Primary production during nutrient-induced blooms at elevated CO2 concentrations

Mesocosms experiments (PeECE II and PeECE III) were carried out in 9 transparent mesocosms. Prior to the experimental period, the seawater carbonate system was manipulated to achieve three different levels of CO2. At the onset of the experimental period, nutrients were added to all mesocosms in order to initiate phytoplankton blooms. Rates of primary production were measured by in-situ incubations using 14C-incorporation and oxygen production/consumption. Particulate primary production by 14C was also size fractionated and compared with phytoplankton species composition. Nutrient supply increased the primary production rates, and a net autotrophic phase with 14C-fixation rates up to 4 times higher than initial was observed midway through the 24 days experiment before net community production returned to near-zero and 14C-fixation rates relaxed back to lower than initial. We found a trend in the 14C-based measurements towards higher cumulative primary production at higher pCO2, consistent with recently published results for DIC removal (Riebesell et al., 2007). There where found differences to the size fractionated primary production response to CO2 treatments. The plankton composition changes throughout the bloom, however, resulted in no significant response until the final phase of the experiment where phytoplankton growth became nutrient limited, and phytoplankton community changed from diatom to flagellate dominance. This opens for the two alternative hypotheses that such an effect is associated with mineral nutrient limited growth, and/or with phytoplankton species composition. The lack of a clear net heterotrophic phase in the last part of the experiment supports the idea that a substantial part of production in the upper layer was not degraded locally, but either accumulated there or was exported vertically

Changes in biogenic carbon flow in response to sea surface warming

Human-induced climate change is causing a warming of the surface ocean. Due to widely differing temperature sensitivities of key biological processes, this may have profound implications for marine food web interactions and the biogeochemical cycling of key elements such as carbon. Using a novel indoor-mesocosm approach, we show that rising sea surface temperature shifts the balance between photosynthetic production and respiratory consumption of organic carbon in a plankton community. This may weaken the ocean’s capacity to sequester atmospheric CO2, hence providing a positive feedback to anthropogenic climate change

Temperature and nutrient stoichiometry interactively modulate organic matter cycling in a pelagic algal-bacterial community

A microcosm experiment was conducted to investigate the interactive effects of rising sea-surface temperature and altered nutrient stoichiometry on the biogeochemical cycling of organic matter in a pelagic algal–bacterial assemblage. Natural seawater, containing a mixed bacterial community, was inoculated with an axenic culture of the bloom-forming diatom species Skeletonema costatum. A factorial combination of three temperatures, simulating weak to strong warming as projected for the end of the 21st century, and either nitrogen (N)-replete or -deficient growth conditions were applied. Depending on the type of nutrient limitation, the mixed algal–bacterial communities displayed pronounced differences in the accumulation and microbial utilization of organic matter in response to warming. Under N-deficient conditions, the build-up of organic matter occurred, irrespective of temperature, dominantly in the particulate pool, and only small amounts of dissolved material accumulated. The subsequent bacterial consumption of organic matter was low, as indicated by measurements of bacterial secondary production and extracellular enzyme activities, and remained also largely unaffected by an increase in temperature from 4°C up to 12°C. Contrastingly, warming resulted in a distinct temperature-dependent increase in the accumulation of dissolved organic carbon compounds under N-replete growth conditions. Moreover, rising temperature notably stimulated the bacterial activity, indicating an enhanced flow of organic matter through the microbial loop. These findings suggest that there will be strong shifts in the biogeochemical cycling of organic matter in the upper ocean in response to increased temperature and nutrient loading that will affect pelagic food-web structures and the biological sequestration of organic matter

Availability of phosphate for phytoplankton and bacteria and of labile organic carbon for bacteria at different pCO2 levels in a mesocosm study

