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

Phytoplankton encompasses a broad size spectrum of phylogenetically diverse microorganisms. Assessing intra- and inter-group variability in CO2-fixation is crucial to understand the carbon pump, as different phytoplankton groups vary in their efficiency of carbon fixation and export. We measured CO2-fixation at the single-cell level for different phytoplankton groups around the naturally iron-fertilized Kerguelen plateau (Southern Ocean), a region known for intense diatom blooms suspected to enhance CO2 sequestration. After the bloom, small cells (<20 µm) composed of phylogenetically distant taxa (prymnesiophytes, prasinophytes, and small diatoms) grew 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 a large proportion of the CO2 fixation (41–70%). The vertical distribution of pigments suggested that grazing may be an important pathway for the export of small phytoplankton. This study highlights the need to further explore the role of small cells in CO2-fixation and export in the Southern Ocean.