The ecological distribution characteristics of ultraplankton (autotrophic
ultraplankton: Synechococcus, Prochlorococcus, picoeukaryotes, nanoeukaryotes; heterotrophic prokaryotes: low and high nucleic acid content heterotrophic prokaryotes) and virioplankton in seamount areas (Yap, Mariana, Caroline and Magellan seamounts) of the tropical western Pacific Ocean were analyzed by flow cytometry in laboratory. We also studied the potential influencing factors on ultraplankton and virioplankton distribution.
The abundance and biomass of ultraplankton were investigated in winter (Yap seamount), spring (Mariana seamount), summer (Caroline seamount) and autumn (Caroline seamount). Synechococcus ranged from 0-3.64×103 cells mL-1, with a seasonal variation of summer > spring > autumn > winter. Prochlorococcus was the most abundant autotrophic ultraplankton and ranged from 0.07-176.25×103 cells mL-1, with a seasonal variation of winter > summer > spring > autumn. Picoeukaryotes
ranged from 0-5.78×103 cells mL-1, with a seasonal variation of winter > summer > spring > autumn. Nanoeukaryotes was the lowest abundant autotrophic ultraplankton and ranged from 0-1.14×103 cells mL-1, with a seasonal variation of autumn > summer > winter > spring. Heterotrophic prokaryotes was the most abundant ultraplankton and ranged from 0.58-7.32×105 cells mL-1, with a seasonal variation of autumn > summer > spring > winter. Low and high nucleic acid content heterotrophic
prokaryotes (LNA and HNA) ranged from 0.09-3.16×105 cells mL-1 and
0.41-5.20×105 cells mL-1. The seasonal variation of LNA and HNA was consistent with that of heterotrophic prokaryotes. The vertical distribution pattern of ultraplankton was related to in situ chlorophyll a fluorescence. High abundance of Synechococcus was observed in the upper deep chlorophyll a maximum (DCM) layer. The depths of high abundance of Synechococcus were shallower in winter (upper 100 m water column) and deeper in spring, summer and autumn (upper 130 m water column). Prochlorococcus and picoeukaryotes exhibited maximum abundance in the DCM layer. The depths of maximum abundance of Prochlorococcus and picoeukaryotes were shallowest in winter (30 m-100 m water column) and deepest in autumn (100 m-150 m water column). Nanoeukaryotes and heterotrophic prokaryotes were found abundant in wide distribution from surface to DCM layer. The depths of high abundance of nanoeukaryotes and heterotrophic prokaryotes were shallowest in
winter (upper 100 m water column) and deepest in autumn (nanoeukaryotes: upper 150 m water column, heterotrophic prokaryotes: upper 175 m water column). For the autotrophic ultraplankton, nanoeukaryotes biomass dominated the upper DCM layer, whereas Prochlorococcus biomass dominated the DCM layer. For the heterotrophic
prokaryotes, the percentage of high nucleic acid content heterotrophic prokaryotes biomass was higher than that of low nucleic acid ones. No obvious “seamount effect”was observed in the distribution of autotrophic ultraplankton. The proportion of low nucleic acid content heterotrophic prokaryotes increased near Caroline seamount, which means that the existence of seamount may affect the proportion of the two groups of heterotrophic prokaryotes. Redundancy analysis (RDA) showed that ultraplankton (except for Prochlorococcus and picoeukaryotes) had a positive relationship with temperature and salinity, and a negative relationship with nutrients. This indicated that the seasonal variation of ultraplankton was related to the seasonal difference of environmental factors (temperature, salinity and nutrients).
The distributions of ultraplankton were investigated in Mariana (11.1-11.5°N, 139.1-139.6°E) and Magellan (17.2-17.6°N, 152.5-155.5°E) seamounts. The abundance and biomass of autotrophic ultraplankton were higher in Mariana seamount than Magellan seamount. However, the trend of heterotrophic prokaryotes was opposite. The depths of high abundance and biomass of ultraplankton (except for Synechococcus) in Magellan seamount were deeper than Mariana seamount. The main
contributors of biomass of autotrophic ultraplankton in Mariana seamount were Prochlorococcus and nanoeukaryotes, whereras Prochlorococcus and picoeukaryotes biomass were dominant in Magellan seamount. The difference of distribution of ultraplankton in two seamounts was affected by geographical location. In addition, the types of seamounts also affected ultraplankton distribution.
The 405 nm violet side scatter (SSC) replaced 488 nm blue SSC in flow cytometer, which greatly improved the accuracy and resolution of detecting virioplankton. This technology was applied to detect virioplankton in seamount areas. The virioplankton distribution was investigated in Caroline (shallow seamount) and Magellan (deep seamount) seamounts. The total abundance of virus-like particles (VLP) in Caroline and Magellan seamounts were in the range of 0.51-21.11×106 particles mL-1 and 0.31-13.01×106 particles mL-1, respectively and the average of VLP were 5.37±3.75×106 particles mL-1 and 4.99±3.26×106 particles mL-1, respectively. The virioplankton abundance of Caroline seamount was higher than that of Magellan seamount. The depth of high abundance of virioplankton in Caroline seamount was shallower than that in Magellan seamount. Three to four distinct viral subclusters with similar side scatter but different green fluorescence intensities were identified in two seamounts. Viral subclusters exhibited differences related to depth. From surface to DCM layers, there were four distinct subclusters classified as low fluorescence viruses (LFV), medium fluorescence viruses a and b (MFV-a and MFV-b) and high fluorescence viruses (HFV). From DCM layers to bottom, only one MFV subcluster was resolved. In full water column of two seamounts, LFV comprised the most abundant subclusters, followed by MFV, and HFV constituted the least abundant subcluster. However in Caroline seamount, the MFV abundance was higher than the LFV abundance in 75 m-150 m water column. The vertical distribution of the ratio of virioplankton/heterotrophic prokaryote (VPR) was same in two seamounts. With the increase of water depth, VPR increased. Shallower subsurface peaks and significant virioplankton abundance enhancements were detected at the summit and seamount stations in Caroline seamount, which formed the “seamount effect” of virioplankton. Interactions between the shallow Caroline seamount and the local current transported nutrients into the euphotic zone and viruses in seamount sediments might also be resuspended into water column, which jointly supported higher virioplankton standing stocks. However, there was no obvious “seamount effect” in the distribution of virioplankton in Magellan seamount. The impact of deep seamount on environmental factors and resuspension of viruses in seamount sediments didn’t reach the epipelagic layer and affect the distribution of virioplankton.