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Reliable net primary production (NPP) estimations of kelp forests are important to evaluate their C-fixation potential. Photosynthetic oxygen measurements can be converted to C-fixation using photosynthetic quotients (PQs). Although there is a known high variability in PQs, the extent and the consequences for NPP is understudied in kelp species. Thus, the present study aimed (i) to quantify the variability of PQs, (ii) to model NPP and (iii) to assess the impact of warming on both. The kelp, Laminaria hyperborea, was studied near the island of Helgoland (North Sea, Germany) along a depth gradient (2, 4, 6 m below mean low water spring tide) across all four seasons. Blade discs were cultivated during at least 6 days per season under simulated ambient photosynthetic photon flux density (PPFD) and temperature conditions and, in parallel, in a warming scenario (+ 4°C). PQs were calculated from parallel oxygen production and 14C-fixation measurements at saturating PPFD at the end of the incubation period. Seasonal PQs varied between 1.7 and 4.4, with highest values in summer due to increased oxygen production. The warming scenario stimulated C-fixation in most seasons, lowering the PQ in comparison to ambient temperature conditions, while collection depth had no significant effect on PQs. The seasonal PQs were used to model daily NPP rates for kelp standing stock at 4 m depth. These daily NPP rates were compared between temperature treatments and with daily NPP rates based on fixed PQs. The warming scenario had a stimulating effect on daily NPP rates in the high-light season spring. In the low-light season autumn, warming resulted in negative daily NPP rates, as the high respiration rates could not be compensated by gross photosynthesis. Overall, annual NPP rate under warming conditions (347 g C m-2 yr-1) was 14% higher than the annual NPP rate under ambient conditions (303 g C m-2 yr-1). Modelling daily NPP with fixed PQs, which neglects the seasonal variation of the PQs, led to a high overestimation of up to 255%. We, therefore, recommend modelling NPP rates not with a fixed PQ, but with seasonal PQs determined under different temperature scenarios in order to obtain reliable future predictions. [ABSTRACT FROM AUTHOR]

SUMMARY: Photosynthetic and chemosynthetic extremophiles have evolved adaptations to thrive in challenging environments by finely adjusting their metabolic pathways through evolutionary processes. A prime adaptation target to allow autotrophy in extreme conditions is the enzyme Rubisco, which plays a central role in the conversion of inorganic to organic carbon. Here, we present an extensive compilation of Rubisco kinetic traits from a wide range of species of bacteria, archaea, algae, and plants, sorted by phylogenetic group, Rubisco type, and extremophile type. Our results show that Rubisco kinetics for the few extremophile organisms reported up to date are placed at the margins of the enzyme's natural variability. Form ID Rubisco from thermoacidophile rhodophytes and form IB Rubisco from halophile terrestrial plants exhibit higher specificity and affinity for CO2 than their non‐extremophilic counterparts, as well as higher carboxylation efficiency, whereas form ID Rubisco from psychrophile organisms possess lower affinity for O2. Additionally, form IB Rubisco from thermophile cyanobacteria shows enhanced CO2 specificity when compared to form IB non‐extremophilic cyanobacteria. Overall, these findings highlight the unique characteristics of extremophile Rubisco enzymes and provide useful clues to guide next explorations aimed at finding more efficient Rubiscos. Significance Statement: Autotrophic extremophiles exhibit unique physiological adaptations, which yielded great advances in basic and applied science. While the CO2‐fixing enzyme Rubisco has been mainly characterized in non‐extremophile organisms, the analysis of the few extremophile Rubisco kinetics reported up to date reveals variants placed at the enzyme's natural variability limits. This foundational knowledge supports future research, aiding photosynthesis genetic engineering and uncovering Earth's life origin. [ABSTRACT FROM AUTHOR]

