SCIENCE: VOL. 386, NO. 6718HIDDEN COMET TAILS OF MARINE SNOW IMPEDE OCEAN-BASED CARBON SEQUESTRATION

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RESEARCH ARTICLE

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

RAHUL CHAJWA HTTPS://ORCID.ORG/0000-0002-6488-1834, ELIOTT FLAUM HTTPS://ORCID.ORG/0000-0001-8737-7967, KAY D. BIDLE HTTPS://ORCID.ORG/0000-0003-4170-412X, BENJAMIN VAN MOOY HTTPS://ORCID.ORG/0000-0002-2804-6508, AND MANU PRAKASH HTTPS://ORCID.ORG/0000-0002-8046-8388 Authors Info & Affiliations

SCIENCE

11 Oct 2024

Vol 386, Issue 6718

DOI: 10.1126/science.adl57671,7641

Editor’s summary

Sinking marine particulate organic matter, commonly called “marine snow,” moves huge amounts of carbon from the new-surface ocean to depth, constituting one of the major components of Earth’s carbon cycle. The biological and physical complexity of these particles, together with the wide range of lengths and timescales that are involved in their motion, have made it exceedingly difficult to determine their dynamics in detail. Chajwa et al. used microscopic imaging to show that these particles universally exhibit a tail-like flow morphology that greatly influences their movement and how much carbon they sequester in the deep ocean (see the Perspective by Cael and Guidi). —Jesse Smith

Structured Abstract

INTRODUCTION

Phytoplankton in the upper layer of the ocean agglomerates and sinks under gravity, giving rise to a natural carbon transport mechanism termed “biological pump.” The perpetual shower of soft and fragile marine snow in the ocean is estimated to be annually sequestering 2 to 4.5 billion tons of carbon from the atmosphere into the abyss, regulating both the atmospheric carbon dioxideand the sustenance of marine ecosystems. A predictive underpinning of marine snow is thus crucial. However, we currently lack a quantitative microphysics-based framework for the formation, sedimentation, and remineralization of marine snow, leading to major uncertainties in the current carbon flux estimates in climate models.

RATIONALE

Because sedimentation physics is at the heart of marine snow phenomena, we took an observation-driven approach to addressing this problem. Based on the 19th-century paradigm of Stokes’ law and its ad hoc generalizations, researchers had been seeking a universal trend in how size is related to sinking speed in marine snow. Because marine snow is a structurally complex soft matter that deforms while sinking under its own weight, it violates key assumptions underlying Stokes’ law and presents a classic two-way fluid-structure coupling that remained unexplored. To directly investigate the sinking dynamics of this complex object, we organized an ocean expedition (Cruise ID: EN667) during an algal bloom in the Gulf of Maine (42.5°N, 69.5°W). Marine snow aggregates were collected through freely hanging sediment traps in multiple deployments at a depth of 80 m. To directly observe sedimentation dynamics of these aggregates at sea, we utilized a new scale-free vertical tracking microscope mounted on a two-axis gimbal that minimized mechanical noise from ship’s motion and allowed us to track small aggregate (equivalent spherical diameter < 750 μm ) sinking over long times. To visualize the flow around sinking aggregates, we used a tracer bead (diameter 700 nm to 2 μm) solution.

RESULTS

By directly measuring the sinking velocities and detailed flows around individual marine snow particles, we discovered a new morphological feature in marine snow: a physical invisible comet tail forming a halo around a visible particulate matter during sedimentation. These hitherto-unseen comet tails are made of viscoelastic transparent exopolymer, which fundamentally modifies the sinking behavior. Our observations guided a new theoretical framework based on Stokesian sedimentation in which we included this previously invisible degree of freedom and constructed a reduced-order model for these compound particles. Furthermore, the combination of field experiments and theory enabled a sedimentation-based measurement of the elastic response of the mucus. We corroborated these findings with three-dimensional volumetric imaging of marine snow particles, which illuminate the heterogeneous microstructure of marine snow.

CONCLUSION

Through detailed analysis of more than 100 marine snow aggregates studied individually, we discovered hidden comet tails that effectively act as “physical” drag lines on sinking marine snow. This mucus-induced impedance almost doubles the estimate of mean residence time of marine snow in the Euphotic zone, nearly halting some particles to a standstill. This suggests a substantial overestimate in the flux inferred by using only the visible size of a marine snow particle. The discovery of multiphase nature of marine snow and a new conceptual framework that incorporates the invisible degrees of freedom in the sedimentation dynamics lays the foundation for understanding the formation, sedimentation, and remineralization of marine snow in the purview of physics. The crucial role of viscoelasticity of marine mucus as one of the knobs of carbon flux opens rich possibilities for studying biological origin of mucus and its complex rheology in the open oceans and potential biogeoengineering remediation.

Hidden comet tails of marine snow.

(A) A simplified depiction of carbon sequestration in the biological pump through marine snow. (B) Experimental data: (Left) Image of sinking marine snow visualized with tracer beads in the background and (right) fluid flow corresponding to the same particle showing the invisible mucus tail (yellow region) that falls along with the particle, greatly increasing the particle’s effective size. (C) Impact of mucus on sedimentation: Mucus greatly increases the time marine snow can spend in the upper layers of the ocean, presenting a natural knob in this carbon flux. ρm, mucus density; ρsw, sea water density; ρp, particulate density; a, semiminor axis of the mucus comet tail; b, semimajor axis of the mucus comet tail; l, size of the visible aggregate.

Abstract

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 phenomenon is obscured by the biological richness of these aggregates and a lack of direct observation of their sedimentation physics. Utilizing a scale-free vertical tracking microscopy in a field setting, we present microhydrodynamic 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 halos around sinking aggregates. These invisible comet tails slow down individual particles, greatly increasing their residence time. Based on these findings, we constructed a reduced-order model for the Stokesian sedimentation of these mucus-embedded two-phase particles, paving the way toward a predictive understanding of marine snow.