Concepción Iñiguez

Mixed meadows of the seagrass Cymodocea nodosa and the green macroalga Caulerpa prolifera are commonly found in Mediterranean coastlines. These two marine macrophytes are increasingly exposed to ocean warming and increased dissolved CO ₂ conditions, but how the interactive effects of these stressors remodel their photosynthetic physiology remains poorly resolved. We conducted a 15-day full factorial flow-through microcosm experiment crossing two temperatures (25 and 30 °C) with two CO ₂ concentrations (400 and 1000 ppm), to analyse the responses in growth, photophysiology and operation of their biophysical CO ₂ -concentrating mechanisms (CCMs). The two species operated fundamentally different CCMs, with proton extrusion coupled to extracellular carbonic anhydrases as the major CCM component in C. nodosa , and anion-exchange HCO ₃ ^- transport in C. prolifera . Warming was the main driver of the reorganization of CCM components in both species, whereas elevated CO ₂ had comparatively minor effects on the individual components. Notably, combined warming and elevated CO ₂ interactively restored several CCM components of the seagrass to control levels, suggesting that CO ₂ enrichment did not deactivate CCMs. Warming stimulated net photosynthesis and leaf elongation in C. nodosa whereas in C. prolifera neither warming nor elevated CO ₂ significantly modified net photosynthesis or rhizoid elongation, and the additional fronds formed under warming did not translate into biomass gain. These divergent physiological trajectories, observed under microcosm conditions after a 15-day incubation, suggest that near-future climate conditions may favour the ecophysiological performance of the seagrass over that of the macroalga in co-occurring Mediterranean meadows.

Aquatic photoautotrophs experience strong physicochemical constraints on inorganic carbon acquisition due to low CO2 and O2 diffusion in water producing a strong reduction in the gas conductance. These limitations have driven the widespread evolution of CO2-concentrating mechanisms (CCMs), which enhance CO2 availability around Rubisco and mitigate its catalytic inefficiencies. A compilation of the Rubisco kinetics and apparent photosynthetic CO2 affinities (1/Km CO2) for a wide range of taxa shows that, despite the structural diversity of CCMs, there is an evolutive convergence among cyanobacteria, diatoms, haptophytes, and some rhodophytes and chlorophytes with similarly high 1/Km CO2, therefore operating almost CO2-saturated photosynthesis under ambient conditions. Notably, none of the compiled species achieves CO2 saturation solely through Rubisco kinetics, underscoring the central role of CCMs in aquatic carbon fixation. CCM effectiveness (the ratio between Rubisco CO2 semi-saturation constant under 21% O2 and apparent photosynthetic semi-saturation constant for CO2, Kcair/Km CO2) varies widely across the compiled species but is consistently higher in organisms possessing Rubisco-containing microcompartments, although pyrenoids are not strictly required for concentrating CO2 around Rubisco above ambient levels. Furthermore, an inverse relationship between Rubisco carboxylation efficiency and 1/Km CO2 supports a divergent CCM-Rubisco coevolutionary trajectory compared with terrestrial plants, likely influenced by intracellular photosynthetic O2 accumulation under submerged conditions. The present review synthesizes current knowledge on CCM diversity and its regulation under future global change scenarios but also reanalyzes CCM effectiveness and its co-evolution with Rubisco kinetic traits across aquatic oxygenic phototrophs, highlighting major knowledge gaps to be filled by future research.

Cyanobacteria are the most ancient oxygenic photosynthetic organisms on Earth and play a pivotal role in the global carbon cycle. Despite their ecological and evolutionary significance, the mechanisms of carbon acquisition and fixation in this phylum remain largely unexplored beyond a few model species. Here, we examined representative taxa spanning the full phylogenetic breadth of Cyanobacteria, assessing in vivo carbon acquisition pathways, the role and effectiveness of CO2-concentrating mechanisms (CCMs), as well as conducting in vitro biochemical characterizations of the kinetic traits and carbon isotope fractionation of Rubisco. We found significant strain-specific differences in Rubisco kinetics and CCM performance, but a common signature of high Rubisco catalytic turnover coupled with low CO2 affinity, consistent with the co-evolution of this enzyme together with effective CCMs. Furthermore, we identified a strong positive correlation between Rubisco intrinsic carbon isotope fractionation and its CO2/O2 specificity factor. Together, these results provide insight into Rubisco catalysis and shed light on its co-evolution with CCMs, underscoring their role in shaping Earth's carbon dynamics.

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.

Cyanobacteria are the most ancient oxygenic photosynthetic organisms on Earth and play a pivotal role in the global carbon cycle. Despite their ecological and evolutionary significance, the mechanisms of carbon acquisition and fixation in this phylum remain largely unexplored beyond a few model species. Here, we examined representative taxa spanning the full phylogenetic breadth of Cyanobacteria, assessing in vivo carbon-acquisition pathways, the role and effectiveness of CO ₂ -concentrating mechanisms (CCMs), as well as conducting in vitro biochemical characterizations of the kinetic traits and carbon isotope fractionation of Rubisco. We found significant lineage-specific differences in Rubisco kinetics and CCM performance, but a common signature of high Rubisco catalytic turnover coupled with low CO ₂ affinity—consistent with the co-evolution of this enzyme together with powerful CCMs. Furthermore, we identified a strong positive correlation between Rubisco carbon isotope fractionation and its CO ₂ /O ₂ specificity factor. Together, these results provide fresh insight into Rubisco catalysis and shed light on its co-evolution with CCMs, underscoring their role in shaping Earth′s carbon dynamics.

Extreme environments challenge fundamental pillars of photosynthesis: light capture and carbon fixation. Organisms thriving in extreme conditions, such as high and low temperatures, extreme pH levels, and high salinity, have evolved remarkable adaptive mechanisms allowing them to sustain photosynthesis. Research into these adaptations has expanded our understanding of the limits and evolution of photosynthesis, while also providing promising biotechnological applications. In this review, we explore the adaptations that tolerant and extremophilic photosynthetic organisms have evolved, overcoming these environmental challenges while maintaining photosynthetic functionality. These adaptations include modifications in photosystems and electron transport chain components, the development of photoprotective mechanisms, the use of unique CO₂‐concentrating mechanisms (CCMs), and fine‐tuning of Rubisco's kinetic properties and concentration. Our aim is to provide the basis for future research in extremophile biology while highlighting its applications in biotechnology.

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.