ALBERT

Description: Marine ecosystems of the Southern Ocean are particularly vulnerable to ocean acidification. Antarctic krill (Euphausia superba; hereafter krill) is the key pelagic species of the region and its largest fishery resource. There is therefore concern about the combined effects of climate change, ocean acidification and an expanding fishery on krill and ultimately, their dependent predators-whales, seals and penguins. However, little is known about the sensitivity of krill to ocean acidification. Juvenile and adult krill are already exposed to variable seawater carbonate chemistry because they occupy a range of habitats and migrate both vertically and horizontally on a daily and seasonal basis. Moreover, krill eggs sink from the surface to hatch at 700-1,000 m, where the carbon dioxide partial pressure (pCO2) in sea water is already greater than it is in the atmosphere. Krill eggs sink passively and so cannot avoid these conditions. Here we describe the sensitivity of krill egg hatch rates to increased CO2, and present a circumpolar risk map of krill hatching success under projected pCO2 levels. We find that important krill habitats of the Weddell Sea and the Haakon VII Sea to the east are likely to become high-risk areas for krill recruitment within a century. Furthermore, unless CO2 emissions are mitigated, the Southern Ocean krill population could collapse by 2300 with dire consequences for the entire ecosystem.

Description: Antarctic krill, Euphausia superba, shapes the structure of the Southern Ocean ecosystem. Its central position in the food web, the ongoing environmental changes due to climatic warming, and increasing commercial interest on this species emphasize the urgency of understanding the adaptability of krill to its environment. Krill has evolved rhythmic physiological and behavioral functions which are synchronized with the daily and seasonal cycles of the complex Southern Ocean ecosystem. The mechanisms, however, leading to these rhythms are essentially unknown. Here, we show that krill possesses an endogenous circadian clock that governs metabolic and physiological output rhythms. We found that expression of the canonical clock gene cry2 was highly rhythmic both in a light-dark cycle and in constant darkness. We detected a remarkable short circadian period, which we interpret as a special feature of the krill's circadian clock that helps to entrain the circadian system to the extreme range of photoperiods krill is exposed to throughout the year. Furthermore, we found that important key metabolic enzymes of krill showed bimodal circadian oscillations (~9–12 h period) in transcript abundance and enzymatic activity. Oxygen consumption of krill showed ~9–12 h oscillations that correlated with the temporal activity profile of key enzymes of aerobic energy metabolism. Our results demonstrate the first report of an endogenous circadian timing system in Antarctic krill and its likely link to metabolic key processes. Krill's circadian clock may not only be critical for synchronization to the solar day but also for the control of seasonal events. This study provides a powerful basis for the investigation into the mechanisms of temporal synchronization in this marine key species and will also lead to the first comprehensive analyses of the circadian clock of a polar marine organism through the entire photoperiodic cycle.

Description: Antarctic krill (Euphausia superba) hold a central position in the Southern Ocean food web, yet little is known about how they might respond to anthropogenic climate change, in particular the projected rise in temperature in their habitat. Krill‘s life cycle and metabolism are successfully adapted and timed closely to their highly seasonal environment. An elevation in sea water temperature has the potential to disrupt this delicate interplay, desynchronizing krill physiology with essential cornerstones in the course of the year. The aim of this study was to elucidate the direct effects of rising sea water temperatures on Antarctic krill metabolism. To this end, krill were exposed to gradually increasing temperatures from 0.5°C to 7°C over a period of four months. Over the course of the experiment, respiration and morphometric parameters including growth and maturation were regularly monitored. These observations supplement the analysis of key enzyme activities in a range of metabolic pathways including glycolysis (pyruvate kinase), beta-oxidation (3-hydroxyacyl-CoA-dehydrogenase), Krebs cycle (malate dehydrogenase, citrate synthase) and cellular respiration (cytochrome C oxidase). In combination with the analysis of elemental composition these data add to our understanding of the response mechanisms of krill to a changing environment. The results are discussed in view of possible implications in the context of climate change, such as ecological mis-matches with Antarctic seasonality.

