Hidden comet-tails of marine snow impede ocean-based carbon sequestration

Gravity-driven sinking of "marine snow" sequesters carbon in the ocean, constituting a key biological pump that regulates earth's climate. A mechanistic understanding of this phenomena is obscured by the biological richness of these aggregates and lack of direct observation of their sedimentation physics. Utilizing a scale-free vertical tracking microscopy in field setting, here we present micro-hydrodynamic measurements of freshly collected marine snow aggregates from sediment-traps. Our observations reveal hitherto unknown comet-like morphology, arising from fluid-structure interactions of transparent exopolymer halo around sinking aggregates. These invisible comet-tails slows down individual particles, dramatically increasing their residence time. Based on these findings, we construct a reduced order model for the Stokesian sedimentation of these mucus-embedded two-phase particles, paving the way towards a predictive understanding of marine snow.

Methods
Sediment trap Sampling:

Data was collected while on board the R/V Endeavor during the cruise using a gravity machine (GM) [for details on the instrument see Krishnamurthy et al. Nat Methods 17, 1040–1051 (2020). https://doi.org/10.1038/s41592-020-0924-7]. We use sediment traps on RIPPLE1 Cruise (Cruise ID: EN667) with mesh size (50 μm) at 80 m depth in the Gulf of Maine on RV Endeavor [design of the net trap is given in Peterson et al. (2005) Limnol. Oceanogr. Methods, 3, doi:10.4319/lom.2005.3.520.]. The nets were recovered after 24 hrs and the exact times of the deployment and recovery are provided in the supplementary of material of the preprint: R. Chajwa et al. 2023 https://arxiv.org/abs/2310.01982. After the cod-end recovery, we pass the material through a quantitative splitter. We use a splitter that is modified from the on the basic design of Lamborg et al., 2008 https://doi.org/10.1016/j.dsr2.2008.04.011, to accommodate collection of twelve samples instead of eight and to allow direct electric drive.

Preparing the GM Wheel

The material collected was gently picked by a wide end pipette and added to a wheel shaped fluid chamber with inner thickness 5mm. We study particles with equivalent spherical diameter below 750 micron. Thus ensuring that the wheels are much wider than the particle dimensions, to avoid wall induced shear. Furthermore, our work is focused on micro-physics of sedimentation, thus we limit ourselves to particle sizes < 750 μm.

PIV and Sedimentation Measurement

We mix 60 μL of 700 nm - 2  μm polystyrene bead solution in 100 mL sea water for doing Particle Imagining Velocimetry (PIV). We load about 1mL of the sediment from the collected sample in the gravity machine wheel containing the bead sea water solution. After loading the aggregates, the marine snow suspension was gently homogenized by manually rotating the wheel clockwise and counter-clockwise multiple times. This homogenization is needed to minimize inter-particle hydrodynamic interactions. We then take tracks of marine snow aggregates in GM for around 2 min at 5fps.