Co-limitation by iron, silicate, and light of three Southern Ocean diatom species

International audienceThe effect of combined iron, silicate, and light co-limitation was investigated in two Southern Ocean diatom species, Chaetoceros dichaeta and Actinocyclus sp. and one cosmopolitan species, Chaetoceros debilis, all isolated in the Southern Ocean (SO). We found species specific differences in the level of nutrient limitation and its effect on physiological and morphological parameters. Growth of all species tested was clearly co-limited by iron and silicate, reflected in a 4 to 40 times higher increase in cell numbers in the high iron, high silicate treatments compared with the controls. However, the effect of iron and silicate availability on chain length and frustules structures was species specific. Most drastic frustule malformation was found under iron and silicate co-limitation in C. dichaeta while Si limitation caused a strong cell elongation in both Chaetoceros species. Additional a significant increase in chain length was observed in these species under high iron conditions. Therefore, species composition in the SO is likely also indirectly affected by these nutrients via different effects on diatom grazing protection. These morphological changes reflect a potential as biological markers in sediments for the growth history of chain forming species. High light conditions, comparable with light intensities found in the upper 28 m of the SO, showed a negative impact on growth of the endemic species C. dichaeta and Actinocyclus sp. This is in contrast to the assumed light limitation of SO diatoms and indicates an adaptation strategy to the deep mixing and resulting low light conditions in the SO. In contrast to that, the cosmopolitan species C. debilis was not negatively affected by increased light intensity, indicating adaptation to a broader light environment. These results suggest that light limitation of SO phytoplankton due to deep wind mixed layers may play a minor role than hitherto assumed

The European Climate Research Alliance (ECRA): collaboration from bottom-up

The European Climate Research Alliance (ECRA) is an association of leading European research institutions in the field of climate research (http://www.ecra-climate.eu/, last access: 6 December 2018). ECRA is a bottom-up initiative and helps to facilitate the development of climate change research, combining the capacities of national research institutions, and inducing closer ties between existing national research initiatives, projects and infrastructures. ECRA works as an open platform to bring together climate researchers, providing excellent scientific expertise for policy makers and of societal relevance. The ECRA Board consists of representatives of ECRA partners and decides on governance, scientific priorities, and organisational matters. Currently organized into four Collaborative Programmes, climate scientists share their knowledge, experience and expertise to identify the most important research requirements for the future, thus developing a foresight approach. The CPs cover the topics: (1) Arctic variability and change, (2) Sea level changes and coastal impacts, (3) Changes in the hydrological cycle and (4) High impact events. The CP activities are planned in workshops and participation is open to all interested scientists from the relevant research fields. In particular, young researchers are actively encouraged to join the network. Each CP develops its joint research priorities for shaping European research into the future. Because scientific themes are interconnected, the four Collaborative Programmes interact with each other, e.g. through the organization of common workshops or joint applications. In addition, the Collaborative Programme leads attend the Board meetings. The different formats of ECRA meetings range from scientific workshops to briefing events and side events at conferences to involve different groups of interests. This facilitates the interaction of scientists, various stakeholder groups and politicians. A biennial open ECRA General Assembly that is organised in Brussels represents an umbrella event and acts as a platform for discussion and contact with stakeholders. This event is an excellent opportunity to jointly discuss research priorities of high societal relevance

Iron, silicate, and light co-limitation of three Southern Ocean diatom species

The effect of combined iron, silicate, and light co-limitation was investigated in the three diatom species Actinocyclus sp. Ehrenberg, Chaetoceros dichaeta Ehrenberg, and Chaetoceros debilis Cleve, isolated from the Southern Ocean (SO). Growth of all species was co-limited by iron and silicate, reflected in a significant increase in the number of cell divisions compared to the control. Lowest relative Si uptake and drastic frustule malformation was found under iron and silicate co-limitation in C. dichaeta, while Si limitation in general caused cell elongation in both Chaetoceros species. Higher light intensities similar to SO surface conditions showed a negative impact on growth of C. dichaeta and Actinocyclus sp. and no effect on C. debilis. This is in contrast to the assumed light limitation of SO diatoms due to deep wind driven mixing. Our results suggest that growth and species composition of Southern Ocean diatoms is influenced by a sensitive interaction of the abiotic factors, iron, silicate, and light

Mechanism of homodimeric cytokine receptor activation and dysregulation by oncogenic mutations

Homodimeric class I cytokine receptors are assumed to exist as preformed dimers that are activated by ligand-induced conformational changes. We quantified the dimerization of three prototypic class I cytokine receptors in the plasma membrane of living cells by single-molecule fluorescence microscopy. Spatial and spatiotemporal correlation of individual receptor subunits showed ligand-induced dimerization and revealed that the associated Janus kinase 2 (JAK2) dimerizes through its pseudokinase domain. Oncogenic receptor and hyperactive JAK2 mutants promoted ligand-independent dimerization, highlighting the formation of receptor dimers as the switch responsible for signal activation. Atomistic modeling and molecular dynamics simulations based on a detailed energetic analysis of the interactions involved in dimerization yielded a mechanistic blueprint for homodimeric class I cytokine receptor activation and its dysregulation by individual mutations.</p

Tools for crushing diatoms – opal teeth in copepods feature a rubber-like bearing composed of resilin

