ALBERT

Description: To understand the thermal plasticity of a coastal foundation species across its latitudinal distribution, we assess physiological responses to high temperature stress in the kelp Laminaria digitata in combination with population genetic characteristics and relate heat resilience to genetic features and phylogeography. We hypothesize that populations from Arctic and cold-temperate locations are less heat resilient than populations from warm distributional edges. Using meristems of natural L. digitata populations from six locations ranging between Kongsfjorden, Spitsbergen (79°N), and Quiberon, France (47°N), we performed a common-garden heat stress experi- ment applying 15°C to 23°C over eight days. We assessed growth, photosynthetic quantum yield, carbon and nitrogen storage, and xanthophyll pigment contents as response traits. Population connectivity and genetic diversity were analyzed with microsatellite markers. Results from the heat stress experiment suggest that the upper temperature limit of L. digitata is nearly identical across its distribution range, but subtle differences in growth and stress responses were revealed for three popu- lations from the species’ ecological range margins. Two populations at the species’ warm distribution limit showed higher temperature tolerance compared to other populations in growth at 19°C and recovery from 21°C (Quiberon, France), and pho- tosynthetic quantum yield and xanthophyll pigment responses at 23°C (Helgoland, Germany). In L. digitata from the northernmost population (Spitsbergen, Norway), quantum yield indicated the highest heat sensitivity. Microsatellite genotyping revealed all sampled populations to be genetically distinct, with a strong hierarchical structure between southern and northern clades. Genetic diversity was lowest in the isolated population of the North Sea island of Helgoland and highest in Roscoff in the English Channel. All together, these results support the hypothesis of moderate local differentiation across L. digitata's European distribution, whereas effects are likely too weak to ameliorate the species’ capacity to withstand ocean warming and marine heatwaves at the southern range edge.

Description: Along temperate to polar rocky shorelines, large brown algae known as kelps form marine forests which provide a three-dimensional habitat for many associated species. Ocean warming is posing an increasing threat to kelps at their warm distributional edges and first range shifts have been recorded. For these sessile species unable to migrate, trait variability due to phenotypic plasticity and genetic variation is an important mechanism of response to environmental change. The aim of this thesis was to produce a comprehensive assessment of the variation and plasticity of thermal traits across populations and life cycle stages of a keystone marine forest species, the cold-temperate to Arctic kelp Laminaria digitata. Using physiological response parameters, population genetics and transcriptomics, I present evidence for four levels of thermal trait variability in a marine forest key species. I describe (1) genetic and physiological differentiation along the species' distribution range, (2) genetic variation for phenotypic plasticity among genotypes, (3) carry-over effects over reproduction and individual ontogeny, and (4) the production of new phenotypes by outbreeding among distant lineages. Integrating these responses into a framework of seasonal temperature variation and predictions of ocean warming showed that L. digitata, as a species with a cold-temperate thermal profile, is adapted well to the current conditions along its distributional range, but may not be equipped to respond to rapid climate change at its warm range edges. The concepts investigated in this thesis provide further insight into trait variability as a mechanism of marine forest resilience and offer intriguing features for mariculture and conservation efforts.

Description: Transgenerational effects (parent environment effects on offspring traits) have recently gained attention as a means of fast response to changing environmental conditions under climate change. In temperate and polar rocky coastal ecosystems, kelps form the base of complexly structured and highly diverse species associations. This study investigates the potential for temperature-related within-generation and transgenerational plasticity in a geographically isolated population of the brown alga Laminaria digitata from the island of Helgoland (North Sea). We obtained separate strains (genetic lines) from fertile field material and assessed sporophyte growth and gametogenesis at 5 and 15 °C. Rearing the resulting sporophytes in a full-factorial design at these two temperatures resulted in four temperature history treatments. A concluding experiment on growth, photosynthetic characteristics (Fv/Fm, rapid P-I curves, pigments) and storage compounds of five parental strains should allow for the separation of phenotypic plasticity from genetic effects in the response to 5 and 15 °C. Initial results indicate seasonality in the temperature response, with higher plasticity in material collected in spring than in summer. Gametogenesis was faster at 15 than at 5 °C, but recruitment was higher at 5 °C. A difference in morphology between sporophytes raised at 5 and 15 °C indicates that the temperature during growth had a stronger impact on morphology than the temperature during gametogenesis. Here we will show preliminary results of whether performance is related to genetic background or phenotypic plasticity among strains.

