by Hannah Kish

Climate change concerns are driven by the consequences of having a high amount of carbon dioxide (CO₂) in the atmosphere, which increases global temperature. The rise in temperature has flow-on effects such as increases in natural disaster frequency, wildfires, and droughts (IPCC 2023). For eons, the presence of carbon dioxide in the atmosphere was balanced by the biological uptake via photosynthesis and the ocean, also known as carbon sequestration (Figure 1). These processes remove CO₂ from the atmosphere and store it in solid or liquid form. Global strategies are discussing how to leverage these naturally occurring processes to increase CO₂ removal and mitigate climate change (Smith et al., 2021).

These strategies include increasing photosynthesis through increasing the biomass of primary producers by restorating rainforests and mangroves or by farming seaweed and kelp (Lefebvre et al.,  2021, Chung et al,  2013).  The deep ocean is a primary focus for carbon sequestration due to its lengthy turnover time (100 of 1000 of years) and huge storage potential, around 50x more than air (Sheps 2009). 

Carbon is introduced to the deep ocean primarily by descending solids, or sinking aggregates. This process helps remove carbon from surface water and contributes to maintaining a stable pH. As the ocean absorbs more atmospheric CO₂ , this process could be leveraged to help increase the amount of carbon being removed from shallower depths. The question is how do we speed up or increase this natural process.

Deepak Krishnamurthy (2024), a researcher from University of California has begun to unravel the complexities of the sinking aggregates. The solids host a multitude of organisms that are important for microbial activity at depth, but how they impact sinking rates, trajectories and mass transfer of aggregates is unknown. Krishnamurthy used Particle Imagery Velocimetry (PIV) to track the flow around the aggregates. PIV uses microscopic beads placed in water and a camera captures their displacement through precise control of exposure times. They are able to place these images into an algorithm that identifies differences in flow patterns (Fig 2).

The results showed that the bacteria and protists living on the aggregates are able to manipulate the flow around themselves in order to feed, causing rotation of the aggregates, bigger plumes, and larger encounter regions. Bigger plumes and larger encounter regions allow for aggregates to either drag or combine with other particulate matter or increase feeding opportunity for the microorganisms. If these microorganisms receive more food, they could grow larger, increasing the weight and therefore speed of the aggregate. The implications of the speed at which aggregate accumulationation occurs at depth could increase the rate at which carbon is being sequestered. These microscale processes should be a focus for future research to determine how carbon removal rates are impacted. 

To read more about the findings: Active sinking particles: sessile suspension feeders significantly alter the flow and transport to sinking aggregates. Krishnamurthy, D (2024).

Literature Cited:

Sixth Assessment Report — IPCC

Assessing the carbon capture potential of a reforestation project David Lefebvre, et al (2021)  

Smith, P. et al. Biophysical and economic limits to negative CO₂ emissions. Nat. Clim. Chang. 6, 42–50 (2016).Return to ref 1 in article

Installing kelp forests/seaweed beds for mitigation and adaptation against global warming: Korean Project Overview  Chung et al 2013

Coral Reefs: Potential Blue Carbon Sinks for Climate Change Mitigation shi 2021

A case for deep-ocean CO2 sequestration Sheps et al 2009