Description: Antarctic krill (Euphausia superba) hold a central position in the Southern Ocean food web, yet little is known about how they might respond to anthropogenic climate change, in particular the projected rise in temperature in their habitat. Krill‘s life cycle and metabolism are successfully adapted and timed closely to their highly seasonal environment. An elevation in sea water temperature has the potential to disrupt this delicate interplay, desynchronizing krill physiology with essential cornerstones in the course of the year. The aim of this study was to elucidate the direct effects of rising sea water temperatures on Antarctic krill metabolism. To this end, krill were exposed to gradually increasing temperatures from 0.5°C to 7°C over a period of four months. Over the course of the experiment, respiration and morphometric parameters including growth and maturation were regularly monitored. These observations supplement the analysis of key enzyme activities in a range of metabolic pathways including glycolysis (pyruvate kinase), beta-oxidation (3-hydroxyacyl-CoA-dehydrogenase), Krebs cycle (malate dehydrogenase, citrate synthase) and cellular respiration (cytochrome C oxidase). In combination with the analysis of elemental composition these data add to our understanding of the response mechanisms of krill to a changing environment. The results are discussed in view of possible implications in the context of climate change, such as ecological mis-matches with Antarctic seasonality.

Description: We investigate how individual growth and population structure of Antarctic krill (Euphausia superba) would be affected by changes in the spatio-temporal dynamics of the sea ice cover. This is of high interest since krill has adapted to a particular environmental regime which is likely to change dramatically over the coming years. The response of krill will in particular depend on its chronobiology: when and why does krill, after a period of decreased metabolic activity during winter, switch back to an active metabolic state? If this switch is purely triggered by the Zeitgeber day length, the metabolically active period of krill and the availability of food resources would become out of phase with potentially drastic consequences for krill populations. Alternatively, the switch might also be triggered by food availability. To explore the consequences of different environmental scenarios and assumptions about krill chronobiology, we developed a spatially explicit individual-based simulation model. The model operates on a daily time step. Each time step ice cover extent and day length for each grid cell in the model are updated. In the model demographic and behavioural processes are simulated every time step. Particularly all modelled krill individuals grow depending on food availability, move, reproduce given their reproductive and metabolic state, and die with a certain probability. Growth and reproduction are modelled according to a simplified version of dynamic energy budget theory (DEBKiss). Simulations run for several years until quasi-stationary population characteristics have emerged. Population metrics such as length distribution and heterogeneity in reproductive state within the population are observed. We will present the model and demonstrate its potential by contrasting results for selected environmental and chronobiological scenarios. The model’s design and implementation are open so that suggestions regarding alternative assumptions and scenarios can easily be implemented and explored.

Description: During evolution, a wide range of organisms from cyanobacteria to humans have adapted to the day-night cycle, caused by the earth’s rotational movements, by developing an endogenous timing system – a circadian clock – that allows synchronization of metabolism, physiology and behaviour with the environment and that also may modulate seasonal responses. Our current molecular understanding of biological rhythms and clocks is largely restricted to circadian and seasonal rhythms in land model species such as the fruit fly, the mouse or the thale cress. In marine organisms in general, little is known about the principles of endogenous clocks and how these clocks interact with environmental cycles. Marine ecosystems are currently experiencing rapid anthropogenic climatic changes. In particular, polar and sub-polar latitudes comprise the fastest warming regions on the planet with profound impacts on the marine environment. I will outline the importance of this research field in polar regions on the Southern Ocean key species Antarctic krill, Euphausia superba. Our seasonal investigations on krill, both field based and in the laboratory, revealed that important physiological functions such as metabolic activity, feeding and growth, as well as maturity are affected by different light-dark cycles, irrespective of food supply, suggesting that the photoperiod acts as the main Zeitgeber for these annual cycles. In recent investigations we identified an endogenous timing system, which is synchronized by the seasonal cycle of photoperiod, and its link to metabolic key processes. We will further outline the implication of these findings for krill stocks in a changing environment and will present a recently started 5 years project on clocks and rhythms in polar pelagic organisms.