Description: Die vorliegende Studie untersucht den Einfluss verschiedener Temperaturen auf das Wachstum und auf physiologische Parameter des BlasentangsFucus vesiculosusaus dem westlichen Baltikum (Kieler Förde). Auf 5 cm Länge zugeschnittene Proben wurden über 21 Tage in einem Temperaturgradienten von 5 bis 29 °C inkubiert, während alle anderen abiotischen Faktoren konstant gehalten wurden. In regelmäßigen Abständen wurde das Wachstum anhand der Parameter Länge, Fläche und Frischgewicht gemessen, zusätzlich wurden Photosynthesemessungen durchgeführt.In einer anschließenden Nachkultur über 10 Tage bei 16 °C wurde die Fähigkeit zur Erholung der Fucus-Stücke geprüft.Abschließende Analysen lieferten Erkenntnisse zu Chlorophyll-, Stickstoff-und Kohlenstoffgehalt der Proben vor und nach Ablauf des Experiments. Ziel dieser Arbeit war es, eine genaue Temperaturcharakteristik für Fucus vesiculosusausdem westlichen Baltikumzu ermitteln, mit besonderem Fokus auf der Reaktion bezüglich erhöhter Temperaturen und der oberen Überlebensgrenze. Anhand der gesammelten Daten zeigt sich ein deutlicher Einfluss der Temperatur auf das Wachstum von Fucus vesiculosus. Während das schnellste Wachstum bei 15 bis 20 °C stattfindet, ist die Alge bei 26 bis 27 °C stark gestresstbei geringen Wachstumsraten. 28 °C führen nach 2 Wochenzum Absterben von mehr als 50 % der Proben, 29 °C liefern diesen Effekt bereits nach 7 Tagen. 15 und 20 °C sorgen ebenfalls für eine konstante effektive Quantenausbeute über die Zeit, während sichbei den anderen Temperaturen negative Effekte zeigen. Das Optimum der relativen Elektronentransportrate in der Photosynthese liegt etwas über dem Wachstumsoptimum bei 20bis 26 °C, während das nichtphotochemische Quenching keine Temperatureffekte zeigt. Die Temperatur hat ebenfalls keinen Einfluss auf den Chlorophyllgehalt der Algen. Das Kohlenstoff-Stickstoff-Verhältnis liefert maximale Werte über einen weiten Temperaturbereich von 10 bis 27 °C.

Description: Seaweeds provide important ecosystem services in coastal areas, and loss of these macrophytes due to anthropogenic global change and warming is a worldwide concern. Fucus vesiculosus L. (Phaeophyceae) is the most abundant and hence ecologically most important primary producer, carbon sink and habitat provider in the western Baltic Sea. Therefore, we used this keystone species to test phenotypic acclimation of physiological performance traits (growth, photosynthesis andmetabolites) of F. vesiculosus apices in a well-defined and highly resolved temperature gradient (5–29 °C), supported by highly temporally resolved measurements. Temperature requirements of growth and photosynthesis were evaluated in three weeks exposure experiments, and changing tolerance ranges for survival over timewere determined. Fucus vesiculosus was able to growand survive over a temperature range from 5 to 26 °C without any injury or visible damage of the apical growing meristem over all three weeks. However, at higherwater temperatures (≥27 °C) growth rapidly decreased fromday three onwards and progressive necrosis was observed at 28 and 29 °C. Stress-induced decrease in growth rate was already indicated by the effective quantum yield of chlorophyll fluorescence of photosystem II (PSII) several days in advance. Optimal temperature for photosynthesis (24 °C),measured as electron transport rate, was higher compared to that for growth (15–20 °C). Accordingly, the concentration ofmannitol, themain product of photosynthesis, increased with higher temperatures. Understanding physiological responses of keystone macroalgae with respect to temperature and time is important, because rising global temperatures and summer heatwave frequencies and duration may affect the ecological functions of F. vesiculosus in the western Baltic Sea.

