Description: Contrasting models predict two different climate change scenarios for the Southern Ocean (SO), forecasting either less or stronger vertical mixing of the water column. To investigate the responses of SO phytoplankton to these future conditions, we sampled a natural diatom dominated (63%) community from today's relatively moderately mixed Drake Passage waters with both low availabilities of iron (Fe) and light. The phytoplankton community was then incubated at these ambient open ocean conditions (low Fe and low light, moderate mixing treatment), representing a control treatment. In addition, the phytoplankton was grown under two future mixing scenarios based on current climate model predictions. Mixing was simulated by changes in light and Fe availabilities. The two future scenarios consisted of a low mixing scenario (low Fe and higher light, low mixing treatment) and a strong mixing scenario (high Fe and low light, strong mixing treatment). In addition, communities of each mixing scenario were exposed to ambient and low pH, the latter simulating ocean acidification (OA). The effects of the scenarios on particulate organic carbon (POC) production, trace metal to carbon ratios, photophysiology and the relative numerical contribution of diatoms and nanoflagellates were assessed. During the first growth phase, at ambient pH both future mixing scenarios promoted the numerical abundance of diatoms (~75%) relative to nanoflagellates. This positive effect, however, vanished in response to OA in the communities of both future mixing scenarios (~65%), with different effects for their productivity. At the end of the experiment, diatoms remained numerically the most abundant phytoplankton group across all treatments (~80%). In addition, POC production was increased in the two future mixing scenarios under OA. Overall, this study suggests a continued numerical dominance of diatoms as well as higher carbon fixation in response to both future mixing scenarios under OA, irrespective of different changes in light and Fe availability.

Description: Contrasting models predict two different climate change scenarios for the Southern Ocean (SO), forecasting either less or stronger vertical mixing of the water column. To investigate the responses of SO phytoplankton to these future conditions, we sampled a natural diatom dominated (63%) community from today's relatively moderately mixed Drake Passage waters with both low availabilities of iron (Fe) and light. The phytoplankton community was then incubated at these ambient open ocean conditions (low Fe and low light, moderate mixing treatment), representing a control treatment. In addition, the phytoplankton was grown under two future mixing scenarios based on current climate model predictions. Mixing was simulated by changes in light and Fe availabilities. The two future scenarios consisted of a low mixing scenario (low Fe and higher light, low mixing treatment) and a strong mixing scenario (high Fe and low light, strong mixing treatment). In addition, communities of each mixing scenario were exposed to ambient and low pH, the latter simulating ocean acidification (OA). The effects of the scenarios on particulate organic carbon (POC) production, trace metal to carbon ratios, photophysiology and the relative numerical contribution of diatoms and nanoflagellates were assessed. During the first growth phase, at ambient pH both future mixing scenarios promoted the numerical abundance of diatoms (~75%) relative to nanoflagellates. This positive effect, however, vanished in response to OA in the communities of both future mixing scenarios (~65%), with different effects for their productivity. At the end of the experiment, diatoms remained numerically the most abundant phytoplankton group across all treatments (~80%). In addition, POC production was increased in the two future mixing scenarios under OA. Overall, this study suggests a continued numerical dominance of diatoms as well as higher carbon fixation in response to both future mixing scenarios under OA, irrespective of different changes in light and Fe availability.

Description: Two Fe-Mn bottle amendment experiments with two natural phytoplankton communities were performed during Polarstern expedition PS97 in 2016 in the Drake Passage. At two locations, sea water was pumped (using trace metals clean techniques) from 25m depth and used to fill polycarbonate bottles after having passed through a cleaned 200 μm mesh (removing large grazers). The Control treatment was 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 2,5L PC bottles. All incubation bottles were maintained at 30 μmol photons m-2 s-1 under a 16:8 (light:dark) hour cycle at 1 ̊C. Autotrophic picoeukaryotes were analyzed via flow cytometry. At the start and the end of the experiments, samples were preserved with 10% buffered formalin, flash-frozen in liquid nitrogen, and analyzed flow cytometrically to assess picoplankton densities. Before running the samples, 2 μL beads (Sperotech - Rainbow Fluorescent Particles (RFPs) - 2.11 μm) were added to each treatment as a size and fluorescence reference. Then picoeukaryotes were identified based on side scatter versus FL-3. Three P subgroups (0.2 – 2 μm) were differentiated according to their size : small (P1), medium (P2) and large (P3), according to sub-cluster of events.

