The Tara schooner travel across planet oceans between 2009 to 2013, and sampled a large number of marine plankton samples. The Genoscope (from François-Jacob Institute from CEA) is a main player of this scientific adventure, along with CNRS, EMBL and ENS. These researchers were involved in the sequencing and bioinformatic analyses, and their results interest the scientific community.
They showed that very different genes are expressed based on sea water temperature or nutrients concentration in the studied oceanic regions.
Half of these genes are still unknown, suggesting that oceans, beyond acting as a major reservoir for biodiversity, also have a huge potential for new gene functions, yet to be elucidated. Using single cells isolation and characterization techniques, they were able to specifically explore roles of the genes present in this mainly uncultivated and yet poorly studied compartment, starting point of the marine food chain.
The largest gene catalogue from a given ecosystem.
Researchers have established the largest gene catalogue ever assembled from a planetary wide ecosystem. With more than 117 million different sequences, it has been established from material gathered from more than
400 samples collected during the expedition. This massive sequencing approach of mRNAs out of environmental samples without prior isolation of organisms is termed metatranscriptomics. It allows to study genes expressed in specific environmental conditions and to show that, for example, many organisms display different genetic response following iron concentration in the environment, and to identify specific gene involved in different organisms groups.
This study also highlight a high proportion (more than the half) of genes with yet unknown biological functions.
What genes for what life function ?
If plankton genes functions can now be studied at a large scale using this catalogue, the identification of specific organisms bearing these genes is yet to be discovered. In a second paper, the researchers from Tara Oceans characterized the genomes of several environmentally key player organisms that are still not cultivated. Individual cells sampled during the expedition have been preserved, and their DNA sequenced using the recent single cell genomics techniques. With this approach, numerous genes from the global catalogue have been assigned to small organisms (smaller than 20 micrometers) feeding from micro-algae and bacteria, and being a key intermediate level in the food web. This study revealed a large yet unexpected diversity of biological functions, indicating the specialization of this plankton compartment.
How did the Ocean evolved, and how is it working ?
These results unveil the importance of these small yet complex organisms that make the largest part of the oceanic plankton. They pave the way for the understanding of oceans ecology through marker genes. Major biogeochemical oceanic processes can know been studied based on these genes, and together with the identification of species and expressed genes, these results from the Tara Oceans project will allow a better understandng of ecological and evolutionary processes of oceanic ecosystems.