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Ocean acidification (OA) due to increased anthropogenic CO₂ emissions is affecting marine ecosystems at an unprecedented rate, altering biogeochemical cycles. Direct empirical studies on natural communities are required to analyse the interactive effects of multiple stressors while spanning multiple trophic levels. We investigated the interactive effects of changes in CO₂ and iron availability on functional plankton groups. We used mesocosms manipulating the carbonate system from the start to achieve present (low concentration, LC) and predicted future pCO₂ levels (high concentration, HC). To manipulate dissolved iron (dFe), half of the mesocosms were amended with 70 nM (final concentration) of the siderophore desferoxamine B (DFB) on Day 7 (+DFB and –DFB treatments). Manipulation of both CO₂ and DFB increased dFe compared to the control. During the 22 experimental days, the plankton community structure showed 2 distinct phases. In phase 1 (Days 1–10), only bacterioplankton abundances increased at elevated pCO₂. In contrast, a strong community response was evident in phase 2 (Days 11–22) due to DFB addition. Biomass of the coccolithophore Emiliania huxleyi increased massively at LC+DFB. HC negatively affected E. huxleyi and Synechococcus sp., and high dFe (+DFB) had a positive effect on both. The rest of the plankton community was unaffected by the treatments. Increased dFe partially mitigated the negative effect of HC imposed on the coccolithophores, indicating that E. huxleyi was able to acclimate better to OA. This physiological iron-mediated acclimation can diminish the deleterious effects of OA on carbon export and the rain ratio, thus affecting food web dynamics and future ecosystem functioning.

Despite the high productivity and ecological importance of seaweeds in polar coastal regions, little is known about their carbon utilization mechanisms, especially the kinetics of the CO₂-fixing enzyme Rubisco. We analyzed Rubisco carboxylation kinetics at 4 °C and 25 °C in 12 diverse polar seaweed species (including cold-temperate populations of the same species) and the relationship with their ability to use bicarbonate, by using 13C isotope discrimination and pH drift experiments. We observed a large variation in Rubisco carboxylation kinetics among the selected species, although no correlation was found between either the Michaelis–Menten constant for CO₂ (K c) or Rubisco content per total soluble protein ([Rubisco]/[TSP]) and the ability to use bicarbonate for non-green seaweeds. This study reports intraspecific Rubisco cold adaptation by means of either higher Rubisco carboxylation turnover rate (k cat c) and carboxylase efficiency (k cat c/K c) at 4 °C or higher [Rubisco]/[TSP] in some of the analyzed species. Our data point to a widespread ability for photosynthetic bicarbonate usage among polar seaweeds, despite the higher affinity of Rubisco for CO₂ and higher dissolved CO₂ concentration in cold seawater. Moreover, the reported catalytic variation within form ID Rubisco might avert the canonical trade-off previously observed between K c and k cat c for plant Rubiscos.

Cyanobacteria largely contribute to the biogeochemical carbon cycle fixing ~ 25% of the inorganic carbon on Earth. However, the carbon acquisition and assimilation mechanisms in Cyanobacteria are still underexplored regardless of being of great importance for shedding light on the origins of autotropism on Earth and providing new bioengineering tools for crop yield improvement. Here, we fully characterized these mechanisms from the polyextremophile cyanobacterium Chroococcidiopsis thermalis KOMAREK 1964/111 in comparison with the model cyanobacterial strain, Synechococcus sp. PCC6301. In particular, we analyzed the Rubisco kinetics along with the in vivo photosynthetic CO 2 assimilation in response to external dissolved inorganic carbon, the effect of CO 2 concentrating mechanism (CCM) inhibitors on net photosynthesis and the anatomical particularities of their carboxysomes when grown under either ambient air (0.04% CO 2 ) or 2.5% CO 2 -enriched air. Our results show that Rubisco from C. thermalis possess the highest specificity factor and carboxylation efficiency ever reported for Cyanobacteria, which were accompanied by a highly effective CCM, concentrating CO 2 around Rubisco more than 140-times the external CO 2 levels, when grown under ambient CO 2 conditions. Our findings provide new insights into the Rubisco kinetics of Cyanobacteria, suggesting that improved S c/o values can still be compatible with a fast-catalyzing enzyme. The combination of Rubisco kinetics and CCM effectiveness in C. thermalis relative to other cyanobacterial species might indicate that the co-evolution between Rubisco and CCMs in Cyanobacteria is not as constrained as in other phylogenetic groups.