Description: Transgenerational effects (effects of parental environment on offspring traits) have recently gained attention as a means of fast response to changing environmental conditions, e.g. under climate change. In temperate and polar rocky coastal ecosystems, kelps form the base of complexly structured and highly diverse species associations. This study investigates the potential for temperature-mediated transgenerational plasticity along the development from haploid parents (gametophytes) to their juvenile diploid offspring (sporophytes) in a geographically isolated population of the brown alga Laminaria digitata from the island of Helgoland (North Sea). We sampled spores from wild donor sporophytes, which we raised at 5 and 15 °C during gametogenesis, and reared the resulting young sporophytes for three months in a full-factorial split design while keeping genetic lineages separate. A concluding 12-day experiment on growth, biochemistry (carbon, mannitol content) and photosynthetic characteristics (maximum quantum yield Fv/Fm, maximum electron transport rate rETRmax) of five genetic lineages allowed for the separation of late temperature (12 day) responses, within-generation plasticity (early temperature) and transgenerational plasticity (gametogenesis temperature) in response to 5 and 15 °C. We observed significant two- and three-way interactions between gametogenesis temperature and early and late sporophyte temperatures for all parameters. While interactive effects between early and late experimental temperatures probably represent acclimation processes, interactions involving gametogenesis temperature indicate transgenerational effects. The direction of these effects differed between parameters. A main effect is that only with a history of 5 °C as gametogenesis and early temperature, sporophytes are growing faster at 5 than 15 °C over 12 days late temperature. According to our results, the temperature experienced by parents during gametogenesis influences temperature reaction norms in three- to four-month-old Laminaria digitata sporophytes. This, to our knowledge, is the first evidence for transgenerational plasticity in kelps.

Description: Phenotypic plasticity (genotype × environment interaction) is an especially important means for sessile organisms to cope with environmental variation. While kelps, the globally most productive group of seaweeds, generally possess a wide thermal performance range, kelp populations at their warm distribution limits are threatened by ocean warming. Here, we investigated effects of temperature during ontogeny of the kelp Laminaria digitata across haploid gametophyte and diploid sporophyte life cycle stages in five distinct genetic lines. We hypothesized that thermal plasticity increases trait performance of juvenile sporophytes in experimental temperatures that match the temperature experienced during gametogenesis and recruitment, and that plasticity differs among genetic lines (genetic variation for plasticity). We applied a full-factorial experimental design to generate different temperature histories by applying 5 and 15°C during meiospore germination, gametogenesis of parental gametophytes and recruitment of offspring sporophytes (19–26 days), and juvenile sporophyte rearing (91–122 days). We then tested for thermal plasticity among temperature history treatments at 5 and 15°C in a final 12-day experiment assessing growth, the storage compound mannitol, carbon and nitrogen contents, and fluorometric responses in 3–4 month old sporophytes for five genetic lines. Our study provides evidence for the importance of cold temperatures at early development on later sporophyte performance of L. digitata. Gametogenesis and recruitment at 5°C promoted higher growth of offspring sporophytes across experimental temperatures. While photosynthetic capacity was higher at 15°C, carbon and nitrogen storage were higher at 5°C, both showing fast acclimation responses. We identified an important role of genetic variation for plasticity in shaping L. digitata’s thermal plasticity. Trait performance at 5 or 15°C (reaction norm slopes) differed among genetic lines, even showing opposite response patterns. Interestingly, genetic variation for plasticity was only significant when sporophytes were reared at 5°C. Thus, we provide evidence that the cold-temperate to Arctic kelp species, L. digitata, which possesses a wide temperature tolerance between 0 and 23°C, is impaired by warm temperature during gametogenesis and recruitment, reducing growth of juvenile sporophytes and expression of variable thermal plasticity in the wild.