Welcome! Click Here for Darkness survival Hack About the Game: They experience over 120 days of darkness during the polar night surviving on storage compounds without conducting photosynthesis. Furthermore, the Arctic is experiencing continuous warming as a consequence of climate change. Such temperature increase may enhance the metabolic activity of kelps, using up storage compounds faster. As the survival strategy of kelps during darkness in the warming Arctic is poorly understood, we studied the physiological and transcriptomic responses of Saccharina latissima, one of the most common kelp species in the Arctic, after a two-week dark exposure at two temperatures (0 and 4℃) versus the same temperatures under low light conditions. Growth rates were decreased in darkness but remained stable at two temperatures. Pigments had higher values in darkness and at 4℃. Darkness had a greater impact on the transcriptomic performance of S. latissima than increased temperature according to the high numbers of differentially expressed genes between dark and light treatments. Darkness repressed the expression of genes coding for glycolysis and metabolite biosynthesis, as well as some energy-demanding processes, such as synthesis of photosynthetic components and transporters. Moreover, increased temperature enhanced these repressions, while the expression of some genes encoding components of the lipid catabolism, glyoxylate cycle and signalling were enhanced in darkness. Our study helps to understand the survival strategy of kelp in the early polar night and its potential resilience to the warming Arctic. Transcriptomic Responses to Darkness and the Survival Strategy of the Kelp Saccharina latissima in the Early Polar Night. Kelps in the Arctic region are facing challenging natural conditions. They experience over 120 days of darkness during the polar night surviving on storage compounds without conducting photosynthesis. Furthermore, the Arctic is experiencing continuous warming as a consequence of climate change. Such temperature increase may enhance the metabolic activity of kelps, using up storage compounds faster. As the survival strategy of kelps during darkness in the warming Arctic is poorly understood, we studied the physiological and transcriptomic responses of Saccharina latissima , one of the most common kelp species in the Arctic, after a 2-week dark exposure at two temperatures (0 and 4°C) versus the same temperatures under low light conditions. Growth rates were decreased in darkness but remained stable at two temperatures. Pigments had higher values in darkness and at 4°C. Darkness had a greater impact on the transcriptomic performance of S. latissima than increased temperature according to the high numbers of differentially expressed genes between dark and light treatments. Darkness generally repressed the expression of genes coding for glycolysis and metabolite biosynthesis, as well as some energy-demanding processes, such as synthesis of photosynthetic components and transporters. Moreover, increased temperature enhanced these repressions, while the expression of some genes encoding components of the lipid and laminaran catabolism, glyoxylate cycle and signaling were enhanced in darkness. Our study helps to understand the survival strategy of kelp in the early polar night and its potential resilience to the warming Arctic. Introduction. Kelps are perennial macroalgae with high economic and ecological significance. They are important primary producers and provide food and shelter for numerous organisms in the marine ecosystem (Bischof et al., 2019). Kelps show a broad geographical distribution from cold-temperate to Arctic coastlines. Compared to the kelps in temperate regions, kelps in the Arctic face a harsher physical environment due to the high latitude. The Arctic features strong, seasonal fluctuating solar radiation. In Kongsfjorden, Svalbard (78°55′N, 11°55′E), the polar night lasts for 129 days, from the end of October to mid-February (Cohen et al., 2015). Due to sea ice and snow cover, kelps in the inner part of Kongsfjorden can be exposed to extended darkness until April–July when the sea ice breaks up (Svendsen et al., 2002). To some kelps originating from cold-temperate regions (e.g., Saccharina latissima ), the low water temperature in the Arctic represents a suboptimal growth environment (Fortes and Lüning, 1980, Hurd et al., 2014). However, such environments are changing due to climate change. The Arctic has warmed at more than twice as fast as the global mean air temperature increase since the mid-1990s (Overland et al., 2019). In Kongsfjorden, the surface water temperature has increased to around 2°C in winter and early spring from 2007 (Hegseth et al., 2019). It might continue to increase since the winter air temperature in the Arctic is predicted to increase by over 7°C at the end of the 21 st century (IPCC, 2007, MacDonald, 2010). Meanwhile, the warming climate largely prevents the formation of sea ice in the inner part of Kongsfjorden since 2006 (Pavlova et al., 2019) and consequently shortens the period of darkness. Hence, it is urgent to understand the survival mechanisms of kelps during the polar night. Some species of seaweeds in the Arctic may potentially benefit from the increase of water temperature if their optimum temperature for growth is above 5°C (Gómez et al., 2009). However, the metabolic activity of organisms increases with temperature rise, potentially leading to shorter survival times of organisms during the polar night. For example, Schaub et al. (2017) demonstrated that the Arctic benthic diatom Navicula perminuta utilized lipid resources significantly faster at 7°C than 0°C during an 8-week dark exposure period. The magnitude of temperature increase also affects the survival period of organisms. The Antarctic sea-ice diatom Fragilariopsis cylindrus showed only a 7-day maximum dark survival time at 10°C, in contrast with a maximum survival time of 60 days at −2 and 4°C (Reeves et al., 2011). Historically, studies on algae exposed to prolonged dark periods were mainly focused on the growth patterns (Lüning, 1969, Smayda and Mitchell-Innes, 1974, Chapman and Lindley, 1980, Dunton, 1985), physiological performances and biochemical mechanisms (Weykam et al., 1997, Lüder et al., 2002, Karsten et al., 2019). Mcminn and Martin (2013) concluded several strategies for algae to survive long periods of darkness. One of them is changing physiological activities, including the reduction of metabolic rate (Palmisano and Sullivan, 1982, Jochem, 1999) and utilization of energy storage products (e.g., laminarans and lipids) to maintain basic metabolism during prolonged darkness (Manoharan et al., 1999, Schaub et al., 2017, Scheschonk et al., 2019). Darkness Survival hack
