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Eukaryotic Phytoplankton Contributing to a Seasonal Bloom and Carbon Export Revealed by Tracking Sequence Variants in the Western North Pacific - PubMed

  • ️Tue Jan 01 2019

Eukaryotic Phytoplankton Contributing to a Seasonal Bloom and Carbon Export Revealed by Tracking Sequence Variants in the Western North Pacific

Takuhei Shiozaki et al. Front Microbiol. 2019.

Abstract

Greater diversity of eukaryotic phytoplankton than expected has been revealed recently through molecular techniques, but little is known about their temporal dynamics or fate in the open ocean. Here, we examined size-fractionated eukaryotic phytoplankton communities from the surface to abyssopelagic zone (5,000 m) throughout the year, by tracking sequence variants of the 18S rRNA gene in the western subtropical North Pacific. The oceanographic conditions were divided into two periods, stratification and mixing, between which the surface phytoplankton community differed. During the mixing period, the abundance of large phytoplankton (≥3 μm) increased, with diatoms and putative Pseudoscourfieldia marina dominating this fraction. Picophytoplankton (<3 μm) also increased during the mixing period and were dominated by Mamiellophyceae. Taxa belonging to prasinophytes (including Ps. marina and Mamiellophyceae) were observed in the epipelagic zone throughout the year, and thus likely seeded the seasonal bloom that occurred during the mixing period. In contrast, diatoms observed during the mixing period mostly represented taxa unique to that period, including coastal species. Numerical particle backtracking experiments indicated that water masses in the surface layer could be transported from coastal areas to the study site. Gene sequences of coastal diatoms were present in the abyssopelagic zone. Therefore, allochthonous species drove the seasonal bloom and could be transported to deep waters. In the abyssopelagic zone, the relative abundance of Ps. marina in deep waters was similar to or higher than that of diatoms during the mixing period. Among picophytoplankton, Mamiellophyceae made up a significant fraction in the abyssopelagic zone, suggesting that prasinophytes are also involved in carbon export. Our molecular survey showed that these previously overlooked phytoplankton species could contribute significantly to the seasonal bloom and biological pump in the subtropical open ocean.

Keywords: 18S rRNA; biological pump; coastal diatoms; eukaryotic phytoplankton; prasinophytes.

Copyright © 2019 Shiozaki, Hirose, Hamasaki, Kaneko, Ishikawa and Harada.

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Figures

FIGURE 1
FIGURE 1

(A) Location of station S1 in the northwestern North Pacific Ocean. (B) Seasonal variations in satellite-derived (MODIS Aqua) chlorophyll a (chl a), sea surface temperature, and Argo-derived mixed layer depth (MLD) (Hosoda et al., 2010) at station S1 (1° × 1° area), averaged between January 2003 and December 2012. Colored bars indicate the months in which the cruises took place. (C) Vertical profiles of temperature, salinity, dissolved oxygen, nitrate, and chl a during each cruise.

FIGURE 2
FIGURE 2

(A) Back trajectories of particles present at station S1 at depths of the 12 (surface layer), 480 (mesopelagic zone), 1,000 (bathypelagic zone), and 5,000 m (abyssopelagic zone) over the 90 days prior to sampling each month. Back trajectories at the other depths are shown in Supplementary Figure S3. (B) Satellite-derived (MODIS Aqua) chl a averaged over the 90 days prior to sampling each month.

FIGURE 3
FIGURE 3

Vertical distributions of the total number of eukaryotic phytoplankton sequences in each fraction during each season. Major groups are shown in color and listed in the legend.

FIGURE 4
FIGURE 4

Relationship between the number of eukaryotic phytoplankton sequences and chl a concentration for each size fraction.

FIGURE 5
FIGURE 5

Grouping of eukaryotic phytoplankton communities based on non-metric multidimensional scaling (nMDS) according to community similarity (Bray–Curtis distance), differentiated by (A) size fraction (color), (B) water depth (shape), and month (color).

FIGURE 6
FIGURE 6

Heatmap of the relative abundance of representative sequence variants (SVs; ≥5% of total reads at a given depth) in the (A) ≥3-μm and (B) <3-μm size fractions. For the <3-μm size fraction, only SVs assigned to Archaeplastida are shown, while other SVs are shown in Supplementary Figure S5. Relative abundance is shown in white when it is >5%. The SVs noted in the text are shown in red.

FIGURE 7
FIGURE 7

Contribution of eukaryotic phytoplankton sequence reads originating in each month, for each size fraction in the surface layer, calculated on the basis of (A) SV sequence reads and (B) SV richness based on the number of sequence reads and presence/absence (binary) data, respectively. SVs observed in November 2010 were set to 100%, shown by the first bar (light blue), and ratios of that value to total reads in each subsequent months are shown by the other bars. SVs that were first detected in February 2011, April 2011, and July 2011 are indicated by blue, dark blue, and green bars, respectively.

FIGURE 8
FIGURE 8

Contribution of eukaryotic phytoplankton sequence reads originating in each layer/zone, for each size fraction during each month. SVs observed in the surface layer were set to 100%, shown by the first bar (light blue), and ratios of that value to total reads in deeper zones are shown below. SVs that were first detected in the mesopelagic, bathypelagic, and abyssopelagic zones are indicated by blue, dark blue, and green bars, respectively.

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