Abstract
Antarctic krill (Euphausia superba), a key species in the Southern Ocean, reduce their metabolism as an energy saving mechanism in response to the harsh environmental conditions during the Antarctic winter. Although the adaptive significance of this seasonal metabolic shift seems obvious, the driving factors are still unclear. In particular, it is debated whether the seasonal metabolic cycle is driven by changes in food availability, or if an endogenous timing system entrained by photoperiod might be involved. In this study, we used different long-term photoperiodic simulations to examine the influence of light regime and endogenous rhythmicity on the regulation of krill seasonal metabolic cycle. Krill showed a seasonal cycle of growth characterized by null-to-negative growth rates during autumn-winter and positive growth rates during spring-summer, which was manifested also in constant darkness, indicating strong endogenous regulation. Similar endogenous cycles were observed for the activity of the key-metabolic enzyme malate dehydrogenase (MDH) and for the expression levels of a selection of metabolic-related genes, with higher values in spring-summer and lower values in autumn-winter. On the other side, a seasonal cycle of oxygen consumption was observed only when krill were exposed to simulated seasonal changes in photoperiod, indicating that light-related cues might play a major role in the regulation of krill oxygen consumption. The influence of light-regime on oxygen consumption was minimal during winter, when light-phase duration was below 8 h, and it was maximal during summer, when light-phase duration was above 16 h. Significant upregulation of the krill clock genes clk, cry2, and tim1, as well as of the circadian-related opsins rh1a and rrh, was observed after light-phase duration had started to decrease in early autumn, suggesting the presence of a signaling cascade linking specific seasonal changes in the Antarctic light regime with clock gene activity and the regulation of krill metabolic dormancy over the winter.
Highlights
Antarctic krill (Euphausia superba, hereafter krill), a shrimp-like crustacean species, plays a central role in the Southern Ocean ecosystem, being both a major grazer of marine phytoplankton and a critical food item for whales, seals, birds, and fish, thereby linking primary production to higher trophic levels
Its circumpolar distribution shows a latitudinal range from 51◦S to 70◦S with more than 50% of Southern Ocean krill stocks located in the southwest Atlantic sector and the region of the West Antarctic Peninsula (Atkinson et al, 2004)
To examine the effect of the seasonal photoperiodic cycle on krill metabolic gene expression, we investigated seasonal changes in the expression levels of six genes involved in carbohydrate metabolism, lipid synthesis, amino acid metabolism, protein synthesis, and energy metabolism (Supplementary Table S1)
Summary
Antarctic krill (Euphausia superba, hereafter krill), a shrimp-like crustacean species, plays a central role in the Southern Ocean ecosystem, being both a major grazer of marine phytoplankton and a critical food item for whales, seals, birds, and fish, thereby linking primary production to higher trophic levels. Its circumpolar distribution shows a latitudinal range from 51◦S to 70◦S with more than 50% of Southern Ocean krill stocks located in the southwest Atlantic sector and the region of the West Antarctic Peninsula (Atkinson et al, 2004). These areas are currently experiencing some of the most rapid anthropogenicdriven warming on Earth, resulting in a 1◦C increase of the surface summer temperature of the adjacent ocean since 1950 (Meredith and King, 2005). The central position of krill in the food web, the ongoing environmental changes in its habitat, and increasing commercial interest emphasize the urgency to understand the adaptability of krill to its environment
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