Diatoms are generally known for superior mechanical properties of their mineralised shells. Nevertheless, many copepod crustaceans are able to crush such shells using their mandibles. This ability very likely requires feeding tools with specific material compositions and properties. For mandibles of several copepod species silica-containing parts called opal teeth have been described. The present study reveals the existence of complex composite structures, which contain, in addition to silica, the soft and elastic protein resilin and form opal teeth with a rubber-like bearing in the mandibles of the copepod Centropages hamatus. These composite structures likely increase the efficiency of the opal teeth while simultaneously reducing the risk of mechanical damage. They are supposed to have coevolved with the diatom shells in the evolutionary arms race, and their development might have been the basis for the dominance of the copepods within today's marine zooplankton

Foraminiferal assemblages as palaeoenvironmental bioindicators in Late Jurassic epicontinental platforms: relation with trophic conditions

Foraminiferal assemblages from the neritic environment reveal the palaeoecological impact of nutrient types in relation to shore distance and sedimentary setting. Comparatively proximal siliciclastic settings from the Boreal Domain (Brora section, Eastern Scotland) were dominated by inner−shelf primary production in the water column or in sea bottom, while in relatively seawards mixed carbonate−siliciclastic settings from the Western Tethys (Prebetic, Southern Spain), nutrients mainly derived from the inner−shelf source. In both settings, benthic foraminiferal assemblages increased in diversity and proportion of epifauna from eutrophic to oligotrophic conditions. The proximal setting example (Brora Brick Clay Mb.) corresponds to Callovian offshore shelf deposits with a high primary productivity, bottom accumulation of organic matter, and a reduced sedimentation rate for siliciclastics. Eutrophic conditions favoured some infaunal foraminifera. Lately, inner shelf to shoreface transition areas (Fascally Siltstone Mb.), show higher sedimentation rates and turbidity, reducing euphotic−zone range depths and primary production, and then deposits with a lower organic matter content (high−mesotrophic conditions). This determined less agglutinated infaunal foraminifera content and increasing calcitic and aragonitic epifauna, and calcitic opportunists (i.e., Lenticulina). The comparatively distal setting of the Oxfordian example (Prebetic) corresponds to: (i) outer−shelf areas with lower nutrient input (relative oligotrophy) and organic matter accumulation on comparatively firmer substrates (lumpy lithofacies group) showing dominance of calcitic epifaunal foraminifera, and (ii) mid−shelf areas with a higher sedimentation rate and nutrient influx (low−mesotrophic conditions) favouring potentially deep infaunal foraminifers in comparatively unconsolidated and nutrient−rich substrates controlled by instable redox boundary (marl−limestone rhythmite lithofacies).This research was carried out with the financial support of projects CGL2005−06636−C0201 and CGL2005−01316/BTE, and University of Oslo, Norway−Statoil cooperation. M.R. holds a Juan de la Cierva grant from the Ministry of Science and Technology of Spain

Carbon sources of Antarctic nematodes as revealed by natural carbon isotope ratios and a pulse-chase experiment

δ13C of nematode communities in 27 sites was analyzed, spanning a large depth range (from 130 to 2,021 m) in five Antarctic regions, and compared to isotopic signatures of sediment organic matter. Sediment organic matter δ13C ranged from −24.4 to −21.9‰ without significant differences between regions, substrate types or depths. Nematode δ13C showed a larger range, from −34.6 to −19.3‰, and was more depleted than sediment organic matter typically by 1‰ and by up to 3‰ in silty substrata. These, and the isotopically heavy meiofauna at some stations, suggest substantial selectivity of some meiofauna for specific components of the sedimenting plankton. However, 13C-depletion in lipids and a potential contribution of chemoautotrophic carbon in the diet of the abundant genus Sabatieria may confound this interpretation. Carbon sources for Antarctic nematodes were also explored by means of an experiment in which the fate of a fresh pulse of labile carbon to the benthos was followed. This organic carbon was remineralized at a rate (11–20 mg C m−2 day−1) comparable to mineralization rates in continental slope sediments. There was no lag between sedimentation and mineralization; uptake by nematodes, however, did show such a lag. Nematodes contributed negligibly to benthic carbon mineralization

Semi-Automated Image Analysis for the Assessment of Megafaunal Densities at the Arctic Deep-Sea Observatory HAUSGARTEN

Megafauna play an important role in benthic ecosystem function and are sensitive indicators of environmental change. Non-invasive monitoring of benthic communities can be accomplished by seafloor imaging. However, manual quantification of megafauna in images is labor-intensive and therefore, this organism size class is often neglected in ecosystem studies. Automated image analysis has been proposed as a possible approach to such analysis, but the heterogeneity of megafaunal communities poses a non-trivial challenge for such automated techniques. Here, the potential of a generalized object detection architecture, referred to as iSIS (intelligent Screening of underwater Image Sequences), for the quantification of a heterogenous group of megafauna taxa is investigated. The iSIS system is tuned for a particular image sequence (i.e. a transect) using a small subset of the images, in which megafauna taxa positions were previously marked by an expert. To investigate the potential of iSIS and compare its results with those obtained from human experts, a group of eight different taxa from one camera transect of seafloor images taken at the Arctic deep-sea observatory HAUSGARTEN is used. The results show that inter- and intra-observer agreements of human experts exhibit considerable variation between the species, with a similar degree of variation apparent in the automatically derived results obtained by iSIS. Whilst some taxa (e. g. Bathycrinus stalks, Kolga hyalina, small white sea anemone) were well detected by iSIS (i. e. overall Sensitivity: 87%, overall Positive Predictive Value: 67%), some taxa such as the small sea cucumber Elpidia heckeri remain challenging, for both human observers and iSIS