The ocean is a carbon sink, with major implications for climate change. Among the phenomena involved in the ocean's capture of CO2, plant plankton (or phytoplankton) absorb CO2 through photosynthesis and produce organic matter made up of carbon, which is transferred along the marine food chain. When the organisms die, some of this carbonaceous matter sediments to the bottom of the oceans, subtracting CO2 from the atmosphere. In scientific terms, this is known as the biological carbon pump.
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Other smaller diazotrophs (1 to 8 µm), known as unicellulars, are also ubiquitous in the (sub)tropical ocean, with the range of some groups extending as far as temperate and polar regions. But if these organisms support biological production in these vast ocean regions, what role do they play in the biological carbon pump? This question is extremely topical, as simulations of the ocean of the future predict a geographical expansion of the (sub)tropical ocean, and with it a probable expansion of the range of diazotrophs. Unfortunately, research on this subject remains scarce, for a number of reasons.
Diazotrophs in the deep ocean?
Firstly, it is generally accepted that diazotrophs do not sediment in the deep ocean, but are recycled in the surface layer, returning their CO2 to the atmosphere. In fact, the size and density of the cells would not be sufficient to cause them to sink to the ocean depths (beyond 100 m).
However, our recent studies as part of the TONGA project (South Pacific) have challenged this paradigm: by taking measurements in the deep ocean (between 100 and 1,000 m) using a combination of tools to collect carbonaceous matter as it settles, we have shown that diazotrophs fall towards the deep ocean, contributing in some places to the bulk of the carbon export flow.
The study also reveals that the organisms are only slightly degraded (Fig. 3), or even virtually intact at this depth, suggesting a rapid fall and therefore little recycling of CO2 during the descent. In a complementary study, we revealed that some of these organisms (Trichodesmium) are still alive at a depth of 1000 m, confirming their rapid fall and therefore their direct export to the deep ocean, where this carbon will be trapped over the long term. Through laboratory studies, we then measured the speed at which this 'marine snow' from diazotrophs sinks (100 to 400 m per day), confirming the observations made in the field. These relatively high falling speeds are thought to be due to the fact that the small cells of diazotrophs (1-8 µm) are able to agglomerate to form aggregates of marine snow that are large enough (50-500 µm) and voluminous enough to sink.
Diazotrophs (Trichodesmium sp. and unicellulars) collected in particle traps in the deep ocean (170, 270, and 1000 m) in the South Pacific (TONGA campaign). A, B, C: Photos taken using epifluorescence microscopy. D, E, F: Photos taken using scanning electron microscopy. S. Bonnet, K. Leblanc, Provided by the author
The inextricable link between diazotrophs and the carbon cycle
In addition to the direct sedimentation of diazotrophs, other possible carbon sequestration pathways derived from diazotrophs exist (indirect pathways), which are extremely complex and difficult to capture using current methods. In fact, in these regions where diazotrophs mainly support biological production at the surface, the resulting marine snow at depth may be composed of diazotrophs (direct export), non-diazotrophic phytoplankton, zooplankton, detritus, excrement or a mixture of these elements, ranging in size from a few µm to several cm.
To date, it is impossible to decipher these different pathways, to quantify the relative efficiency of each, or to know which physico-chemical and biological parameters control them. To complicate matters, the biological processes that control the production and sedimentation of carbon derived from diazotrophs occur on a wide range of spatial and temporal scales, which are often difficult to grasp in the ocean. In particular, current observation systems lack the temporal resolution to assess how rapid environmental changes (hourly, daily or seasonal) influence the surface diazotroph community and, consequently, the quantity and quality of carbon exported to the deep ocean. There is an urgent need to develop appropriate approaches to decipher these pathways if we are to understand how and to what extent diazotrophs export carbon to the deep ocean.
The HOPE project, funded by the ERC to the tune of €2.5 million, aims to help overcome these technological hurdles by coupling approaches at the interface between microbial oceanography, geochemistry and autonomous sensor technology, which examine processes occurring at different spatio-temporal scales, and are capable of capturing the transient and seasonal characteristics of the biological pump supported by diazotrophs.
In its final phase, HOPE aims to produce global and spatially resolved maps of the contribution of diazotrophs to global carbon export, and the metrics needed to feed the marine component of climate models. These models predict a future ocean that is warmer and more stratified, in which the range of diazotrophs could expand even further. Exploring in detail their role in the biological carbon pump is therefore extremely topical.