Raquel Carmona

As a consequence of warming, many high Arctic fjords are now free from ice cover all year round changing the environmental conditions of their ecosystems. Seasonal physiology of the macroalgal community in Kongsfjorden is expected to be affected by this change. We characterized the photosynthetic performance and biochemical composition of five common macroalgal species of Kongsfjorden, at different seasons (spring, summer, late summer and autumn). The studied species were the ochrophytes Alaria esculenta and Saccharina latissima, the rhodophytes Phycodrys rubens and Ptilota gunneri, and the chlorophyte Monostroma arcticum. The two kelps species had an early start with highest maximum photosynthesis rates (Pmax) and photosynthetic efficiencies (a) in spring, while the two rodophytes and the chlorophyte showed their highest values later in the year, when recovering from excess light. The decrease in PAM-related parameters (maximum electron transport rate, ETR, and optimal irradiance, Eopt) of kelps in summer points to a high sensitivity of their photosynthetic apparatus to continuous light. The loss of photosynthetic performance in summer could be attributed to decreased pigment concentration and lack of recovery (dark) periods. In general, 14C fixation at saturating light was higher in September, except for A. esculenta that was in March. In late summer, the two kelps and the chlorophyte species accumulated more soluble carbohydrates, while the rhodophytes did in early autumn. In general, the higher N and C contents in spring and autumn, respectively, reveal a seasonal pattern in elemental composition, related to nutrient and light availability during the year. The new scenario of absence of ice in this high Arctic fjord has resulted in an extended period of macroalgal productivity. The remaining limitations to productivity are the excess of continuous solar irradiance during the polar day, the nitrogen depletion in summer and the long polar night.

The high variability of natural environments poses significant challenges to photosynthetic organisms, which must adapt to constant fluctuations. Mechanisms such as acclimation and adaptation are essential for mitigating stress and ensuring survival. Rugulopteryx okamurae , an invasive alien seaweed recently introduced into Mediterranean and Atlantic waters, displays a remarkable ecological success, forming dense monospecific populations throughout diverse environments. This persistence suggests highly efficient acclimation mechanisms, possibly driven by seasonal physiological performance. The aim of this study was to evaluate the seasonal physiological response of R. okamurae to seasonal environmental fluctuations within a Posidonia oceanica meadow in the Alboran Sea, to better understand the photosynthetic acclimation strategies underlying its invasiveness. Photosynthesis–irradiance curves, pigment concentrations and elemental composition were analysed in individuals sampled bimonthly from July 2021 to July 2022 at a −10 m depth. Results showed significant seasonal variation in physiological parameters, highlighting a consistent acclimation capacity and robust photosynthetic performance. Fluctuations in pigment content and photosynthetic variables reflected the species' ability to optimise metabolic activity in response to environmental changes. Light compensation and saturation parameters further indicated strong photoadaptive capacity, enabling the species to thrive under both low and high light conditions. Its tolerance to a broad range of environmental factors, combined with mechanisms that prevent photoinhibition, may explain its persistence across seasons and wide depth ranges in the Alboran Sea. Although further research along depth gradients is needed, these findings underscore the role of physiological plasticity in the ecological success of R. okamurae and highlight the value of field‐based studies in understanding invasion processes of marine macrophytes.

The success of invasive species can be measured by invasiveness, which depicts intrinsic characteristics that enable them to thrive in new environments. In invasive seaweeds, for example, the persistence of multiple overlapping cohorts throughout the year plays a key role in increasing plant cover and exerting unrelenting pressure on invaded areas. The marine brown macroalgae Rugulopteryx okamurae has recently established abundant populations in the Mediterranean Sea and Atlantic Ocean, negatively affecting both biodiversity and socioeconomic factors by unprecedently aggressive invasive behaviour. The objective of the study is to understand the invasiveness of R. okamurae through its popu lation dynamics. For this, a year-round study was conducted in a protected habitat of Posidonia oceanica in southern Spain, revealing that R. okamurae uses alternating mechanisms for population maintenance. It achieves high density of young individuals in late summer and autumn, peaking at 3285 individuals per square metre. In spring and early summer, the population shifts towards fewer – but larger – individuals, with densities dropping to 888 individuals per square metre and biomass reaching a peak of 170 g dry weight (DW) per square metre. Six overlapping cohorts were identified by Gaussian curves. They persisted throughout the year, but they were not related to environmental factors, which indicates adaptive physiological mechanisms that sustain dense monospecific populations. Additionally, the association between cohorts and different morphotypes suggests that R. okamurae phenotypic plasticity enables its persistence in introduced areas. These findings provide valuable insights into the biological traits underpinning its invasiveness in P. oceanica meadows, revealing temporal windows of invasiveness driven by different mechanisms. This knowledge is crucial for developing effective conservation and management strategies aimed at mitigating the impact of this invasive species.

In Southern European estuaries and associated salt marshes, the anthropogenic nutrient inputs, together with longer drought periods, are leading to increasing eutrophication and salinization of these coastal ecosystems. In this study, uptake kinetics of ammonium, nitrate, and phosphate by three common plants in Palmones salt marsh (Southern Spain), Sarcocornia perennis ssp. alpini, Atriplex portulacoides, and Arthrocnemum macrostachyum were measured in hydroponic cultures. We also determined how these uptakes could be modified by increasing salinity, adding NaCl to the incubation medium (from 170 to 1,025 mM). Kinetic parameters are analyzed to understand the competition of the three species for nutrient resources under realistic most frequent concentrations in the salt marsh. These results may also be useful to predict the possible changes in the community composition and distribution if trends in environmental changes persist. Atriplex portulacoides showed the highest V_max for ammonium, the most abundant nutrient in the salt marsh, while the highest affinity for this nutrient was observed in A. macrostachyum. Maximum uptake rates for nitrate were much lower than for ammonium, without significant differences among species. The highest V_max value for phosphate was observed in A. macrostachyum, whereas A. portulacoides presented the highest affinity for this nutrient. High salinity drastically affected the physiological response of these species, decreasing nutrient uptake. Sarcocornia perennis ssp. alpini and A. macrostachyum were not affected by salinity up to 510 mM NaCl, whereas A. portulacoides notably decreased its uptake capacity at 427 mM and even withered at 1,025 mM NaCl. At current most frequent concentrations of ammonium and phosphate in the salt marsh, S. perennis ssp. alpini is the most favored species, from the nutritional point of view. However, A. portulacoides could enhance its presence if the increasing ammonium load continues, although a simultaneous salinization would negatively affect its nutritional physiology.

Marine macroalgae possess a range of mechanisms to increase the availability of CO₂ for fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase. Of these, possession of a periplasmic or external carbonic anhydrase and the ability to use bicarbonate ions is widely distributed. The mechanisms of carbon acquisition were studied in two estuarine red macroalgae Bostrychia scorpioides and Catenella caespitosa using a range of techniques. pH-drift and CO₂-depletion experiments at constant pH suggested that CO₂ is the main source of inorganic carbon in both species. Inhibitors indicated that internal and external carbonic anhydrase were present in both species. Inhibitors also suggested that uptake of bicarbonate is unlikely to be present (P < 0.05).