Fecal Pellet-Like Gyrodinium Species in Sinking Particles: Newly Found Potential Contributors for Carbon Export in the Antarctic Seasonal Ice Zone

Fecal pellets (FPs) are generated by various species and have gained attention as contributors to the biological carbon pump. Metazoans and protozoans are known as FP and minipellet-producers, respectively. Herein, we discovered fecal pellet-like dinoflagellates (FLDs) in the seasonal sea ice zone in the Southern Ocean. The size and form of these FLDs were similar to those of zooplankton oval FPs. However, due to their appearance, they have been misclassified as FPs rather than dinoflagellates, leading to potential oversight of their role in the carbon cycle. Thus, we aimed to identify FLD cells at the species level and examine the impact of FLDs on flux estimation of particulate organic carbon (POC). Our findings are as follows: first, FLD cells were identified as Gyrodinium rubrum and Gyrodinium heterogrammum through 18S rRNA gene sequencing. Second, FLDs can potentially excrete larger FPs than minipellets. Third, the sinking rate of FLDs is higher than that of other protozoa and dinoflagellate cysts. Finally, a maximum of 12 mgC m(-2) day(-1) of the POC flux can be attributed to FLDs (representing 32% of POC flux). These results suggest that FLDs are important drivers not only for the microbial loop but also for the biological carbon pump. In future projections of carbon sequestration, the contribution of metazoans to carbon export must be considered, but not that of FLDs. Their unknown physiological and ecological characteristics, especially including the responses to climate changes, must be urgently investigated for future projections of carbon sequestration in the Southern Ocean. Plain Language Summary Zooplankton feed on phytoplankton cells and excrete fecal pellets (FPs). FPs have gained attention as carbon carriers from the surface to the deep ocean because of their high carbon contents and sinking rate. Previous studies have sampled FPs using sediment traps and estimated carbon fluxes through microscopic observations to better understand the ocean carbon cycle. Here, we report the discovery of a dinoflagellate that closely resembles FP and was captured using a drifting sediment trap. This dinoflagellate has been named fecal-pellet-like dinoflagellate (FLD). It is believed that FLD cells have been misclassified as FPs given their appearance. Our findings indicated that a maximum of 12 mgC m(-2) day(-1) (representing 32% of the particulate organic carbon flux at 50 m depths during the summer productive season) may be attributed to FLD carbon flux. This result suggests that FLD cells serve as potential contributors for carbon export, contrary to the knowledge that heterotrophic dinoflagellates had been known as one of the dominant microbial loop components. Studies on FLDs are essential to understand the Southern Ocean carbon cycle.

Automated summary

In the Southern Ocean, dinoflagellates that resemble fecal pellets (FLDs) were discovered, which have been mistakenly classified as fecal pellets and overlooked in their role in the carbon cycle. By gene sequencing, FLD cells were identified as Gyrodinium rubrum and Gyrodinium heterogrammum, and it was found that FLDs can potentially excrete larger fecal pellets and have a higher sinking rate compared to other protozoa and dinoflagellate cysts. FLDs can contribute up to 32% of the particulate organic carbon flux.