Submerged angiosperms sustain some of the most productive and diverse ecosystems worldwide. However, their carbon acquisition and assimilation mechanisms remain poorly explored, missing an important step in the evolution of photosynthesis during the colonization of aquatic environments by angiosperms. Here we reveal a convergent kinetic adaptation of Rubisco in phylogenetically distant seagrass species that share catalytic efficiencies and CO2 and O2 affinities up to three times lower than those observed in phylogenetically closer angiosperms from terrestrial, freshwater and brackish-water habitats. This Rubisco kinetic convergence was found to correlate with the effectiveness of seagrass CO2-concentrating mechanisms (CCMs), which probably evolved in response to the constant CO2 limitation in marine environments. The observed Rubisco kinetic adaptation in seagrasses more closely resembles that seen in eukaryotic algae operating CCMs rather than that reported in terrestrial C4 plants. Our results thus demonstrate a general pattern of co-evolution between Rubisco function and biophysical CCM effectiveness that traverses distantly related aquatic lineages.Rubisco in seagrasses converged on lower catalytic efficiencies and CO2/O2 affinities than in their terrestrial and freshwater relatives, as a result of correlative adaptation between Rubisco and the effectiveness of their CO2-concentrating mechanisms.

Ocean acidification and warming are affecting polar regions with particular intensity. Rocky shores of the Antarctic Peninsula are dominated by canopy-forming Desmarestiales. This study investigates the physiological and transcriptomic responses of the endemic macroalga Desmarestia anceps to a combination of different levels of temperature (2 and 7 °C), dissolved CO₂ (380 and 1000 ppm), and irradiance (65 and 145 μmol photons m−2 s−1). Growth and photosynthesis increased at high CO₂ conditions, and strongly decreased at 2 °C plus high irradiance, in comparison to the other treatments. Photoinhibition at 2 °C plus high irradiance was evidenced by the photochemical performance and intensive release of dissolved organic carbon. The highest number of differentially regulated transcripts was observed in thalli exposed to 2 °C plus high irradiance. Algal 13C isotopic discrimination values suggested an absence of down-regulation of carbon-concentrating mechanisms at high CO₂. CO₂ enrichment induced few transcriptomic changes. There was high and constitutive gene expression of many photochemical and inorganic carbon utilization components, which might be related to the strong adaptation of D. anceps to the Antarctic environment. These results suggest that increased temperature and CO₂ will allow D. anceps to maintain its productivity while tolerating higher irradiances than at present conditions.

The coccolithophore Emiliania huxleyi shows a variety of responses to ocean acidification (OA) and to high-CO 2 concentrations, but there is still controversy on differentiating between these two factors when using different strains and culture methods. A heavily calcified type A strain isolated from the Norwegian Sea was selected and batch cultured in order to understand whether acclimation to OA was mediated mainly by CO 2 or H + , and how it impacted cell growth performance, calcification, and physiological stress management. Emiliania huxleyi responded differently to each acidification method. CO 2 -enriched aeration (1200 µatm, pH 7.62) induced a negative effect on the cells when compared to acidification caused by decreasing pH alone (pH 7.60). The growth rates of the coccolithophore were more negatively affected by high pCO 2 than by low pH without CO 2 enrichment with respect to the control (400 µatm, pH 8.1). High CO 2 also affected cell viability and promoted the accumulation of reactive oxygen species (ROS), which was not observed under low pH. This suggests a possible metabolic imbalance induced by high CO 2 alone. In contrast, the affinity for carbon uptake was negatively affected by both low pH and high CO 2 . Photochemistry was only marginally affected by either acidification method when analysed by PAM fluorometry. The POC and PIC cellular quotas and the PIC:POC ratio shifted along the different phases of the cultures; consequently, calcification did not follow the same pattern observed in cell stress and growth performance. Specifically, acidification by HCl addition caused a higher proportion of severely deformed coccoliths, than CO 2 enrichment. These results highlight the capacity of CO 2 rather than acidification itself to generate metabolic stress, not reducing calcification.

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.

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 14 C-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.

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.

Ocean acidification affects with special intensity Arctic ecosystems, being marine photosynthetic organisms a primary target, although the consequences of this process in the carbon fluxes of Arctic algae are still unknown. The alteration of the cellular carbon balance due to physiological acclimation to an increased CO sub(2) concentration (1300 ppm) in the common Arctic brown seaweeds Desmarestia aculeata and Alaria esculenta from Kongsfjorden (Svalbard) was analysed. Growth rate of D. aculeata was negatively affected by CO sub(2) enrichment, while A. esculenta was positively affected, as a result of a different reorganization of the cellular carbon budget in both species. Desmarestia aculeata showed increased respiration, enhanced accumulation of storage biomolecules and elevated release of dissolved organic carbon, whereas A. esculenta showed decreased respiration and lower accumulation of storage biomolecules. Gross photosynthesis (measured both as O sub(2) evolution and super(14)C fixation) was not affected in any of them, suggesting that photosynthesis was already saturated at normal CO sub(2) conditions and did not participate in the acclimation response. However, electron transport rate changed in both species in opposite directions, indicating different energy requirements between treatments and species specificity. High CO sub(2) levels also affected the N-metabolism, and super(13)C isotopic discrimination values from algal tissue pointed to a deactivation of carbon concentrating mechanisms. Since increased CO sub(2) has the potential to modify physiological mechanisms in different ways in the species studied, it is expected that this may lead to changes in the Arctic seaweed community, which may propagate to the rest of the food web.