What if the tropical oceans are trapping more CO₂ than expected?

The ocean is a carbon sink, with major implications for climate change. Among the phenomena involved in this sequestration of CO2 by the ocean, the plant plankton (or phytoplankton) absorbs CO2 by photosynthesis, manufactures organic matter consisting of carbon, which is transferred along the marine food chain. When the organisms die, some of this carbonaceous material sediments on the ocean floor, thus removing CO2 from the atmosphere. This is called the biological carbon pump in scientific terms.

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Other smaller diazotrophs (1 to 8 µm), called unicellulars, are also ubiquitous in the (sub)tropical ocean, with the range of some groups extending even to temperate and polar regions. But then, if these organisms support biological production in these vast ocean regions, what is their role in the biological carbon pump? This question is of the utmost topicality because 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 topic remains scarce for several reasons.

Diazotrophs in the deep ocean?

First, it is generally accepted that diazotrophs do not sediment to the deep ocean, but are recycled in the surface layer, returning their CO2 to the atmosphere. Indeed, the size and density of the cells would not be sufficient to generate a fall to the deep ocean (beyond 100 m)

However, our recent studies conducted in the framework of the TONGA project (South Pacific) have challenged this paradigm: by making measurements in the deep ocean (between 100 and 1000 m) using a coupling of tools collecting sedimenting carbonaceous material, we have shown that diazotrophs fall towards the deep ocean, contributing in some places to the major part of the carbon export flux.

The study further reveals that the organisms are minimally degraded (Fig. 3) or even nearly intact at this depth, suggesting rapid fall and thus low CO2 recycling during descent. In a complementary study, we reveal that some of these organisms (Trichodesmium) are still alive at 1000 m depth, confirming their rapid fall and thus their direct export to the deep ocean, where this carbon will be trapped on the long term. Through laboratory studies, we have subsequently measured the speed at which this "marine snow" from diazotrophs sinks (100 to 400 m per day), confirming field observations. These relatively high sink rates would be due to the fact that small (1-8 µm) diazotroph cells have the ability to agglomerate to form marine snow aggregates large enough (50-500 µm) and voluminous to sink.

The inextricable link between diazotrophs and the carbon cycle

In addition to the direct sedimentation of diazotrophs, other possible diazotroph-derived carbon sequestration pathways exist (indirect pathways), which are extremely complex and difficult to capture with current methods. Indeed, in those regions where diazotrophs support the majority of biological production at the surface, the resulting marine snow at depth may be composed of diazotrophs (direct export), non-diazotrophic phytoplankton, zooplankton, detritus, droppings, 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, and to know what physicochemical and biological parameters control them. To complicate matters, the biological processes that control the production and deposition of diazotroph-derived carbon occur over a wide range of spatial and temporal scales, often difficult to capture in the ocean. In particular, current observing 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 with 2.5 million Euros, aims to contribute to overcome these technological barriers through a coupling of approaches at the interface between microbial oceanography, geochemistry and autonomous sensor technology, which examine processes occurring at different spatio-temporal scales, and are able to capture 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 warmer and more stratified future ocean, in which the range of diazotrophs may expand further. Exploring in detail their role in the biological carbon pump is therefore of the utmost relevance.