The hidden driving force in CO₂ transport to the deep sea: Phaeodaria's carbon transport quantified for the first time.

1. Key Points

• Phaeodaria (a group of single-celled zooplankton)^※1 has been reported to exhibit high biomass depending on the sea area. Still, little was known about their contribution to the transport of CO₂ by marine organisms from the surface to the deep sea.
• We have quantified for the first time in the world the carbon transport to the deep sea by small phaeodarians less than 1 mm in size (hereinafter referred to as <1-mm phaeodarians), which had been challenging to measure.
• By combining ultra-high sensitivity trace element analysis technology^※2 and the relationship between the volume and carbon content of phaeodarians^※3, the carbon transport of <1 mm phaeodarians, which had previously been overlooked, can now be estimated.
• As a result, <1 mm phaeodarians accounted for approximately 60% of the carbon transport of the total phaeodarians.
• This method can be applied to other sea areas and plankton and is expected to contribute to elucidating the role of single-celled plankton on a global scale in the carbon cycle within the ocean (affecting atmospheric CO₂ concentrations).

Figure 1　Schematic diagram of the study

2. Overview

Takahito Ikenoue, a researcher at the Research Institute for Global Change (RIGC), Marine Biodiversity and Environmental Assessment Research Center (BioEnv), Marine Plastics Research Group (M-Plastics), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), and his colleagues have, for the first time in the world, quantitatively clarified how much carbon of <1-mm phaeodarians (a group of single-celled zooplankton), which had been challenging to measure, is transported to the deep sea using ultra-high sensitivity trace element analysis technology (Fig. 1).

Some of the CO₂ in the atmosphere is converted into organic matter through photosynthesis by phytoplankton dwelling in the ocean's surface layer. Eventually, this organic matter combines with the carcasses and fecal pellets of organisms that feed on it, aggregating into marine sinking particles^※4, which are transported to the deep sea. These marine sinking particles are one of the main mechanisms for transporting CO₂ from the atmosphere to the deep sea. The western North Pacific subarctic region is one of the areas where marine sinking particles are efficiently transported to the deep sea, and high biomass of phaeodarians has been observed in the past ^{Reference 3}. However, only a small amount of carbon remains in the carcasses of phaeodarians in marine sinking particles compared to when they were alive. Therefore, it was difficult to quantify the carbon transport of <1-mm phaeodarians using conventional elemental analyzers.

In this study, we overcame this problem by combining a highly sensitive analysis technique using an ultra-high-sensitivity elemental analyzer with existing volume-carbon content relationships for phaeodarians. First, we clarified the habitat depth distribution^※5 of various species of Phaeodaria in the western North Pacific subarctic region using plankton net observations. We also collected marine sinking particles using sediment traps^※6 moored at a water depth of 1,000 m in the same sea area. We then picked out the carcasses of phaeodarians from the particles and measured the amount of residual carbon using a highly sensitive elemental analyzer. Furthermore, by comparing the carbon content of living phaeodarians, calculated from the volume-carbon content equation, with the carbon content remaining after sinking, we estimated the percentage of carbon that can be transported to the deep sea (remaining carbon ratio). Even for species with few specimens and for which carbon content cannot be measured using ultra-high sensitivity elemental analyzers, the remaining carbon content can be estimated by multiplying the carbon content of the living species by the remaining carbon ratio corresponding to the depth at which it dwells; this enabled us to quantify the carbon transport of <1-mm phaeodarians at a water depth of 1,000 m for the first time.

We found that the contribution of carbon in <1-mm phaeodarians to the organic carbon transport of marine sinking particles ranged from 1.1 to 12.5%, with an average of 3.5%, which is not negligible; this accounts for 60% of the total carbon transport of phaeodarians. This study has revealed that quantifying the carbon transport of <1-mm phaeodarians, which had previously been overlooked, is essential for understanding the movement of carbon within the ocean, which influences atmospheric CO₂ concentrations. Furthermore, the methodology used in this study can be applied to other marine regions and plankton and is expected to contribute to elucidating the role of single-celled plankton in the global carbon cycle within the ocean.

This finding was published in Progress in Earth and Planetary Science on August 10, 2025 (JST). This study was supported by a Sasakawa Scientific Research Grant (29-701) from The Japan Science Society and Grants-in-Aid for Scientific Research (12J04155, 17K00539, and 23K11414).