Summary: The kinetic properties of Rubisco, the most important carbon‐fixing enzyme, have been assessed in a small fraction of the estimated existing biodiversity of photosynthetic organisms. Until recently, one of the most significant gaps of knowledge in Rubisco kinetics was marine macrophytes, an ecologically relevant group including brown (Ochrophyta), red (Rhodophyta) and green (Chlorophyta) macroalgae and seagrasses (Streptophyta). These organisms express various Rubisco types and predominantly possess CO2‐concentrating mechanisms (CCMs), which facilitate the use of bicarbonate for photosynthesis. Since bicarbonate is the most abundant form of dissolved inorganic carbon in seawater, CCMs allow marine macrophytes to overcome the slow gas diffusion and low CO2 availability in this environment. The present review aims to compile and integrate recent findings on the biochemical diversity of Rubisco and CCMs in the main groups of marine macrophytes. The Rubisco kinetic data provided demonstrate a more relaxed relationship among catalytic parameters than previously reported, uncovering a variability in Rubisco catalysis that has been hidden by a bias in the literature towards terrestrial vascular plants. The compiled data indicate the existence of convergent evolution between Rubisco and biophysical CCMs across the polyphyletic groups of marine macrophytes and suggest a potential role for oxygen in shaping such relationship. [ABSTRACT FROM AUTHOR]

Summary: Seaweeds have a wide ecophysiological and phylogenetic diversity with species expressing different Rubisco forms that frequently coexist with biophysical CO2 concentrating mechanisms (CCMs), an adaptation that overcomes the low CO2 availability and gas diffusion in seawater.Here, we assess the possible coevolution between the Rubisco catalysis and the type and effectiveness of CCMs present in six upper subtidal macroalgal species belonging to three phylogenetic groups of seaweeds.A wide diversity in the Rubisco kinetic traits was found across the analyzed species, although the specificity factor was the only parameter explained by the expressed Rubisco form. Differences in the catalytic trade‐offs were found between Rubisco forms, indicating that ID Rubiscos could be better adapted to the intracellular O2 : CO2 ratio found in marine organisms during steady‐state photosynthesis. The biophysical components of the CCMs also differed among macroalgal species, resulting in different effectiveness to concentrate CO2 around Rubisco active sites. Interestingly, an inverse relationship was found between the effectiveness of CCMs and the in vitro Rubisco carboxylation efficiency, which possibly led to a similar carboxylation potential across the analyzed macroalgal species.Our results demonstrate a coevolution between Rubisco kinetics and CCMs across phylogenetically distant marine macroalgal species sharing the same environment. [ABSTRACT FROM AUTHOR]

Cold-temperate and Arctic hard bottom coastal ecosystems are dominated by kelp forests, which have a high biomass production and provide important ecosystem services, but are subject to change due to ocean warming. However, the photophysiological response to increasing temperature of ecologically relevant species, such as Laminaria digitata, might depend on the local thermal environment where the population has developed. Therefore, the effects of temperature on growth rate, biochemical composition, maximum quantum yield, photosynthetic quotient and carbon budget of young cultured sporophytes of Laminaria digitata from the Arctic at Spitsbergen (SPT; cultured at 4, 10 and 16 °C) and from the cold-temperate North Sea island of Helgoland (HLG; cultured at 10, 16 and 22 °C) were comparatively analyzed. Temperature significantly affected growth rates of L. digitata from SPT and HLG, with the highest rates occurring at 10 °C, but growth did not differ between both isolates neither at 10 °C nor at 16 °C. Nevertheless, maximum quantum yield and carbon fixation rate were highest at 4 °C for the Arctic and at 16 °C for the cold-temperate L. digitata. Significantly higher rates of oxygen production and carbon fixation were observed in the cold-temperate relative to the Artic L. digitata at 10 and 16 °C, respectively. Neither temperature nor biogeographic region of origin affected the photosynthetic quotient, and release rates of dissolved or particulate organic carbon. Total carbon and mannitol content were significantly higher in the Arctic compared to the cold-temperate L. digitata at 10 °C, revealing an increased accumulation of storage compounds in the high latitude L. digitata. We conclude that L. digitata from SPT and HLG differ in their sensitivity to increasing temperatures and that the Arctic population from Spitsbergen is likely to benefit from ocean warming, while the temperate population from Helgoland will be negatively affected by further increases in ambient temperature., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025 The Authors. Published by Elsevier GmbH.. All rights reserved.)