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To better predict ecological consequences of changing Arctic sea ice environments, we aimed to quantify the contribution of ice algae-produced carbon (α Ice) to pelagic food webs in the central Arctic Ocean. Eight abundant under-ice fauna species were submitted to fatty acid (FA) analysis, bulk stable isotope analysis (BSIA) of nitrogen (δ 15N) and carbon (δ 13C) isotopic ratios, and compound-specific stable isotope analysis (CSIA) of δ 13C in trophic marker FAs. A high mean contribution α Ice was found in Apherusa glacialis and other sympagic (ice-associated) amphipods (BSIA: 87% to 91%, CSIA: 58% to 92%). The pelagic copepods Calanus glacialis and C. hyperboreus, and the pelagic amphipod Themisto libellula showed substantial, but varying α Ice values (BSIA: 39% to 55%, CSIA: 23% to 48%). Lowest α Ice mean values were found in the pteropod Clione limacina (BSIA: 30%, CSIA: 14% to 18%). Intra-specific differences in FA compositions related to two different environmental regimes were more pronounced in pelagic than in sympagic species. A comparison of mixing models using different isotopic approaches indicated that a model using δ 13C signatures from both diatomspecific and dinoflagellate-specific marker FAs provided the most conservative estimate of α Ice. Our results imply that ecological key species of the central Arctic Ocean thrive significantly on carbon synthesized by ice algae. Due to the close connectivity between sea ice and the pelagic food web, changes in sea ice coverage and ice algal production will likely have important consequences for food web functioning and carbon dynamics of the pelagic system.

Ocean acidification, caused by elevated seawater carbon dioxide levels, may have a deleterious impact on energetic processes in animals. Here we show that high PCO(2) can suppress metabolism, measured as oxygen consumption, in the pteropod, L. helicina forma antarctica, by ∼20%. The rates measured at 180-380 µatm (MO(2)  =  1.25 M(-0.25), p  =  0.007) were significantly higher (ANCOVA, p  =  0.004) than those measured at elevated target CO(2) levels in 2007 (789-1000 µatm,  =  0.78 M(-0.32), p  =  0.0008; Fig. 1). However, we further demonstrate metabolic plasticity in response to regional phytoplankton concentration and that the response to CO(2) is dependent on the baseline level of metabolism. We hypothesize that reduced regional Chl a levels in 2008 suppressed metabolism and masked the effect of ocean acidification. This effect of food limitation was not, we postulate, merely a result of gut clearance and specific dynamic action, but rather represents a sustained metabolic response to regional conditions. Thus, pteropod populations may be compromised by climate change, both directly via CO(2)-induced metabolic suppression, and indirectly via quantitative and qualitative changes to the phytoplankton community. Without the context provided by long-term observations (four seasons) and a multi-faceted laboratory analysis of the parameters affecting energetics, the complex response of polar pteropods to ocean acidification may be masked or misinterpreted.

This study aimed at a better understanding of the fatty acid (FA) turnover in Arctic pteropods. Thecosome pteropods, i.e. Limacina helicina (juveniles and adults) and L. retroversa (adults), were collected in summer/autumn in Kongsfjorden and Isfjorden (Svalbard, 78° N) and, for the first time, successfully fed with 13C-labeled algae for 6 d. The gymnosome pteropod Clione limacina was sampled in summer in northern Svalbard and fed with 13C-labeled L. retroversa for 23 d. FA compositions were determined by gas chromatography, and 13C enrichment of FAs was analyzed by compound-specific isotope analysis. Among the thecosomes, maximum lipid turnover occurred in L. retroversa adults (1.3% d−1). L. helicina adults and juveniles showed lower lipid turnover rates (0.1 and 0.2% d−1, respectively). The thecosomes exhibited the ability to assimilate omega-3 FAs (up to 8.0% d−1). The lipid turnover rate of C. limacina averaged at only 0.07% d−1. However, C. limacina clearly showed the unusual capacity of de novo synthesis of odd-chain FAs (up to 1.2% d−1). Lipid turnover rates of pteropods were lower than those reported for Arctic copepods. However, pteropods may play a substantial role in the transfer of lipids to higher trophic levels, especially in autumn, when copepods have descended from the upper layers of the water column. The pteropods also showed the capacity to channel particular compounds such as omega-3 and odd-chain FAs, and therefore could be important for the functional diversity of the Arctic zooplankton community.

