Abstract
Seasonal variations in the picophytoplankton community structure (Synechococcus spp. and picoeukaryotes) were studied by flow cytometry in the coastal ecosystem of the subtropical western Pacific from October 2019 to September 2020. Synechococcus spp. was dominant in abundance during the study period, with its density ranging from 0.05 to 5.6 × 104 cells mL−1; its maximum occurred in July 2020. Picoeukaryotes were less abundant, with their density ranging from 0.2 to 13.6 × 103 cells mL−1. Their highest abundance was recorded in January 2020. The growth rates of Synechococcus spp. and picoeukaryotes ranged from −0.39 to 1.42 d−1 and 0.38 to 2.46 d−1, respectively, throughout the study period. Overall, the growth rate of the picoeukaryotes was significantly higher than that of Synechococcus spp. It is interesting to note that the grazing mortality of Synechococcus spp. and picoeukaryotes during the warmer period (April to September) was relatively low. Based on this study, we suggest that mixotrophic nanoflagellates lowered their feeding activity that obtained nutrients from prey and instead used additional nutrients during the incubation experiments. Our study demonstrated that a shift in the picophytoplankton community composition and grazing activity of predacious nanoflagellates in cold and warm periods can impact on the seasonal dynamics of the microbial food web.
Highlights
Picophytoplankton mainly consist of picocyanobacteria (e.g., Synechococcus spp. and Prochlorococcus) and picoeukaryotes, and form an important component of the phytoplankton community in oligotrophic warm environments [1,2,3,4]
In subtropical western Pacific coastal ecosystems, the seasonal trends of Synechococcus spp. abundance and its growth rate approximately follow the annual cycle of temperatures [12,18]
Tsai et al [15] found that pigmented nanoflagellates consume approximately 63% of the mean Synechococcus spp. production during the warmer seasons; pigmented nanoflagellates are the key grazers of the Synechococcus spp. population in subtropical western Pacific coastal waters
Summary
Picophytoplankton mainly consist of picocyanobacteria (e.g., Synechococcus spp. and Prochlorococcus) and picoeukaryotes, and form an important component of the phytoplankton community in oligotrophic warm environments [1,2,3,4]. The estimates of growth and mortality rates (from grazing or viral lysis) of Synechococcus and Prochlorococcus are from widely disparate oceanographic regimes [12,13,14,15,16,17], few growth and mortality rate measurements are available for photosynthetic picoeukaryotes, especially for seasonal observations [17]. In subtropical western Pacific coastal ecosystems, the seasonal trends of Synechococcus spp. abundance and its growth rate approximately follow the annual cycle of temperatures [12,18]. Tsai et al [16] confirmed that nanoflagellate grazing was a significant cause of Synechococcus spp. mortality, viral lysis was an important source of mortality, especially at night-time. There is much information regarding the influence of these processes on the seasonal and diel dynamics of Synechococcus spp., little is known about their influence on picoeukaryotes in this coastal environment
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