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This study aimed to examine interactive effects between ocean acidification and temperature on the photosynthetic and growth performance of Neosiphonia harveyi. N. harveyi was cultivated at 10 and 17.5 °C at present (~380 µatm), expected future (~800 µatm), and high (~1500 µatm) pCO2. Chlorophyll a fluorescence, net photosynthesis, and growth were measured. The state of the carbon-concentrating mechanism (CCM) was examined by pH-drift experiments (with algae cultivated at 10 °C only) using ethoxyzolamide, an inhibitor of external and internal carbonic anhydrases (exCA and intCA, respectively). Furthermore, the inhibitory effect of acetazolamide (an inhibitor of exCA) and Tris (an inhibitor of the acidification of the diffusive boundary layer) on net photosynthesis was measured at both temperatures. Temperature affected photosynthesis (in terms of photosynthetic efficiency, light saturation point, and net photosynthesis) and growth at present pCO2, but these effects decreased with increasing pCO2. The relevance of the CCM decreased at 10 °C. A pCO2 effect on the CCM could only be shown if intCA and exCA were inhibited. The experiments demonstrate for the first time interactions between ocean acidification and temperature on the performance of a non-calcifying macroalga and show that the effects of low temperature on photosynthesis can be alleviated by increasing pCO2. The findings indicate that the carbon acquisition mediated by exCA and acidification of the diffusive boundary layer decrease at low temperatures but are not affected by the cultivation level of pCO2, whereas the activity of intCA is affected by pCO2. Ecologically, the findings suggest that ocean acidification might affect the biogeographical distribution of N. harveyi.

Global warming and enhanced UV-radiation due to stratospheric ozone depletion could drastically affect Arctic coastal ecosystems. Previous research revealed that the UV-susceptibility and impact of increased UV-radiation on Arctic kelp zoospores are highly variable, potentially due to seasonal acclimation. Accordingly, for a better understanding of climate change effects on Arctic kelp, we need to determine the fertility period of Arctic kelps and to systematically examine the seasonal differences of increased UV-radiation during the fertility period. We examined the fertility period of Laminaria digitata and Alaria hyperborea, by evaluating sorus maturation, zoospore release and germination. Zoospore germination was studied under photosynthetic active radiation (PAR 400-700 nm) and PAR and increased UVAB-radiation (280-700 nm) from July to September at 2 and 7 degree C. Furthermore, we tested whether differences in the zoosporic UV-susceptibility were related to the content of UV-screening phlorotannins. The fertility period of A. esculenta is uniform from July to mid-August and ends in September. Within the fertility period, the UV-susceptibility of A. esculenta zoospores was highest at 2 degree C and the beginning of July, whereas it was not affected by seasonality at 7 degree C. The fertility period of L. digitata starts in late July and lasts at least until September, and no seasonal differences in the UV-susceptibility were found. However, UV-susceptibility was significantly lower at 7 degree C. In both species, the zoosporic phlorotannin content did not affect the UV-susceptibility. We conclude that seasonality strongly influences the UV-susceptibility of A. esculenta but at low water temperatures only. Higher seawater temperatures help both species to cope with increasing UV-radiation.

Recent research indicates that synchronicity of sexual reproduction in coral spawning events is breaking down, leading to aging populations and decreased recruitment success. In this perspective, we develop a hypothesis that this phenomenon could be caused by ongoing ocean acidification (OA). We hypothesize, that the underlying physiological machinery could be the carbon concentrating mechanism (CCM). The endosymbiotic zooxanthellae of corals could use this mechanism to sense calm water motion states in a comparable way to that known from macroalgae. In macroalgae, it is well-established that dissolved inorganic carbon (DIC) acts as the trigger for signaling low water motion. Hence, evolutionarily developed signals of low water motion, suited for gamete-release, may be misleading in the future, potentially favoring opportunistic species in a broad range of marine organisms.