Description: Determination of dissolved trace metals concentration: dFe and dMn concentrations were estimated from the initially sampled seawater. To this end, 100 mL of seawater were filtered through HCl-cleaned polycarbonate filters (0.2 μm pore size) using a TMC Nalgene filtration system and the filtrate was collected into PE bottle and stored triple bagged at 2 ̊C until analysis. Concentrations of the dFe and dMn were determined on a SeaFast system (Elemental Scientific, Omaha, NE, USA) coupled to an inductively coupled plasma mass spectrometer (ICP-MS, Element2, Thermo Fisher Scientific, resolution of R = 2000). During the pre-concentration step, an iminodiacetate (IDA) chelation column (part number CF-N-0200, Elemental Scientific) was used. The pre-filtered seawater samples were acidified to pH=1.7 with a double distilled nitric acid (HNO~3~) and were UV-treated using a 450 W photochemical UV power supply (ACE GLASS Inc., Vineland N. J., USA) to minimize adsorption of TMs onto the bottle walls and to reduce the formation of Mn and Fe hydroxides during storage. During each UV digestion step, two blanks were taken. The ICP-MS was optimized daily to achieve oxide forming rates below 0.3%. Each seawater sample was analyzed via standard addition to minimize any matrix effects, which might influence the quality of the analysis. To assess the accuracy and precision of the method, a NASS-7 (National Research Council of Canada) reference standard was analyzed in a 1:10 dilution (corresponding to environmentally representative concentrations) at the beginning, in the middle and at the end of each run (two batch runs; n = 18). The measured values were in the limits of the certified NASS-7 reference material, with a concentration of 351 ± 26 ng L^-1^ for dFe and 750 ± 60 ng L^-1^ for dMn (mean ± strandard deviation). The detection limits for Mn and Fe were 8.1 pM and 81.8 pM, respectively. Determination of the chlorophyll a fluorescence: For the 18 additional stations, chlorophyll a fluorescence measurements were collected using a Fast Repetition Rate Fluorometer (FRRf) coupled to a FastAct Laboratory system (FastOcean PTX), both from Chelsea Technologies Group. Samples were first dark acclimated for 1h before the meeasurement was perfomed.

Description: Two Fe-Mn bottle amendment experiments with two natural phytoplankton communities were performed during Polarstern expedition PS97 in 2016 in the Drake Passage. At two locations, sea water was pumped (using trace metals clean techniques) from 25m depth and used to fill polycarbonate bottles after having passed through a cleaned 200 μm mesh (removing large grazers). The Control treatment was 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 2,5L PC bottles. All incubation bottles were maintained at 30 μmol photons m-2 s-1 under a 16:8 (light:dark) hour cycle at 1 ̊C. Chlorophyll a samples were taken at the beginning and the end of both experiments. In order to compare the contribution of large (〉2 μm) relative to small cells (0.2-2 μm), 250 mL (on average) of samples were filtered onto 0.2 μm (for the total fraction) and 2 µm (for the large fraction) polycarbonate filters, hence the small fraction was calculated as the difference of the total and the large fraction. All samples were directly flash frozen into liquid nitrogen (N~2~) and then stored at −80 ̊C in the dark until further analysis. After being homogenized, samples were extracted in 90% acetone for 24h at 4 ̊C in the dark and analyzed fluorometrically on a Trilogy Fluorometer.

Description: This study highlights the importance of manganese (Mn) next to iron (Fe) for growth of specific Southern Ocean phytoplankton groups. Two Fe-Mn bottle amendment experiments with two natural phytoplankton communities were performed during Polarstern expedition PS97 in 2016 in the Drake Passage. At two locations, sea water was pumped (using trace metals clean techniques) from 25m depth and used to fill polycarbonate bottles after having passed through a cleaned 200 μm mesh (removing large grazers). The Control treatment was 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 2,5L PC bottles. All incubation bottles were maintained at 30 μmol photons m-2 s-1 under a 16:8 (light:dark) hour cycle at 1 ̊C. After on average 15 days, samples for chlorophyll a content, flow cytometry and light macroscopy were taken in order to detect FeMn co-limitation effect on species composition. In addition to the two experiments, 9 in situ stations of PS97 were also sampled for dissolved Fe, dissolved Mn as well as photophysiology and to complete this dataset, data from PS112 (2018) were also used. The results showed that only some members of the phytoplankton community were Fe-Mn co-limited, with the biogeochemical important diatom group Fragilariopsis and one subgroup of picoeukaryotes.

