Marine phytoplankton are responsible for approximately half of Earth's primary production, with about 20% of this production exported to the deep ocean via the biological pump. The size structure of phytoplankton communities significantly influences the amount and type of carbon exported. Large phytoplankton (>20 µm) are traditionally associated with high carbon export due to their rapid sinking rates or incorporation into fecal pellets. However, recent research suggests that small phytoplankton also contribute to carbon export through aggregation or grazing. This study focuses on the Southern Ocean (SO), a region contributing significantly to oceanic CO2 uptake. The SO is characterized by high-nutrient, low-chlorophyll (HNLC) areas where iron limits primary production. While large diatom blooms in naturally iron-fertilized regions like the Kerguelen plateau receive much attention, the role of smaller phytoplankton in post-bloom conditions remains understudied. This research aims to characterize the diversity and quantify the contribution of small phytoplankton to CO2 fixation in the SO after a diatom bloom, using the Kerguelen plateau as a case study.

Previous studies have highlighted the disproportionate contribution of certain phytoplankton groups to biogeochemical cycles, despite their low abundance or biomass. For example, flow cytometry studies have shown that picoeukaryotes, though less abundant than cyanobacteria, can contribute similarly or even more to CO2 fixation. Secondary ion mass spectrometry (SIMS), particularly nanoscale SIMS (nanoSIMS), allows for single-cell level measurements, revealing that rare taxa can account for a large portion of carbon uptake. This technique has demonstrated high intra-group heterogeneity in carbon uptake across various phytoplankton groups (diazotroph-associated diatoms, chain-forming diatoms, and specific pico-phytoplankton). This heterogeneity could be due to genetic diversity, gene expression differences, or cell life history. The Southern Ocean's significance in global carbon cycling makes it an ideal location to study phytoplankton CO2 fixation and the specific role of different size classes, particularly after large diatom blooms that are known to occur in iron-fertilized areas.

The study was conducted during the MOBYDICK cruise in March 2018, sampling on and off the Kerguelen plateau. Four stations were selected (M1, M2, M3, M4), with M2 serving as a representative iron-fertilized plateau station. Various measurements were taken: dissolved inorganic nutrients (silicic acid, nitrate, phosphate, ammonium) and pigment analysis via HPLC and CHEMTAX for taxonomic group determination; metabarcoding of the 18S rRNA gene to characterize phytoplankton communities; and CO2 fixation measurements using stable isotope (13C) tracer experiments. For single-cell analysis, samples were processed using flow cytometry to sort small phytoplankton into three populations (Pico, Nano1, Nano2) based on fluorescence and size characteristics. Large (>20 µm) and small (<20 µm) phytoplankton were analyzed with large geometry SIMS and nanoSIMS, respectively, to determine 13C fixation at the single-cell level. Bulk CO2 fixation was calculated from 13C enrichment in community samples. Cell-specific division rates were also calculated to assess metabolic activity. Phaeopigment analysis helped assess grazing impacts.

After the diatom bloom, small phytoplankton (<20 µm) displayed significantly faster growth rates (0.37 ± 0.13 and 0.22 ± 0.09 division d⁻¹ on and off-plateau, respectively) compared to larger diatoms (0.11 ± 0.14 and 0.09 ± 0.11 division d⁻¹ on and off-plateau, respectively). A large proportion (19 ± 13%) of diatoms were inactive. Small phytoplankton accounted for a substantial portion (41–70%) of total CO2 fixation. The vertical distribution of pigments suggested that grazing is a primary mechanism for exporting small phytoplankton cells from the surface waters. Detailed single-cell analysis using SIMS revealed significant variation in 13C-fixation rates within and between size groups.

The study's results challenge the traditional view that large phytoplankton dominate carbon export in the Southern Ocean. The significant contribution of small phytoplankton to CO2 fixation, even after a diatom bloom, highlights their crucial role in the carbon cycle. The observed faster growth rates and high contribution to CO2 fixation in small cells likely reflect their adaptation to nutrient-limited conditions after the diatom bloom consumes most available silica and iron. Grazing appears to be a key mechanism for exporting small phytoplankton from the euphotic zone. This study emphasizes the need to integrate small phytoplankton into models of carbon cycling and biogeochemical processes in the Southern Ocean.

This research demonstrates the substantial contribution of small phytoplankton to CO2 fixation in the post-bloom Southern Ocean. Their higher growth rates and significant contribution to carbon cycling highlight the importance of considering their role in biogeochemical models. Future research could explore other mechanisms of small phytoplankton export and the long-term impact of environmental changes on their contribution to carbon sequestration. Highlighting the need to incorporate these smaller organisms in more comprehensive models.

The study's spatial and temporal scope might limit the generalizability of findings to other regions or seasons. The use of incubation experiments, while useful for quantifying CO2 fixation, might not perfectly capture in situ conditions. The assumption that DIC was the sole carbon source for growth in cell-specific division rate calculations might introduce some uncertainty.