[A-OS17] 海洋微生物生態系

[AOS17-04] Spatiotemporal variations of eukaryotic phytoplankton communities and the association with seawater pCO₂ levels in the North Pacific Ocean

*江 思宇¹、中岡 慎一郎²、中岡 慎治³、鈴木 光次⁴、遠藤 寿¹ (1.京都大学化学研究所、2.国立環境研究所　地球システム領域、3.北海道大学　大学院先端生命科学研究院、4.北海道大学　大学院地球環境科学研究院)

キーワード：二酸化炭素、生物ポンプ、北太平洋、18S rRNA遺伝⼦、植物プランクトン、珪藻類

The increasing anthropogenic CO₂ emissions and the induced climate change have become one of the biggest global crises. The ocean has been estimated to absorb about 25% of this anthropogenic CO₂; thus, it plays a critical role in the global carbon cycle. The marine biological pump is one of the most important mechanisms of carbon sequestration in the ocean. However, the efficiency of the biological pump (i.e., whether the carbon fixed by phytoplankton primary production can be exported to the deep ocean) largely depends on phytoplankton species and the quality and sinking speed of organic matters. Here, we introduced a comprehensive dataset consisting of the partial pressure of seawater CO₂ (pCO_2,sw) and eukaryotic 18S rRNA gene composition across the surface North Pacific Ocean covering two years (from Sept. 2014 to Oct. 2016). By analyzing this dataset, we derived an association between oceanic CO₂ levels and eukaryotic phytoplankton communities, which enables the identification of phytoplankton groups that were potentially responsible for oceanic CO₂ absorption. Our results showed at most sampling sites, especially in the northwestern region, the delta pCO₂ (pCO₂ in seawater minus that in the atmosphere) was negative, indicating that the North Pacific Ocean generally appears as a sink of atmospheric CO₂. Furthermore, the delta pCO₂ was negatively correlated with chlorophyll a concentrations (p < 0.01), suggesting the low delta pCO₂ levels were possibly induced by phytoplankton photosynthesis. To further elucidate which phytoplankton groups are responsible for pCO₂ drawdown, the weighted correlation network analysis (WGCNA) was applied to the genus-level taxonomic profiles evaluated with 18S rRNA gene amplicon sequencing. This method clustered the phytoplankton genus into 12 modules according to their cooccurrences. Three modules showed significantly negative correlations to delta pCO₂ (p < 0.05), and two of them were mainly composed of diatoms (classes Mediophyceae and Bacillariophyceae). The positive correlations of these two modules to both chlorophyll a and fucoxanthin (a chemotaxonomic marker pigment of diatoms and haptophytes in oceanic waters) concentrations (p < 0.01) demonstrate that our method successfully identified these algal taxa that accounted for low delta pCO₂ levels. Finally, the diatom modules were negatively correlated with sea surface temperature, whereas were positively correlated with wind stress curl (an indicator of upwelling) and nutrient concentrations. These results collectively suggested that the upwelling-induced nutrient enrichments during cold seasons possibly supported the surface diatom blooms and the intense CO₂ absorption in the study area. In summary, our dataset derives an association between eukaryotic phytoplankton communities at a finer taxonomic resolution with the oceanic CO₂ absorption in the North Pacific Ocean.