The previous research demonstrated that warming and ocean acidification (OA) affect the biochemical composition of Arctic (Spitsbergen; SP) and cold-temperate (Helgoland; HL) Saccharina latissima differently, suggesting ecotypic differentiation. This study analyses the responses to different partial pressures of CO₂ (380, 800, and 1500 latm pCO₂) and temperature levels (SP population: 4, 10 °C; HL population: 10, 17 °C) on the photophysiology (O₂ production, pigment composition, D1-protein content) and carbon assimilation [Rubisco content, carbon concentrating mechanisms (CCMs), growth rate] of both ecotypes. Elevated temperatures stimulated O₂ production in both populations, and also led to an increase in pigment content and a deactivation of CCMs, as indicated by ¹³C isotopic discrimination of algal biomass (ep) in the HL population, which was not observed in SP thalli. In general, pCO₂ effects were less pronounced than temperature effects. High pCO₂ deactivated CCMs in both populations and produced a decrease in the Rubisco content of HL thalli, while it was unaltered in SP population. As a result, the growth rate of the Arctic ecotype increased at elevated pCO₂ and higher temperatures and it remained unchanged in the HL population. Ecotypic differentiation was revealed by a significantly higher O₂ production rate and an increase in Chl a, Rubisco, and D1 protein content in SP thalli, but a lower growth rate, in comparison to the HL population. We conclude that both populations differ in their sensitivity to changing temperatures and OA and that the Arctic population is more likely to benefit from the upcoming environmental scenario than its Atlantic counterpart.

Previous research suggested that the polar and temperate populations of the kelp Saccharina latissima represent different ecotypes. The ecotypic differentiation might also be reflected in their biochemical composition (BC) under changing temperatures and pCO₂. Accordingly, it was tested if the BC of Arctic (Spitsbergen) and temperate S. latissima (Helgoland) is different and if they are differently affected by changes in temperature and pCO₂. Thalli from Helgoland grown at 17 °C and 10 °C and from Spitsbergen at 10 °C and 4 °C were all tested at either 380, 800, or 1,500 µatm pCO₂, and total C-, total N-, protein, soluble carbohydrate, and lipid content, as well as C/N-ratio were measured. At 10 °C, the Arctic population had a higher content of total C, soluble carbohydrates, and lipids, whereas the N- and protein content was lower. At the lower tested temperature, the Arctic ecotype had particularly higher contents of lipids, while content of soluble carbohydrates increased in the Helgoland population only. In Helgoland-thalli, elevated pCO₂ caused a higher content of soluble carbohydrates at 17 °C but lowered the content of N and lipids and increased the C/N-ratio at 10 °C. Elevated pCO₂ alone did not affect the BC of the Spitsbergen population. Conclusively, the Arctic ecotype was more resilient to increased pCO₂ than the temperate one, and both ecotypes differed in their response pattern to temperature. This differential pattern is discussed in the context of the adaptation of the Arctic ecotype to low temperature and the polar night.

The Arctic population of the kelp Saccharina latissima differs from the Helgoland population in its sensitivity to changing temperature and CO sub( 2 ) levels. The Arctic population does more likely benefit from the upcoming environmental scenario than its Atlantic counterpart. The previous research demonstrated that warming and ocean acidification (OA) affect the biochemical composition of Arctic (Spitsbergen; SP) and cold-temperate (Helgoland; HL) Saccharina latissima differently, suggesting ecotypic differentiation. This study analyses the responses to different partial pressures of CO sub(2) (380, 800, and 1500 mu atm pCO sub(2)) and temperature levels (SP population: 4, 10 degree C; HL population: 10, 17 degree C) on the photophysiology (O sub(2) production, pigment composition, D1-protein content) and carbon assimilation [Rubisco content, carbon concentrating mechanisms (CCMs), growth rate] of both ecotypes. Elevated temperatures stimulated O sub(2) production in both populations, and also led to an increase in pigment content and a deactivation of CCMs, as indicated by super(13)C isotopic discrimination of algal biomass ( epsilon sub(p)) in the HL population, which was not observed in SP thalli. In general, pCO sub(2) effects were less pronounced than temperature effects. High pCO sub(2) deactivated CCMs in both populations and produced a decrease in the Rubisco content of HL thalli, while it was unaltered in SP population. As a result, the growth rate of the Arctic ecotype increased at elevated pCO sub(2) and higher temperatures and it remained unchanged in the HL population. Ecotypic differentiation was revealed by a significantly higher O sub(2) production rate and an increase in Chl a, Rubisco, and D1 protein content in SP thalli, but a lower growth rate, in comparison to the HL population. We conclude that both populations differ in their sensitivity to changing temperatures and OA and that the Arctic population is more likely to benefit from the upcoming environmental scenario than its Atlantic counterpart.