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Abstract

The diel variations of viruses (< 0.2 μm), picoplankton (0.2-2 μm; heterotrophic bacteria, Synechococcus spp., and picoeukaryotes), and nanoflagellates (2-20 μm; heterotrophic and pigmented nanoflagellates) were examined for 2 days with approximately 4 h of temporal sampling in the spring of 2024 in coastal surface waters of the subtropical western Pacific. The abundance of bacteria did not follow a consistent diel pattern, and viral abundance was positively correlated with bacterial abundance. Synechococcus spp. was found to grow during the light period and with peak abundance at night, exhibiting a remarkably diel pattern; however, the pattern was opposite in picoeukaryotes. In the size-fractionation experiments, after 32 h of incubation, the effects of grazing pressure on bacteria were examined because bacterial abundance generally decreased and was significantly lower in the unfiltered treatment than in the 1 µm filtration treatment. Furthermore, we found a significant increase in picoeukaryotic abundance after 32 h of incubation; however, the opposite pattern was observed in bacteria, with a decline in their abundance, probably because of a predator-prey relationship. The results of our study suggest that some species of picoeukaryotes may act as bacterial predators by being mixotrophic. By analyzing these findings, we gain a more comprehensive understanding of picoplankton in coastal oceans, their functions, and the effects of environmental changes on their structure.

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