Small phytoplankton contribute greatly to CO2-fixation after the diatom bloom in the Southern Ocean

Abstract

Phytoplankton is composed of a broad-sized spectrum of phylogenetically diverse microorganisms. Assessing CO2-fixation intra- and inter-group variability is crucial in understanding how the carbon pump functions, as each group of phytoplankton may be characterized by diverse efficiencies in carbon fixation and export to the deep ocean. We measured the CO2-fixation of different groups of phytoplankton at the single-cell level around the naturally iron-fertilized Kerguelen plateau (Southern Ocean), known for intense diatoms blooms suspected to enhance CO2 sequestration. After the bloom, small cells (<20 µm) composed of phylogenetically distant taxa (prymnesiophytes, prasinophytes, and small diatoms) were growing faster (0.37 ± 0.13 and 0.22 ± 0.09 division d−1 on- and off-plateau, respectively) than larger diatoms (0.11 ± 0.14 and 0.09 ± 0.11 division d−1 on- and off-plateau, respectively), which showed heterogeneous growth and a large proportion of inactive cells (19 ± 13%). As a result, small phytoplankton contributed to a large proportion of the CO2 fixation (41–70%). The analysis of pigment vertical distribution indicated that grazing may be an important pathway of small phytoplankton export. Overall, this study highlights the need to further explore the role of small cells in CO2-fixation and export in the Southern Ocean.

Origin and fate of long-chain polyunsaturated fatty acids in the Kerguelen Islands region (Southern Ocean) in late summer

“…Chl-a at 250 m depth (Irion et al, 2021). In addition, we noted variable vertical trends of n-3 C18 PUFA and C16 PUFA as observed with pigments (Irion et al, 2021).…”

“…followed by diatoms (27-40% and 18-33% of total Chl-a on-and off-plateau, respectively), and finally by pico-phytoplankton (prasinophytes) (Irion et al, 2021(Irion et al, , 2020. This latter group was found in low proportions at all stations except on the plateau where it reached up to 16% of total…”

“…Chl-a was low in the ML (<0.6 µg.L -1 ), and POC and TFA concentrations were <45 mg.L -1 and <4.5 µg.L -1 , respectively. The phytoplanktonic community, as described using 18S rDNA amplicon sequencing, pigments, flow cytometry, and microscopic enumeration (Irion et al, 2021(Irion et al, , 2020Lafond et al, 2020), revealed a mixed composition with three main groups, prymnesiophytes, diatoms and prasinophytes showing variable proportions on-and off the Plateau. Nano-phytoplankton (prymnesiophytes) were the most abundant group (37-53% and 59-70% of total Chl-a on-and off-plateau, respectively)…”

Highly-resolved interannual phytoplankton community dynamics of the coastal Northwest Atlantic

“…Those in common include: Bathycoccus , Micromonas , Chaetoceros , Phaeocystis , Teleaulax , and Thalassiosira , which were all identified as key phytoplankton found in the off-shore NWA with both Micromonas and Bathycoccus especially relevant to the NWA subtropic zone [ 6 , 70 ]. Members of these two taxa are also widespread [ 71 ], and Micromonas and P. antarctica both occur in the Southern Ocean [ 72 ]. In addition to genera mentioned above, Ostreococcus , Dictyocha , Florenciella, Fragilariopsis, Minidiscus , and Braarudosphaera bigelowii were also observed on the Scotian Shelf [ 69 ].…”

Ecological Importance of Viral Lysis as a Loss Factor of Phytoplankton in the Amundsen Sea

“…The Southern Ocean is a major sink for anthropogenic carbon dioxide (CO 2 ), and summer phytoplankton blooms contribute to the efficiency of the biological pump [ 1 , 2 , 3 ]. Especially, the more abundant smaller-sized phytoplankton (<20 µm cell diameter, [ 4 ]) are responsible for a large portion of the primary production (up to 41–70%) [ 5 , 6 ], illustrating their ecological importance for carbon cycling. Productivity in the Southern Ocean varies regionally [ 7 , 8 ] with coastal polynyas (reoccurring open water regions surrounded by sea ice) in particular being areas of high productivity [ 9 , 10 ].…”