our planet are Earth's largest ecosystem. About 50% of the primary productivity is attributable to them. The most important photosynthetic eukaryotes are the diatoms; they contribute about 40% of the primary marine production, and thus produce almost 1/4 of the oxygen we breathe. Although they form only a small part of the photosynthetic biomass, in some regions of the ocean, they can fix the same amount of carbon per day as a forest of terrestrial plants.
There are at least 100,000 species of diatoms, which makes them the most abundant photosynthetic group after the angiosperms. Diatoms, therefore occupy a central position in the control of marine resources, and for prediction of climatic change. However, we possess very little knowledge about the biology of diatoms today. Furthermore, algal blooms are often caused by diatoms, sometimes with production of toxins (a cause of anmesiac intoxication by shellfish), and they may have harmful effects on the local ecosystem, fishing, aquaculture and tourism. Finally, diatoms are the principal cause of "biofouling", because of their capacity to adhere to surfaces and to produce extracellular polymeric mucilaginous substances (EPS). The total direct costs of "antifouling" treatments in the world is estimated at 3 billion dollars.
In parallel to their ecology, the diatoms are also interesting for the study of evolution, because their genomes are the result of a fusion of 3 genomes derived from their cyanobacterial and eukaryotic ancestors. Furthermore, they have applications in biotechnology: in nanotechnology because of their silica, as a nutrient in aquaculture and as biofactors for the production of molecules such as omega-3 type fatty acids.

Information on gene expression in response to environmental signals, associated with EST sequence alignments between the genomes of the two diatoms, T. pseudonana and P. tricornutum, will provide basic knowledge for the study of important questions in diatom biology, for example :

• Carbon fixation : diatoms are the principal carbon-fixing organisms in the marine environment. The annotation of the diatom genome will provide information of the mechanisms of carbon fixation and to determine whether they fix CO2 via a C4 or C3 cycle.
• Acquisition of nutrients and their assimilation : the nutrients which limit the primary productivity of diatoms are nitrate, phosphate, silica and iron. The annotation of the genome will help in determining the pathways of acquisition and assimilation, and will provide information on the mechanisms of formation of the cell wall of diatoms, which is composed of silica.
• Life and death in the oceans : diatoms commonly traverse periods of bloom and crash. Very few details are known about cell division and the life cycle of diatoms, and information is also lacking about the biotic and abiotic forces which stimulate the development of a bloom.
• Adhesion and motility : "Biofouling" results from adhesion of diatoms to inert surfaces through the production of extracellular polymeric substances (EPS). ESPs also contribute to "ocean snow", a process of mass sedimentation of fixed carbon. The biosynthetic pathways involved and their regulation are largely unknown.