Each year, the ocean absorbs between 5 and 10 gigatons of atmospheric carbon through biological processes, thus playing a crucial role in climate regulation.
This carbon is transported to the depths in the form of organic particles, constituting what scientists refer to as the "biological carbon pump". The efficiency of this pump depends largely on the activity of microbes and zooplankton, which transform and degrade carbonaceous material during its transfer to the deep ocean. However, the precise mechanisms controlling this process have been poorly understood until now.
To uncover this mystery, researchers deployed an innovative device called C-RESPIRE in six oceanic regions with contrasting characteristics. This instrument, attached to a drifting mooring line, allows for unprecedentedly precise measurements of the degradation of organic particles by marine microorganisms in the mesopelagic zone, situated between 328 and 3,280 feet (100 and 1,000 meters) deep. Measurements were conducted under in situ pressure and temperature conditions, ensuring their ecological relevance.
The results reveal significant regional variability in the respective roles of microbes and zooplankton. Within the first 984 feet (300 meters) of the mesopelagic zone, microbial contribution to the degradation of organic carbon does not exceed 30% of the total particle flux attenuation. This unexpected finding suggests a predominant role of zooplankton in transforming particles at these depths. However, the study indicates that the relative importance of microbes increases with depth, potentially becoming dominant in deeper layers.
Even more surprisingly, the study highlights a diversity of factors regulating microbial activity depending on the oceanic region. In (sub)tropical areas, where temperature gradients are prominent, temperature plays a key role in regulating microbial activity. Conversely, in mid and high latitudes, where thermal gradients are less pronounced, other factors come into play. The biochemical composition of particles, the physiology and ecology of microbial communities, and their complex interactions with zooplankton appear to be key elements in these regions.
These findings challenge current models of the biological carbon pump, which have so far been based on a simplified empirical relationship known as the "Martin curve". This curve, widely used in biogeochemical models, does not capture the diversity of mechanisms highlighted by this study.
The results emphasize the need to account for the diversity of regional biological pumps, each with its own characteristics in terms of microbial influence and controlling factors. This newly revealed complexity paves the way toward a more nuanced and realistic representation of carbon flux attenuation in ocean models.
This major scientific advance provides new ways to improve our understanding and predictive capacity regarding the evolution of the ocean carbon cycle in the face of climate change. It also underscores the importance of continuing research on the complex interactions between microorganisms, zooplankton, and organic particles in the deep ocean—a field still largely underexplored but crucial for the future of our planet.
References:
Bressac, M. et al. Decoding drivers of carbon flux attenuation in the oceanic biological pump.
https://doi.org