No abstract

Marine macrophytes, including seagrasses and macroalgae, form the basis of diverse and productive coastal ecosystems that deliver important ecosystem services. Moreover, western countries increasingly recognize macroalgae, traditionally cultivated in Asia, as targets for a new bio-economy that can be both economically profitable and environmentally sustainable. However, seagrass meadows and macroalgal forests are threatened by a variety of anthropogenic stressors. Most notably, rising temperatures and marine heatwaves are already devastating these ecosystems around the globe, and are likely to compromise profitability and production security of macroalgal farming in the near future. Recent studies show that seagrass and macroalgae can become less susceptible to heat events once they have been primed with heat stress. Priming is a common technique in crop agriculture in which plants acquire a stress memory that enhances performance under a second stress exposure. Molecular mechanisms underlying thermal priming are likely to include epigenetic mechanisms that switch state and permanently trigger stress-preventive genes after the first stress exposure. Priming may have considerable potential for both ecosystem restoration and macroalgae farming to immediately improve performance and stress resistance and, thus, to enhance restoration success and production security under environmental challenges. However, priming methodology cannot be simply transferred from terrestrial crops to marine macrophytes. We present first insights into the formation of stress memories in both seagrasses and macroalgae, and research gaps that need to be filled before priming can be established as new bio-engineering technique in these ecologically and economically important marine primary producers.

Most kelp species are of high ecological and economic importance worldwide, but are highly susceptible to rising ocean temperatures due to their sessile lifestyle. Due to interference with reproduction, development and growth, natural kelp forests have vanished in multiple regions after extreme summer heat waves. Furthermore,

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.

Chloroplast DNA is methylated in the kelp Saccharina japonica, in contrast to most plants. Its function is yet largely unexplored. We detected methylation in the chloroplast DNA of the congener Saccharina latissima, a non – model macroalgal species of high ecological (wild populations) and economical (wild and cultured populations) importance in the North Atlantic. To the functional relevance of chloroplast DNA methylation, we compared for the first time methylation patterns between wild and cultured kelp from different climatic regions (High-Arctic (79 °N) and temperate (53 °N), laboratory samples at 5 °C, 10 °C and 15 °C). Our results suggest genome –wide differences in methylated sites, and methylation level, between the climatic regions. At gene level, our data found functions related to photosynthesis to be the predominant affected case only for differential methylation between origins, but not between growth conditions. Here, sample origin led to significant differences between cultivated and wild samples due to differential methylation of genes related to DNA replication in the Spitsbergen samples. Both findings indicate that origin and cultivation strongly affected the chloroplast methylome, but differently. Similar methylomes for samples from the same origin — independent from whether they grow in the wild or in the lab — suggest that origin– specific methylation marks on the chloroplast genome are inherited. However, the capacity for rapid adaptation (to cultivation conditions) could be shown for Saccharina latissima during this study. Given that DNA – methylation affects gene expression, our study suggests that lab – cultivation alters epigenetically determined kelp chloroplast characteristics at least to the same degree as ecotypic differentiation does.