The oceanic biological carbon pump (BCP) regulates the Earth carbon cycle by transporting part of the photosynthetically fixed CO2 into the deep ocean. The BCP occurs mainly in the form of particles rich in organic carbon (POC) sinking out the surface ocean. Atmospheric CO2 concentrations are highly sensitive to depth at which POC flux penetrates in the mesopelagic ocean (the so-called POC flux attenuation between 100 and 1000m depth). Initially, the POC flux attenuation was interpreted as resulting from the consumption of POC by heterotrophs only (bacterias and zooplankton). However, direct consumption of POC by heterotrophs appears to explain only a limited part of the observed flux attenuation in the mesopelagics. Recent developments in semi-autonomous observations have highlighted that the missing part in POC flux attenuation could originate from fragmentation of sinking particles into small non-sinking particles. Fragmentation of sinking particles could explain up to half of the observed POC flux attenuation, making particles breakdown the major process controlling biological CO2 sequestration by the ocean.
The mechanisms leading to particles fragmentation in the mesopelagic zone are however unclear. Two main mechanisms have been proposed. 1) zooplankton activity on sinking particles may break them apart into smaller non-sinking particles. Alternatively, 2) the presence of shear driven turbulence also could cause particles to fragment into smaller non-sinking particles especially when they are fragile. Both, the organic nutrients content and the fragility of marine particles seem therefore crucial to consider when assessing mechanisms leading to particles fragmentation. Yet, no experiments assessing the importance of turbulence vs zooplankton activity on changes in particles size spectra have been performed.
Compounds such as C-rich transparent exopolymer (TEP) and other protein-based gel particles stainable with Coomassie Blue (CSP) are both considered as important glues binding marine particles together. Both TEP and CSP are produced by phytoplankton and prokaryote exudation. This potentially occurs under various stresses such as N limitation for TEP production and P limitation for CSP. Being an essential component of the organic matrix of marine particles and acting as binding glues, TEP and CSP could play a major role in setting the level of fragmentation induced by the mechanisms described above (turbulence and zooplankton activity).