Description: The Western Antarctic Peninsula (WAP), one of the most productive regions of the Southern Ocean, is currently undergoing rapid environmental changes such as ocean acidification (OA) and increased daily irradiances from enhanced surface‐water stratification. To assess the potential for future biological CO2 sequestration of this region, we incubated a natural phytoplankton assemblage from Ryder Bay, WAP, under a range of pCO2 levels (180 μatm, 450 μatm, and 1000 μatm) combined with either moderate or high natural solar radiation (MSR: 124 μmol photons/m**2/s and HSR: 435 μmol photons/ m**2/s, respectively). The initial and final phytoplankton communities were numerically dominated by the prymnesiophyte Phaeocystis antarctica, with the single cells initially being predominant and solitary and colonial cells reaching similar high abundances by the end. Only when communities were grown under ambient pCO2 in conjunction with HSR did the small diatom Fragilariopsis pseudonana outcompete P. antarctica at the end of the experiment. Such positive light‐dependent growth response of the diatom was, however, dampened by OA. These changes in community composition were caused by an enhanced photosensitivity of diatoms, especially F. pseudonana, under OA and HSR, reducing thereby their competitiveness toward P. antarctica. Moreover, community primary production (PP) of all treatments yielded similar high rates at the start and the end of the experiment, but with the main contributors shifting from initially large to small cells toward the end. Even though community PP of Ryder Bay phytoplankton was insensitive to the changes in light and CO2 availability, the observed size‐dependent shift in productivity could, however, weaken the biological CO2 sequestration potential of this region in the future.

Description: The rise in anthropogenic CO2 and the associated ocean acidification (OA) will change trace metal solubility and speciation, potentially altering Southern Ocean (SO) phytoplankton productivity and species composition. As iron (Fe) sources are important determinants of Fe bioavailability, we assessed the effect of Fe-laden dust versus inorganic Fe (FeCl3) enrichment under ambient and high pCO2 levels (390 and 900 μatm) in a naturally Fe-limited SO phytoplankton community. Despite similar Fe chemical speciation and net particulate organic carbon (POC) production rates, CO2-dependent species shifts were controlled by Fe sources. Final phytoplankton communities of both control and dust treatments were dominated by the same species, with an OA-dependent shift from the diatom Pseudo nitzschia prolongatoides towards the prymnesiophyte Phaeocystis antarctica. Addition of FeCl3 resulted in high abundances of Nitzschia lecointei and Chaetoceros neogracilis under ambient and high pCO2, respectively. These findings reveal that both the characterization of the phytoplankton community at the species level and the use of natural Fe sources are essential for a realistic projection of the biological carbon pump in the Fe-limited pelagic SO under OA. As dust deposition represents a more realistic scenario for the Fe-limited pelagic SO under OA, unaffected net POC production and dominance of P. antarctica can potentially weaken the export of carbon and silica in the future.

Description: This study highlights the importance of manganese (Mn) next to iron (Fe) for the growth of specific Southern Ocean phytoplankton groups with important implications for carbon export in the Weddell Sea. A Fe-Mn bottle amendment experiment and an aggregation experiment were performed with a natural phytoplankton community during Polarstern expedition PS124 in 2021 in the Weddell Sea. At this location, seawater was pumped (using trace metals clean techniques) from 25 m depth to fill polycarbonate bottles. The Control treatment consist of the sampled seawater without any trace metals addition, while the other three treatments were enriched with either FeCl₃ alone (0.5 nM; +Fe treatment) or MnCl₂ alone (1 nM; +Mn treatment) or both trace metals together (+FeMn treatment). All treatments were done in triplicate 2.5 L PC bottles. After on average 7 days, samples for chlorophyll a content, particulate organic carbon and primary production were taken to detect FeMn co-limitation effect on species composition, primary production and carbon export. Results showed that when Fe and Mn were added together, primary production rates were highest due to an increased abundance of the prymnesiophyte Phaeocystis antarctica. This change within the phytoplankton community led to highly carbon-enriched aggregates and a four-fold increase in the carbon export potential compared to the doubling in the only Fe treatment. Our findings highlight that even small changes in plankton community composition can have significant effects on the carbon export potential of the Southern Ocean, a region of critical importance for anthropogenic carbon dioxide drawdown.

Description: Organic ligands such as exopolymeric substances (EPS) are known to form complexes with iron (Fe) and modulate phytoplankton growth. However, the effect of organic ligands on bacterial and viral communities remains largely unknown. Here, we assessed how Fe associated with organic ligands influences phytoplankton, microbial, and viral abundances and their diversity in the Southern Ocean. While the particulate organic carbon (POC) was modulated by Fe chemistry and bioavailability in the Drake Passage, the abundance and diversity of microbes and viruses were not governed by Fe bioavailability. Only following amendments with bacterial EPS did bacterial abundances increase, while phenotypic alpha diversity of bacterial and viral communities decreased. The latter was accompanied by significantly enhanced POC, pointing toward the relief of C limitation or other drivers of the microbial loop. Based on the literature and our findings, we propose a conceptual framework by which EPS may affect phytoplankton, bacteria, and viruses. Given the importance of the Southern Ocean for Earth’s climate as well as the prevalence of viruses and their increasingly recognized impact on marine biogeochemistry and C cycling; the role of microbe–virus interactions on primary productivity in the Southern Ocean needs urgent attention.

