ALBERT

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: Photophysiological processes as well as uptake characteristics of iron and inorganic carbon were studied in inshore phytoplankton assemblages of the Western Antarctic Peninsula (WAP) and offshore assemblages of the Drake Passage. Chlorophyll a concentrations and primary productivity decreased from in- to offshore waters. The inverse relationship between low maximum quantum yields of photochemistry in PSII (Fv/Fm) and large sizes of functional absorption cross sections (sigma PSII) in offshore communities indicated iron-limitation. Congruently, the negative correlation between Fv/Fm values and iron uptake rates across our sampling locations suggest an overall better iron uptake capacity in iron-limited pelagic phytoplankton communities. Highest iron uptake capacities could be related to relative abundances of the haptophyte Phaeocystis antarctica. As chlorophyll a-specific concentrations of humic-like substances were similarly high in offshore and inshore stations, we suggest humic-like substances may play an important role in iron chemistry in both coastal and pelagic phytoplankton assemblages. Regarding inorganic carbon uptake kinetics, the measured maximum short-term uptake rates (Vmax(CO2)) and apparent half-saturation constants (K1/2(CO2)) did not differ between offshore and inshore phytoplankton. Moreover, Vmax(CO2) and K1/2(CO2) did not exhibit any CO2-dependent trend over the natural pCO2 range from 237 to 507 µatm. K1/2(CO2) strongly varied among the sampled phytoplankton communities, ranging between 3.5 and 35.3 µmol/L CO2. While in many of the sampled phytoplankton communities, the operation of carbon-concentrating mechanisms (CCMs) was indicated by low K1/2(CO2) values relative to ambient CO2 concentrations, some coastal sites exhibited higher values, suggesting down-regulated CCMs. Overall, our results demonstrate a complex interplay between photophysiological processes, iron and carbon uptake of phytoplankton communities of the WAP and the Drake Passage.

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: 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: 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: 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: 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: The potential interactive effects of iron (Fe) limitation and Ocean Acidification in the Southern Ocean (SO) are largely unknown. Here we present results of a long-term incubation experiment investigating the combined effects of CO2 and Fe availability on natural phytoplankton assemblages from the Weddell Sea, Antarctica. Active Chl a fluorescence measurements revealed that we successfully cultured phytoplankton under both Fe-depleted and Fe-enriched conditions. Fe treatments had significant effects on photosynthetic efficiency (Fv/Fm; 0.3 for Fe-depleted and 0.5 for Fe-enriched conditions), non-photochemical quenching (NPQ), and relative electron transport rates (rETR). pCO2 treatments significantly affected NPQ and rETR, but had no effect on Fv/Fm. Under Fe limitation, increased pCO2 had no influence on C fixation whereas under Fe enrichment, primary production increased with increasing pCO2 levels. These CO2-dependent changes in productivity under Fe-enriched conditions were accompanied by a pronounced taxonomic shift from weakly to heavily silicified diatoms (i.e. from Pseudo-nitzschia sp. to Fragilariopsis sp.). Under Fe-depleted conditions, this functional shift was absent and thinly silicified species dominated all pCO2 treatments (Pseudo-nitzschia sp. and Synedropsis sp. for low and high pCO2, respectively). Our results suggest that Ocean Acidification could increase primary productivity and the abundance of heavily silicified, fast sinking diatoms in Fe-enriched areas, both potentially leading to a stimulation of the biological pump. Over much of the SO, however, Fe limitation could restrict this possible CO2 fertilization effect.

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.