Description: Two Fe-Mn bottle amendment experiments with two natural phytoplankton communities were performed during Polarstern expedition PS97 in 2016 in the Drake Passage. At two locations, sea water was pumped (using trace metals clean techniques) from 25m depth and used to fill polycarbonate bottles after having passed through a cleaned 200 μm mesh (removing large grazers). The Control treatment was 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 2,5L PC bottles. All incubation bottles were maintained at 30 μmol photons m-2 s-1 under a 16:8 (light:dark) hour cycle at 1 ̊C. To determine the effects of the different treatments on the microplankton composition for the two Fe-Mn enrichment experiments, unfiltered seawater was collected at the start and the end of both experiments for later analysis via light microscopy in the home laboratory. Briefly, samples were fixed with hexamine-buffered formalin solution (2% final concentration) and Lugol's solution (1% final concentration) and stored at 2 °C in the dark until taxonomic analysis. All samples were allowed to settle in Utermöhl sedimentation chambers for at least 24 hours and were analyzed on an inverted light microscope, according to the method of Utermöhl (1958). Species were counted and identified according to taxonomic literature. Each aliquot was examined until at least 400 cells had been counted.

Description: It is widely accepted that iron (Fe)-binding organic ligands play a crucial role in Fe distribution in the marine environment and thus in Fe biogeochemistry. Although Competitive Ligand Equilibration – Adsorptive Cathodic Stripping Voltammetry (CLE-AdCSV) is a well-established technique to investigate Fe chemical speciation in marine samples, several impediments still need to be addressed. These include the extrapolation of laboratory measurements to in-situ conditions, the harmonization of the analytical procedures used, and the applicability of the methods over salinity ranges wider than seawater (e.g., sea ice). This work focusses on the calibration of 2-(2-thiazolylazo)-p-cresol (TAC), salicylaldoxime (SA) and 1-nitroso-2-naphthol (NN), along the salinity range 1–90, and titration of natural samples at two different temperatures (4 °C and 20 °C). The artificial ligand concentration was 10 μM for TAC and 5 μM for SA and NN. Calibrations showed that increasing salinity caused a decrease in the conditional stability constants (logK'Fe’AL) for NN and SA (although different behaviours were noted for the two species FeSA and FeSA2). Less accuracy was noted using TAC, which behaved inconsistently outside the 21 〈 S 〈 35 range, and its use is therefore discouraged in fresh and highly saline waters. Titrations of natural samples showed that only SA covered the salinity range selected, up to 78, and its use is therefore recommended in sea-ice studies. The side reaction coefficient (logα'Fe’AL) of each artificial ligand was found to be influenced by temperature differently: logα'Fe’SA was higher at lower temperature (4 °C), whereas logα'Fe’SA2 and logα'Fe’NN3 increased with increasing temperature (to 20 °C). Although titrations performed at 4 °C highlighted that the uncomplexed Fe fraction was 14% lower than at 20 °C, with potential consequences on primary productivity, the percentage of natural Fe complexed was 〉99%. Future investigations should consider the analysis of the samples at a temperature as close as possible to in-situ conditions to reduce the potential temperature effects.

Description: Over the last decades, it has been reported that the habitat of the Southern Ocean (SO) key species Antarctic krill (Euphausia superba) has contracted to high latitudes, putatively due to reduced winter sea ice coverage, while salps as Salpa thompsoni have extended their dispersal to the former krill habitats. To date, the potential implications of this population shift on the biogeochemical cycling of the limiting micronutrient iron (Fe) and its bioavailability to SO phytoplankton has never been tested. Based on uptake of fecal pellet (FP)- released Fe by SO phytoplankton, this study highlights how efficiently krill and salps recycle Fe. To test this, we collected FPs of natural populations of salps and krill, added them to the same SO phytoplankton community, andmeasured the community’s Fe uptake rates. Our results reveal that both FP additions yielded similar dissolved iron concentrations in the seawater. Per FP carbon added to the seawater, 4.8 ± 1.5 times more Fe was taken up by the same phytoplankton community from salp FP than from krill FP, suggesting that salp FP increased the Fe bioavailability, possibly through the release of ligands. With respect to the ongoing shift from krill to salps, the potential for carbon fixation of the Fe-limited SO could be strengthened in the future, representing a negative feedback to climate change.

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.