Thecosomatous pteropods are pelagic molluscs with aragonitic shells. They are considered to be especially vulnerable among plankton to ocean acidification, but to recognize changes due to anthropogenic forcing a baseline understanding of their life history is needed. In the present study, adult Limacina retroversa were collected on five cruises from multiple sites in the Gulf of Maine (between 42 degree 22.1'-42 degree 0.0'N and 69 degree 42.6'-70 degree 15.4'W; water depths of ca. 45-260 m) from October 2013 to November 2014. They were maintained in the laboratory under continuous light at 8 degree C. There was evidence of year-round reproduction and an individual life span in the laboratory of 6 months. Eggs laid in captivity were observed throughout development. Hatching occurred after 3 d, the veliger stage was reached after 6-7 d, and metamorphosis to the juvenile stage was after ~1 month. Reproductive individuals were first observed after 3 months. Calcein staining of embryos revealed calcium storage beginning in the late gastrula stage. Staining was observed in the shell gland, shell field, mantle and shell margin in later stages. Exposure of two batches of larvae at the gastrula stage to elevated CO sub(2) levels (800 and 1200 ppm) resulted in significantly increased mortality in comparison with individuals raised under ambient (~400 ppm) conditions and a developmental delay in the 1200 ppm treatment compared with the ambient and 800 ppm treatments.

Contemporary genetic constitution of marine species carries signatures of Pliocene-Pleistocene glacial cycles. Molecular studies of polar organisms show that isolation in refugia during glaciation often results in loss of genetic diversity. However, recent studies of marine organisms from the Southern Ocean have highlighted their remarkably high level of infraspecific genetic differentiation and the presence of cryptic species. Thus, demographic responses to climate change vary substantially with geography and life history. To elucidate the relative role of glacial period in driving the evolution of Antarctic and Arctic fauna we examined the genetic diversity and historical demography of the pelagic marine gastropods Limacina antarctica from Drake Passage in the Antarctic and Limacina helicina from Spitsbergen fjords in the Arctic. Diversity was assessed by comparing nucleotide sequences from part of the mitochondrial gene encoding the cytochrome c oxidase subunit I (COI). Sequences from 60 individuals of L. antarctica collected at 7 stations along Drake Passage were compared with those of 67 individuals of L. helicina from the fjords Hornsund and Isfjorden. We identified 47 different haplotypes for L. antarctica and 25 for L. helicina. No spatial genetic structure was found in either species, indicating that studied populations in each species belong to a single evolutionary unit. Demographic analyses of haplotype networks and significant negative Tajima’s D and Fu’s F S indices suggest recent rapid population expansion in both species. However, L. antarctica populations displayed a higher level of haplotype and nucleotide diversity than L. helicina populations, which suggests that the impact of glaciations was less prominent in L. antarctica.

Background Ocean acidification (OA), a change in ocean chemistry due to the absorption of atmospheric CO 2 into surface oceans, challenges biogenic calcification in many marine organisms. Ocean acidification is expected to rapidly progress in polar seas, with regions of the Southern Ocean expected to experience severe OA within decades. Biologically, the consequences of OA challenge calcification processes and impose an energetic cost. Results In order to better characterize the response of a polar calcifier to conditions of OA, we assessed differential gene expression in the Antarctic pteropod, Limacina helicina antarctica . Experimental levels of p CO 2 were chosen to create both contemporary pH conditions, and to mimic future pH expected in OA scenarios. Significant changes in the transcriptome were observed when juvenile L. h. antarctica were acclimated for 21 days to low-pH (7.71), mid-pH (7.9) or high-pH (8.13) conditions. Differential gene expression analysis of individuals maintained in the low-pH treatment identified down-regulation of genes involved in cytoskeletal structure, lipid transport, and metabolism. High pH exposure led to increased expression and enrichment for genes involved in shell formation, calcium ion binding, and DNA binding. Significant differential gene expression was observed in four major cellular and physiological processes: shell formation, the cellular stress response, metabolism, and neural function. Across these functional groups, exposure to conditions that mimic ocean acidification led to rapid suppression of gene expression. Conclusions Results of this study demonstrated that the transcriptome of the juvenile pteropod, L. h. antarctica , was dynamic and changed in response to different levels of p CO 2 . In a global change context, exposure of L. h. antarctica to the low pH, high p CO 2 OA conditions resulted in a suppression of transcripts for genes involved in key physiological processes: calcification, metabolism, and the cellular stress response. The transcriptomic response at both acute and longer-term acclimation time frames indicated that contemporary L. h. antarctica may not have the physiological plasticity necessary for adaptation to OA conditions expected in future decades. Lastly, the differential gene expression results further support the role of shelled pteropods such as L. h. antarctica as sentinel organisms for the impacts of ocean acidification.