Availability of phosphate for phytoplankton and bacteria and of glucose for bacteria at different pCO2 levels were studied in a mesocosm experiment (PeECE III). Using nutrient-depleted SW Norwegian fjord waters, three different levels of pCO2 (350 μatm: 1×CO2; 700 μatm: 2×CO2; 1050 μatm: 3×CO2) were set up, and nitrate and phosphate were added at the start of the experiment in order to induce a phytoplankton bloom. Despite similar responses of total particulate P concentration and phosphate turnover time at the three different pCO2 levels, the size distribution of particulate P and 33PO4 uptake suggested that phosphate transferred to the >10 μm fraction was greater in the 3×CO2 mesocosm during the first 6–10 days when phosphate concentration was high. During the period of phosphate depletion (after Day 12), specific phosphate affinity and specific alkaline phosphatase activity (APA) suggested a P-deficiency (i.e. suboptimal phosphate supply) rather than a P-limitation for the phytoplankton and bacterial community at the three different pCO2 levels. Specific phosphate affinity and specific APA tended to be higher in the 3×CO2 than in the 2×CO2 and 1×CO2 mesocosms during the phosphate depletion period, although no statistical differences were found. Glucose turnover time was correlated significantly and negatively with bacterial abundance and production but not with the bulk DOC concentration. This suggests that even though constituting a small fraction of the bulk DOC, glucose was an important component of labile DOC for bacteria. Specific glucose affinity of bacteria behaved similarly at the three different pCO2 levels with measured specific glucose affinities being consistently much lower than the theoretical maximum predicted from the diffusion-limited model. This suggests that bacterial growth was not severely limited by the glucose availability. Hence, it seems that the lower availability of inorganic nutrients after the phytoplankton bloom reduced the bacterial capacity to consume labile DOC in the upper mixed layer of the stratified mesocosms

Build-up and decline of organic matter during PeECE III

Increasing atmospheric carbon dioxide (CO2) concentrations due to anthropogenic fossil fuel combustion are currently changing the ocean's chemistry. Increasing oceanic [CO2] and consequently decreasing seawater pH have the potential to significantly impact marine life. Here we describe and analyze the build-up and decline of a natural phytoplankton bloom initiated during the 2005 mesocosm Pelagic Ecosystem CO2 Enrichment study (PeECE III). The draw-down of inorganic nutrients in the upper surface layer of the mesocosms was reflected by a concomitant increase of organic matter until day t11, the peak of the bloom. From then on, biomass standing stocks steadily decreased as more and more particulate organic matter was lost into the deeper layer of the mesocosms. We show that organic carbon export to the deeper layer was significantly enhanced at elevated CO2. This phenomenon might have impacted organic matter remineralization leading to decreased oxygen concentrations in the deeper layer of the high CO2 mesocosms as indicated by deep water ammonium concentrations. This would have important implications for our understanding of pelagic ecosystem functioning and future carbon cycling

Effects of rising temperature on pelagic biogeochemistry in mesocosm systems: a comparative analysis of the AQUASHIFT Kiel experiments

A comparative analysis of data, obtained during four indoor-mesocosm experiments with natural spring plankton communities from the Baltic Sea, was conducted to investigate whether biogeochemical cycling is affected by an increase in water temperature of up to 6 °C above present-day conditions. In all experiments, warming stimulated in particular heterotrophic bacterial processes and had an accelerating effect on the temporal development of phytoplankton blooms. This was also mirrored in the build-up and partitioning of organic matter between particulate and dissolved phases. Thus, warming increased both the magnitude and rate of dissolved organic carbon (DOC) build-up, whereas the accumulation of particulate organic carbon (POC) and phosphorus (POP) decreased with rising temperature. In concert, the observed temperature-mediated changes in biogeochemical components suggest strong shifts in the functioning of marine pelagic food webs and the ocean’s biological carbon pump, hence providing potential feedback mechanisms to Earth’s climate system

Genome-wide significant association with seven novel multiple sclerosis risk loci

Objective: A recent large-scale study in multiple sclerosis (MS) using the ImmunoChip platform reported on 11 loci that showed suggestive genetic association with MS. Additional data in sufficiently sized and independent data sets are needed to assess whether these loci represent genuine MS risk factors. Methods: The lead SNPs of all 11 loci were genotyped in 10 796 MS cases and 10 793 controls from Germany, Spain, France, the Netherlands, Austria and Russia, that were independent from the previously reported cohorts. Association analyses were performed using logistic regression based on an additive model. Summary effect size estimates were calculated using fixed-effect meta-analysis. Results: Seven of the 11 tested SNPs showed significant association with MS susceptibility in the 21 589 individuals analysed here. Meta-analysis across our and previously published MS case-control data (total sample size n=101 683) revealed novel genome-wide significant association with MS susceptibility (p<5×10−8) for all seven variants. This included SNPs in or near LOC100506457 (rs1534422, p=4.03×10−12), CD28 (rs6435203, p=1.35×10−9), LPP (rs4686953, p=3.35×10−8), ETS1 (rs3809006, p=7.74×10−9), DLEU1 (rs806349, p=8.14×10−12), LPIN3 (rs6072343, p=7.16×10−12) and IFNGR2 (rs9808753, p=4.40×10−10). Cis expression quantitative locus effects were observed in silico for rs6435203 on CD28 and for rs9808753 on several immunologically relevant genes in the IFNGR2 locus. Conclusions: This study adds seven loci to the list of genuine MS genetic risk factors and further extends the list of established loci shared across autoimmune diseases

