ALBERT

Beschreibung: The Southern Ocean (SO) is an important sink for anthropogenic carbon dioxide (CO2). Climate change will cause changes in various environmental parameters, which in turn can affect growth and productivity of SO phytoplankton. Due to global warming, sea surface temperatures will increase and lead to a more stratified and shallower mixed layer resulting potentially in higher light availability and enhanced primary production of iron-limited phytoplankton. On the other hand, ocean acidification may reduce the bioavailability of iron to phytoplankton. To examine the influence of iron availability in combination with current and future higher CO2 concentrations under low and high irradiance on SO phytoplankton physiology, bottle manipulation experiments with a natural phytoplankton assemblage from the Drake Passage were conducted. Ocean acidification led to lowered abundances of Pseudo-nitzschia species at both irradiances. While higher irradiance stimulated daily particulate organic carbon production, this stimulating effect, however, was reduced under high pCO2, but only under iron-limitation. Moreover, the ratio of biogenic silicate to particulate organic carbon remained unchanged by high pCO2 for both iron treatments under high light, but declined under low light. Gaining more insight on the complex interplay of multiple environmental factors is valuable to predict future responses of SO phytoplankton to climate change.

Beschreibung: In many regions of the Southern Ocean, surface concentrations of the trace metal iron are very low. Iron is an essential nutrient, required for numerous metabolic pathways in phytoplankton cells. Atmospheric dust is an important source for iron input into the ocean. An insufficient supply of iron can lead to reduced growth and alterations in the photophysiology. Therefore, iron is a key factor in controlling Antarctic phytoplankton productivity and species composition. However, in experiments looking at the effects of iron on phytoplankton physiology, iron is commonly added as iron chloride and not in the form of dust. This PhD project will focus on the effects of inorganic iron in comparison to iron-containing dust as iron sources in combination with current and future elevated CO2 concentrations on Southern Ocean phytoplankton ecology and physiology. Rising CO2 concentrations in the atmosphere will reduce the pH of the world’s oceans. Ocean acidification will affect Southern Ocean phytoplankton by potentially altering the availability of iron. In order to study the impact of different climate change scenarios on Southern Ocean phytoplankton, laboratory experiments with selected species as well as shipboard experiments with natural phytoplankton assemblages during an expedition to the Southern Ocean will be conducted.

Beschreibung: 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.

Beschreibung: In some parts of the Southern Ocean (SO), even though low surface concentrations of iron (Fe) and manganese (Mn) indicate FeMn co-limitation, we still lack an understanding on how Mn and Fe availability influences SO phytoplankton ecophysiology. Therefore, this study investigated the effects of Fe and Mn limitation alone as well as their combination on growth, photophysiology and particulate organic carbon production of the bloom-forming Antarctic diatom Chaetoceros debilis. Our results clearly show that growth, photochemical efficiency and carbon production of C. debilis were co-limited by Fe and Mn as highest values were only reached when both nutrients were provided. Even though Mn-deficient cells had higher photochemical efficiencies than Fe-limited ones, they, however, displayed similar low growth and POC production rates, indicating that Mn limitation alone drastically impeded the cell’s performance. These results demonstrate that similar to low Fe concentrations, low Mn availability inhibits growth and carbon production of C. debilis. As a result from different species-specific trace metal requirements, SO phytoplankton species distribution and productivity may therefore not solely depend on the input of Fe alone, but also critically on Mn acting together as important drivers of SO phytoplankton ecology and biogeochemistry.

Beschreibung: The ‘Iron Hypothesis’ suggests a fertilization of the Southern Ocean by increased dust deposition in glacial times. This promoted high primary productivity and contributed to lower atmospheric pCO2. In this study, the diatom Pseudo-nitzschia subcurvata, known to form prominent blooms in the Southern Ocean, was grown under simulated glacial and interglacial climatic conditions to understand how iron (Fe) availability (no Fe or Fe addition) in conjunction with different pCO2 levels (190 and 290 μatm) influences growth, particulate organic carbon (POC) production and photophysiology. Under both glacial and interglacial conditions, the diatom grew with similar rates. In comparison, glacial conditions (190 μatm pCO2 and Fe input) favored POC production by P. subcurvata while under interglacial conditions (290 μatm pCO2 and Fe deficiency) POC production was reduced, indicating a negative effect caused by higher pCO2 and low Fe availability. Under interglacial conditions, the diatom had, however, thicker silica shells. Overall, our results show that the combination of higher Fe availability with low pCO2, present during the glacial ocean, was beneficial for the diatom P. subcurvata, thus contributing more to primary production during glacial compared to interglacial times. Under the interglacial ocean conditions, on the other hand, the diatom could have contributed to higher carbon export due to its higher degree of silicification.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Science communication is becoming increasingly important to connect academia and society, and to counteract fake news among climate change deniers. Online video platforms, such as YouTube, offer great potential for low-threshold communication of scientific knowledge to the general public. In April 2020 a diverse group of researchers from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research launched the YouTube channel "Wissenschaft fürs Wohnzimmer" (translated to "Sitting Room Science") to stream scientific talks about climate change and biodiversity every Thursday evening. Here we report on the numbers and diversity of content, viewers, and presenters from 2 years and 100 episodes of weekly livestreams. Presented topics encompass all areas of polar research, social issues related to climate change, and new technologies to deal with the changing world and climate ahead. We show that constant engagement by a group of co-hosts, and presenters from all topics, career stages, and genders enable a continuous growth of views and subscriptions, i.e. impact. After 783 days the channel gained 30,251 views and 828 subscribers and hosted well-known scientists while enabling especially early career researchers to improve their outreach and media skills. We show that interactive and science-related videos, both live and on-demand, within a pleasant atmosphere, can be produced voluntarily while maintaining high quality. We further discuss challenges and possible improvements for the future. Our experiences may help other researchers to conduct meaningful scientific outreach and to push borders of existing formats with the overall aim of developing a better understanding of climate change and our planet.

Beschreibung: 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) and a strong mixing scenario (high Fe and low light). 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.