Differences in Sinking Processes and Biological Pump Contribution Among Phytoplankton Groups in the Mesopelagic Layer

The differing contributions of phytoplankton groups to biological pump have been insufficiently explored. We evaluated the sinking of phytoplankton in the mesopelagic layer using 16S rRNA gene amplicon sequencing. Sinking particles were collected from June to August 2022 in the Sea of Japan using sediment traps moored at depths of 387 and 890 m. Morphologically categorized fecal pellets—ellipsoidal, cylindrical, spherical, and tabular types—were analyzed for their carbon content and phytoplankton assemblages as well as the bulk and non-fecal particles. Fecal pellets contributed ≤4.1% and ≤8.0% of the total particulate organic carbon (POC) flux at 387 and 890 m depths, respectively. Ellipsoidal pellets, likely of appendicularian origin, accounted for 59.3%–78.5% of the fecal pellets' carbon fluxes. Diatoms, particularly Chaetocerotales, were the dominant phytoplankton group across all sinking types and depths, as indicated by eukaryotic chloroplast and cyanobacteria gene proportions. Cyanobacteria Synechococcales were most prevalent in ellipsoidal and cylindrical fecal pellets at 890 m depth. Amplicon sequence variant richness positively correlated with fecal pellet's POC content, with Synechococcales and Chaetocerotales exhibiting the highest diversity in ellipsoidal fecal pellets at both depths. Non-Chaetocerotales diatoms showed comparable or lower diversity levels than the non-fecal particles. These findings suggest that Chaetocerotales and Synechococcales were the most effectively transported phytoplankton groups into the mesopelagic layer through zooplankton grazing and repackaging, particularly by appendicularians. In contrast, other phytoplankton groups, including non-Chaetocerotales diatoms, played a less significant role in this process.