Modern shallow water radiolarians with photosynthetic microbiota in the western North Pacific

To study the photosynthetic associations of modern radiolarians, we analyzed plankton samples from waters shallower than 200 m at 30 stations in the western North Pacific. A total of 328 taxa, including morphotypes, were identified from 2091 specimens. The fluorescence patterns under an epifluorescence microscope were mainly classified into five groups: 1) R&D; 2) R-G; 3) Y-B; 4) Periph-R; and 5) Pale-R. There were 37 species/morphotypes with the R&D pattern in the orders Collodaria, Entactinaria, Nassellaria, and Spumellaria. The R-G and Y-B patterns were confirmed in only four species (Dictyocoryne profunda, D. truncata, Spongaster tetras tetras, S. tetras irregularis) of our identified 328 taxa, suggesting a strong species-specific effect on the presence of PE-containing Synechococcus-type cyanobacteria. In the Periph-R pattern, red fluorescence is emitted from the peripheral part inside flat spumellarians, suggesting that the source of this pale red fluorescence is photosynthetic organisms digested by radiolarians. In particular, taxa with the R&D pattern can be classified into three groups: 1) obligate associations; 2) facultative associations; and 3) non-associations. Many taxa belonging to Acanthodesmiidae and Lophospyris have a great number of symbiotic dinoflagellates outside the central capsule. However, the Acanthodesmiidae includes non-photosynthetic holobionts (Amphispyris and Ceratobotrys borealis), suggesting different environmental adaptability in some acanthodesmiid species. In contrast with the obligate symbiosis tendency in Acanthodesmiidae, the Lophophaenidae tends to have non-photosynthetic holobionts. In Spumellaria, one significant tendency was recognized in Tetrapyle and Phorticium as obligate dinoflagellate holobionts. Thus, Tetrapyle and Phorticium are useful for tracing oceanographic conditions in the euphotic zone. Dictyocoryne truncata and D. profunda are classified into the R-G pattern, whereas D. muelleri was grouped into the R&D pattern, indicating that the difference in the photosynthetic association may be closely related to the dwelling depth. The Q-mode cluster analysis showed that the 30 stations were classified into four clusters: A1, A2, B, and C. The geographical distributions of the four cluster groups are consistent with the climate zone and water masses, suggesting that the species composition of these shallow taxa is influenced by the distribution of water masses. In particular, the Kuroshio Current may play an important role in the migration of shallow living radiolarian species from the subtropical climatic zone to the subarctic climatic zone.