Guidi's lecture revolved around findings from the Tara Oceans expedition, which investigated the unexplored world of marine plankton and microorganisms that form the basis of the global ecosystem. The results shed light on the communities of plankton that export organic carbon – created from atmospheric carbon dioxide via photosynthesis – as sinking particles to the ocean depths and seabed where it is stored. This system is known as the biological carbon pump.
A huge international data collection mission oversaw the largest DNA sequencing effort ever carried out for the ocean. Around 40 million genes were revealed, most new to science. This suggests a wider diversity of plankton – from viruses to prokaryotes (unicellular organisms) and eukaryotes (more complex organisms with cell nucleus) – than recognized in the literature.
All sequenced plankton was analysed to create an 'interactome', a kind of planktonic social network, which shows how the plankton interact with each other through processes such predation and competition. Instead of looking at specific biota, researchers focussed on key players in parts of the network that were correlated with the carbon pump. One interesting result was that the role played by unicellular parasites, cyanobacteria and viruses in exporting carbon was greater than previously thought.
"To me it is a bit like the revolution that is happening in the medical sector. People have started to study the gut microbiome, changing the view of how the microbial community is important in understanding a disease, how the microbiome interaction has an impact. In the ocean for it's the same: the microbiome is part of the system, so we see how the what the community interactions are and how they impact the process," said Guidi.
The team also characterized a network of functions based on analyses of bacteria and virus genes. It was determined that the relative abundance of a small number of these genes can predict a significant amount of variations in carbon export to the deep ocean. Some of these genes are involved in photosynthesis and membrane transport, supporting for example sediment formation and breakdown of aggregated organic material.
"We found two sets of genes in the prokaryotes that were linked to carbon export and related to primary production. So even though we don't know the actor, we know that because we have primary production, and a lot of gene related primary production, we're able to explain the variation of carbon export," said Guidi
"We also found genes related to coagulations and bindings. You need to create material and aggregates to be able to sink and sequester carbon. You don't know exactly where the genes have come from or which organism had those genes. It's like you take everything, put it in a blender and extract a gene. You don't know in which cell it was, but you know that it's there."
Another objective of Tara Ocean was the creation of a vast bank of data, and it was achieved: 7000 datasets are now in the public domain, featuring 35 000 biological samples and masses of in situ environmental readings, remote sensing data, and a library of 50 million images.
"What we're trying to do now is bring in computer scientists, because they're the ones who really know what to do with large amounts of data," said Guidi. This is where the next step is. I'm trying to develop a connection between computer science work and visualization of data."
"We have the data that has been published and we are sequencing more data to fit the database. We're trying to bring additional researchers to the field and we're also trying to develop new cruises in a similar way to fit the database."
Other future objectives include repeating the sampling in different sea areas to see whether the networks differ between regions. As the ocean is a major sink for global carbon, the findings will enable researchers to better understand the sensitivity of this network to a changing ocean and predict the effects of climate change on the carbon pump.
Lionel Guidi, of Sorbonne Universités, UPMC Université Paris, gave his talk 'The planktonic social network of the biological carbon pump'