Description: Antarctic krill (Euphausia superba) from South Georgia comprise one of the most northern and abundant krill stocks. South Georgia waters are undergoing rapid warming, as a result of climate change, which in turn could alter the oxygen concentration of the water. We investigated gene expression in Antarctic krill related to aerobic metabolism, antioxidant defence, and heat-shock response under severe (2.5% O2 saturation or 0.6 kPa) and threshold (20% O2 saturation or 4 kPa) hypoxia exposure compared to in situ levels (normoxic; 100% O2 saturation or 21 kPa). Biochemical metabolic and oxidative stress indicators complemented the genic expression analysis to detect in vivo signs of stress during the hypoxia treatments. Expression levels of the genes citrate synthase (CS), mitochondrial manganese superoxide dismutase (SODMn-m) and one heat-shock protein isoform (E) were higher in euphausiids incubated 6 h at 20% O2 saturation than in animals exposed to control (normoxic) conditions. All biochemical antioxidant defence parameters remained unchanged among treatments. Levels of lipid peroxidation were raised after 6 h of severe hypoxia. Overall, short-term exposure to hypoxia altered mitochondrial metabolic and antioxidant capacity, but did not induce anaerobic metabolism. Antarctic krill are swarming organisms and may experience short periods of hypoxia when present in dense swarms. A future, warmer Southern ocean, where oxygen saturation levels are decreased, may result in smaller, less dense swarms as they act to avoid greater levels of hypoxia.

Description: The high Arctic Kongsfjorden (79°N) is influenced mainly by cold Arctic but also warmer Atlantic water masses. In recent years, the proportion of Atlantic inflow increased, attributed to climate change. Concurrently, two boreal and one subtropical krill species are now being found regularly in Kongsfjorden – in addition to the previously prevailing arcto-boreal species Thysanoessa inermis and T. raschii. Generally, krill occupy a central trophic position as secondary producers linking primary production to higher trophic levels. Many adult fish, seabirds and marine mammals rely on them as a food source. Although a change in a krill population may have a significant impact on the ecosystem, knowledge on the physiological performance of the species inhabiting Arctic waters is still scarce. We aim at determining the ecophysiology of this key group within the challenging Arctic ecosystem. Using non-invasive optical oxygen sensors, respiration measurements helped to characterize the species’ metabolic reaction to temperature variation, i.e. to reveal the thermal limits of metabolic adaptability. Thysanoessa spp. appear more stenotherm than the boreal and the subtropical krill species: the upper level of respiration is reached at temperatures 〈 12°C while mortality increases and the animals are less tolerant to ambient oxygen concentrations. The species’ thermal limit of adaptive capacity found may explain its current arcto-boreal distribution. In contrast, the other boreal and a subtropical krill species show a higher tolerance to temperature changes, which may support the species’ success in northward expansion as reported through increasing abundances at lower latitudes. Accordingly, at least one of the latter species may profit from the increasing “Atlantification” of the Kongsfjord ecosystem due to its superior physiological plasticity.

Description: Recent studies have indicated a metabolic temperature sensitivity in both the arcto-boreal krill species Thysanoessa inermis and Thysanoessa raschii that may determine these species' abundance and population persistence at lower latitudes (up to 40° N). T. inermis currently dominates the krill community in the Barents Sea and in the high Arctic Kongsfjord. We aimed to increase the knowledge on the upper thermal limit found in the latter species by estimating the CT50 value (19.7 °C) (critical temperature at which 50 % of animals are reactive) and by linking metabolic rate measurements with molecular approaches. Optical oxygen sensors were used to measure respiration rates in steps of 2 °C (from 0 to 16 °C). To follow the temperature-mediated mechanisms of passive response, i.e., as a proxy for molecular stress, molecular chaperone heat shock protein 70 (Hsp70) sequences were extracted from a transcriptome assembly, and the gene expression kinetics were monitored during an acute temperature exposure to 6 or 10 °C with subsequent recovery at 4 °C. Our results showed upregulation of hsp70 genes, especially the structurally constitutive and mitochondrial isoforms. These findings confirmed the temperature sensitivity of T. inermis and showed that the thermal stress took place before reaching the upper temperature limit estimated by respirometry at 12 °C. This study provides a baseline for further investigations into the thermal tolerances of arcto-boreal Thysanoessa spp. and comparisons with other krill species under different climatic regimes, especially Antarctica.