Description: Deposition of airborne ash from volcanic eruptions has the potential to inject a significant amount of bioavailable metals into seawater, affecting marine primary productivity and increasing or decreasing phytoplankton biomass in the open ocean. These effects can be the result of the release of fertilizing trace elements e.g. iron in limited areas, like high nutrient low chlorophyll areas, like the Southern Ocean or potentially toxic elements like cadmium, copper or lead. In previous studies, the diatom Thalassiosira pseudonanahas been shown to grow in contact with volcanic ash, whilst the growth of the coccolithophore Emiliania huxleyicame to a halt which was unclear why though and what the effects and thresholds could be 1. Different trace metals have been shown to be vital for different processes in coccoliths 2.Changes in coccolithophore productivity and coccolith numbers or sizes as a reaction to increased metal concentrations, could influence sinking rates and CO2uptake and could therefore alter the efficiency of organic carbon export to deep waters. For example, if volcanic material acts as a fertilizer in open ocean waters, it might stimulate phytoplankton growth, increasing the flux of CO2between the atmosphere and the surface ocean organic pool, and result in significant carbon sequestration.To clarify the potential impacts of volcanic ash on coccolithophores, we performed culturing experiments with two coccolithophore strains and different volcanic ashes at a range of concentrations. Here we present results from these experiments, showing the release of an array of metals from the volcanic ashes in Antarctic seawater and the physiological (growth, Fv/Fm) and morphological responses of both coccolithophore strains (SEM).Finally, from a broader perspective, we compare our results with paleo-data to increase the applicability of calcareous nannofossils (coccolithophore remains) as proxies for trace metal concentrations. In particular, the modeling of nannoplankton species-specific reactions/adaptations to excess volcanic trace metal released during different geological episodes is expected to open new scenarios on the characterization of critical past events.

Description: The effects of climate change, including ocean acidification (OA), on future Southern Ocean phytoplankton community’s species composition and consequent impacts on primary production and carbon export are largely unknown. Further, changes in light availability induced by decreased vertical mixing of surface waters can lead to higher light availability enhancing potentially phytoplankton productivity in Antarctic coastal and shelf areas. Therefore, a suite of laboratory and field experiments with the two ecologically relevant Antarctic diatom species as well as a natural phytoplankton community from the Western Antarctic Peninsula were conducted under different pCO2 levels and irradiance regimes. In the two diatom species Fragilariopsis curta and Odontella weisflogii, OA did not stimulate, but inhibited growth and carbon fixation under low and medium light whereas this effect was amended under high growth irradiances. Under different dynamic light regimes, however, OA stimulated carbon fixation in the two diatoms. Yet, reduced photosynthetic efficiencies in both species indicated that this was likely a stress response to OA. In CO2-light incubation experiments with a natural community of the West Antarctic Peninsula, OA led to a decline of overall diatom abundances, including Fragilariopsis and Odontella, triggering thereby the dominance of the prymnesiophyte Phaeocystis antarctica, an inefficient vector for carbon export. In summary, these findings highlight that, under different irradiance regimes, OA is likely to induce a species shift, away from diatoms, within phytoplankton communities and to alter primary productivity of the coastal Southern Ocean with important implications for biogeochemical cycles in the future.

Description: In many regions of the Southern Ocean, surface concentrations of the trace metals iron and manganese are very low. These elements are required for numerous metabolic pathways in phytoplankton cells and an insufficient supply can lead to reduced growth and alterations in photophysiology. However, little is known about the effects of manganese limitation alone on Antarctic phytoplankton species and its interaction with iron limitation. Therefore, this study focused on the effects of manganese and iron limitation alone as well as their combination on growth, elemental composition and photophysiology of the bloom-forming Antarctic diatom Chaetoceros debilis. Our experimental treatments consisted of four combinations of the two trace metals with two iron-limited treatments and two iron-rich treatments with and without manganese addition, respectively. Limitation by iron alone lowered carbon fixation and photochemical efficiency whereas the limitation with both metals resulted in the highest concentrations of the light-harvesting pigment fucoxanthin. Highest values for growth and carbon fixation were only observed after addition of both trace metals. These findings suggest that C. debilis is co-limited under low iron and manganese concentrations. Gaining more inside on the interplay of various trace metals and their potential co-limitation are valuable to better understand the spatial distribution of phytoplankton key species in the present and the future Southern Ocean.