Pteropods are an abundant group of pelagic gastropods that, although temporally and spatially patchy in the Southern Ocean, can play an important role in food webs and biochemical cycles. We found that the metabolic rate inLimacina helicina antarcticais depressed (~23%) at lower mean chlorophylla(chla) concentrations in the Ross Sea. To assess the specific impact of food deprivation on these animals, we quantified aerobic respiration and ammonia (NH₃) production for 2 dominant Antarctic pteropods,L. helicina antarcticaandClione limacina antarctica. Pteropods collected from sites west of Ross Island, Antarctica were held in captivity for a period of 1 to 13 d to determine their metabolic response to laboratory-induced food deprivation.L. helicina antarcticareduced oxygen consumption by ~20% after 4 d in captivity. Ammonia excretion was not significantly affected, suggesting a greater reliance on protein as a substrate for cellular respiration during starvation. The oxygen consumption rate of the gymnosome,C. limacina antarctica, was reduced by ~35% and NH₃ excretion by ~55% after 4 d without prey. Our results indicate that there is a link between the large scale chlaconcentrations of the Ross Sea and the baseline metabolic rate of pteropods which impacts these animals across multiple seasons.

Thecosomata is a marine zooplankton group, which played an important role in the carbonate cycle in oceans due to their shell composition. So far, there is important discrepancy between the previous morphological-based taxonomies, and subsequently the evolutionary history of Thecosomata. In this study, the remarkable planktonic sampling of TARA Oceans expedition associated with a set of various other missions allowed us to assess the phylogenetic relationships of Thecosomata using morphological and molecular data (28 S and COI genes). The two gene trees showed incongruities (e.g. Hyalocylis, Cavolinia), and high congruence between morphological and 28S trees (e.g. monophyly of Euthecosomata). The monophyly of straight shell species led us to reviving the Orthoconcha, and the split of Limacinidae led us to the revival of Embolus inflata replacing Limacina inflata. The results also jeopardized the Euthecosomata families that are based on plesiomorphic character state as in the case for Creseidae which was not a monophyletic group. Divergence times were also estimated, and suggested that the evolutionary history of Thecosomata was characterized by four major diversifying events. By bringing the knowledge of palaeontology, we propose a new evolutionary scenario for which macro-evolution implying morphological innovations were rhythmed by climatic changes and associated species turn-over that spread from the Eocene to Miocene, and were shaped principally by predation and shell buoyancy.

Thecosome pteropods (pelagic mollusks) can play a key role in the food web of various marine ecosystems. They are a food source for zooplankton or higher predators such as fishes, whales and birds that is particularly important in high latitude areas. Since they harbor a highly soluble aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO(2) emissions. The effect of changes in the seawater chemistry was investigated on Limacina helicina, a key species of Arctic pelagic ecosystems. Individuals were kept in the laboratory under controlled pCO(2) levels of 280, 380, 550, 760 and 1020 microatm and at control (0 degrees C) and elevated (4 degrees C) temperatures. The respiration rate was unaffected by pCO(2) at control temperature, but significantly increased as a function of the pCO(2) level at elevated temperature. pCO(2) had no effect on the gut clearance rate at either temperature. Precipitation of CaCO(3), measured as the incorporation of (45)Ca, significantly declined as a function of pCO(2) at both temperatures. The decrease in calcium carbonate precipitation was highly correlated to the aragonite saturation state. Even though this study demonstrates that pteropods are able to precipitate calcium carbonate at low aragonite saturation state, the results support the current concern for the future of Arctic pteropods, as the production of their shell appears to be very sensitive to decreased pH. A decline of pteropod populations would likely cause dramatic changes to various pelagic ecosystems.

As a result of ocean acidification, aragonite may become undersaturated by 2050 in the upper layers of the Southern Ocean. Analyses of sea snail specimens, extracted live from the Southern Ocean in January and February 2008, show that the shells of these organisms are already dissolving. The carbonate chemistry of the surface ocean is rapidly changing with ocean acidification, a result of human activities 1 . In the upper layers of the Southern Ocean, aragonite—a metastable form of calcium carbonate with rapid dissolution kinetics—may become undersaturated by 2050 (ref.  2 ). Aragonite undersaturation is likely to affect aragonite-shelled organisms, which can dominate surface water communities in polar regions 3 . Here we present analyses of specimens of the pteropod Limacina helicina antarctica that were extracted live from the Southern Ocean early in 2008. We sampled from the top 200 m of the water column, where aragonite saturation levels were around 1, as upwelled deep water is mixed with surface water containing anthropogenic CO 2 . Comparing the shell structure with samples from aragonite-supersaturated regions elsewhere under a scanning electron microscope, we found severe levels of shell dissolution in the undersaturated region alone. According to laboratory incubations of intact samples with a range of aragonite saturation levels, eight days of incubation in aragonite saturation levels of 0.94–1.12 produces equivalent levels of dissolution. As deep-water upwelling and CO 2 absorption by surface waters is likely to increase as a result of human activities 2 , 4 , we conclude that upper ocean regions where aragonite-shelled organisms are affected by dissolution are likely to expand.