Genome-wide significant association with seven novel multiple sclerosis risk loci

Objective: A recent large-scale study in multiple sclerosis (MS) using the ImmunoChip platform reported on 11 loci that showed suggestive genetic association with MS. Additional data in sufficiently sized and independent data sets are needed to assess whether these loci represent genuine MS risk factors. Methods: The lead SNPs of all 11 loci were genotyped in 10 796 MS cases and 10 793 controls from Germany, Spain, France, the Netherlands, Austria and Russia, that were independent from the previously reported cohorts. Association analyses were performed using logistic regression based on an additive model. Summary effect size estimates were calculated using fixed-effect meta-analysis. Results: Seven of the 11 tested SNPs showed significant association with MS susceptibility in the 21 589 individuals analysed here. Meta-analysis across our and previously published MS case-control data (total sample size n=101 683) revealed novel genome-wide significant association with MS susceptibility (p<5×10−8) for all seven variants. This included SNPs in or near LOC100506457 (rs1534422, p=4.03×10−12), CD28 (rs6435203, p=1.35×10−9), LPP (rs4686953, p=3.35×10−8), ETS1 (rs3809006, p=7.74×10−9), DLEU1 (rs806349, p=8.14×10−12), LPIN3 (rs6072343, p=7.16×10−12) and IFNGR2 (rs9808753, p=4.40×10−10). Cis expression quantitative locus effects were observed in silico for rs6435203 on CD28 and for rs9808753 on several immunologically relevant genes in the IFNGR2 locus. Conclusions: This study adds seven loci to the list of genuine MS genetic risk factors and further extends the list of established loci shared across autoimmune diseases

Effects of sea surface warming on elemental cycling in a pelagic system

The present thesis investigates the effects of climate change-induced sea surface warming on elemental cycling in a pelagic system. Human activities, e.g. burning of fossil fuels, changes in land-use practices and deforestation, are changing Earth’s climate at an unprecedented rate in its history. While the ocean mitigates the progress of climate change by taking up and storing atmospheric carbon dioxide (CO2) and heat, thus acting as a natural climate buffer, these anthropogenic perturbations are also affecting the ocean itself in its chemical and physical properties, e.g. through increased warming and acidification of the upper ocean layers. While these abiotic deviations from contemporary surface ocean conditions are predictable with relatively high certainty, present knowledge concerning their effects on marine organisms and any potentially emerging biotic feedbacks to the climate system is still in its infancy. The aim of this thesis is to elucidate such effects of rising sea surface temperature on the biogeochemical cycling of the key elements carbon, nitrogen, and phosphorus during the spring bloom of planktonic communities in a pelagic system.Die vorliegende Arbeit befasst sich mit den Auswirkungen der durch den Klimawandel bedingten Erwärmung des Oberflächenozeans auf biogeochemische Stoffkreisläufe in einem pelagischen System. Menschliche Aktivitäten, wie die Verbrennung fossiler Energieträger, Änderungen in der Landnutzung sowie Entwaldung, verändern das Klima der Erde in einer in der jüngeren Erdgeschichte beispiellosen Geschwindigkeit. Während der Ozean das Voranschreiten des Klimawandels durch die Aufnahme und Lagerung von Kohlendioxid und Wärme abmildert und damit als natürlicher Klimapuffer fungiert, ändern diese anthropogen bedingten Störungen gleichzeitig auch die chemische und physikalische Beschaffenheit des Ozeans selbst, z.B. durch zunehmende Erwärmung und Versauerung der oberen Ozeanschichten. Während diese abiotischen Veränderungen der marinen Umwelt mit recht hoher Sicherheit prognostiziert werden können, ist unser Wissen um deren Auswirkungen auf marine Organismen und daraus erwachsende potentielle Rückkopplungen auf das Klimasystem der Erde noch sehr begrenzt. Das Ziel dieser Arbeit ist es, derartige Auswirkungen einer zunehmenden Erwärmung des Oberflächenozeans auf den biogeochemischen Kreislauf der Schlüsselelemente Kohlenstoff, Stickstoff und Phosphor während der Frühjahrsblüte des Planktons in einem pelagischen System näher zu beleuchten