Biochemical and biophysical CO2 concentrating mechanisms in two species of freshwater macrophyte within the genus Ottelia (Hydrocharitaceae)

Impacts of phosphoenolpyruvate carboxylase gene silencing on photosynthetic efficiency and carbon fixation in Skeletonema costatum

Diatoms play a crucial role in marine primary productivity through carbon fixation, which is essential for understanding the operation of marine biological pumps and carbon sinks. This study focuses on the phosphoenolpyruvate carboxylase (PEPC) gene, a key enzyme in the carbon assimilation pathway of diatoms, by investigating the consequences of its silencing in Skeletonemacostatum. Through this approach, we aimed to clarify the distinct contributions of PEPC to the overall carbon fixation process. The mutant strains of S. costatum were subjected to thorough analysis to identify any shifts in physiological behavior, alterations in the gene expression of key carbon-fixing enzymes, and changes in the associated enzyme activities. Notably, the inhibition of the PEPC gene did not significantly affect the growth rate of S. costatum; however, it did have a notable impact on the photosynthetic apparatus, as evidenced by a reduction in the maximal electron transport rate and a decline in light utilization efficiency. A significant decrease was observed in both the enzymatic activity and gene expression of PEPCase. This down-regulation also affected other enzymes integral to the carbon fixation pathway, such as phosphoenolpyruvate carboxykinase and pyruvate-phosphate dikinase, indicating a wider metabolic perturbation. In contrast, the expression and activity of the Rubisco enzyme suggested that some facets of carbon fixation remained resilient. Furthermore, the substantial upregulation of carbonic anhydrase expression and activity probably represented an adaptive mechanism to sustain the inorganic carbon supply necessary for the carboxylation process of Rubisco. This research not only underscores the pivotal role of the PEPC gene in the carbon fixation of S. costatum but also expands our comprehension of carbon fixation mechanisms in diatoms.

Inorganic carbon utilization strategies of plateau aquatic plants in response to native habitats

Aquatic plants are a crucial component of the aquatic ecosystem in the Tibetan Plateau region. Researching the adaptability of plateau aquatic plants in photosynthesis to the plateau environment can enhance understanding of the operational mechanisms of plateau ecosystems, thereby providing a scientific basis for the protection and management of plateau aquatic ecosystems. This study presents an investigation of photosynthetic inorganic carbon utilization strategies and photosynthetic efficiency of 17 aquatic plants under natural growing conditions in Niyang River basin on the Tibetan Plateau. In pH-drift experiments, 10 of 17 species were able to utilize HCO3-, and environmental factors like water pH were shown to have a significant effect on the ability of the tested species to utilize HCO3-. Titratable acidity in the leaves of Stuckenia filiformis, Zannichellia palustris, Batrachium bungei, and Myriophyllum spicatum showed significant diurnal fluctuations at certain sampling sites, indicating the presence of CAM. In B. bungei, water pH positively correlated with CAM activity, while CO2 concentration negatively correlated with CAM activity. The chlorophyll fluorescence analysis revealed that aquatic plants inhabiting the Tibetan Plateau exhibited photosynthetic adaptations. In conclusion, the aquatic plants on the Tibetan Plateau employ diverse strategies for utilizing inorganic carbon during photosynthesis, exhibiting their flexible adaptability to the native high-altitude habitats of the Tibetan Plateau.

The distinctive level of interaction between carbon and nitrogen metabolisms in the leaves of submerged macrophytes plays a key role in ammonium detoxification

Possible ammonium detoxification mechanisms have been proposed recently, on submerged macrophytes, evidently illustrating that glutamate dehydrogenase (GDH) plays a greater role in ammonium detoxification compared to the primary glutamine synthetase/glutamate oxaloacetate transaminase (GS/GOGAT) pathway. In the current investigation, we cultured three submerged macrophytes to extreme concentrations of [NH4+-N] of up to 50 mg/L with the aim of clarifying the interaction between carbon and nitrogen metabolisms. The activities of carboxylation enzymes pyruvate orthophosphate dikinase (PPDK) and phosphoenolpyruvate carboxylase (PEPC), in lieu of Rubisco, increased almost two-fold for ammonium tolerant species P. maackianus and M. spicatum, compared with the sensitive species P. lucens. While these enzymes are well known for their central role in CO2 fixation, their inference in conferring resistance to ammonium stress has not been well elucidated before. In this study, we demonstrate that the overproduction of PEPC and PPDK led to improved photosynthesis, better ammonium assimilation and overall ammonium detoxification in M. spicatum and P. maackianus. These findings propose likelihood for the existence of a complementary ammonium detoxification pathway that targets carbon metabolism, thus, presenting a relatively efficient linkage between nitrogen and carbon metabolisms and identify candidate species for practical restoration of fresh water resources.

Morphological and physiological response of amphibious Rotala rotundifolia from emergent to submerged form

Rotala rotundifolia is an amphibious aquatic plant that can live in submerged and emergent forms. It is superior in nitrogen and phosphorus removal. To elucidate its adaptation strategies from emergent to submerged conditions, phenotypic and physiological responses of R. rotundifolia were investigated during three months of submergence, at water levels of 0 cm (CK), 50 cm (W50), and 90 cm (W90). Results showed that submergence stress reduced the relative growth rate of plant height, fresh weight, and biomass accumulation, leading to root degradation and a significant decline in the root-shoot ratio. The amounts of soluble protein (SP), soluble sugar (SS), and starch in the aerial leaves of W50 and W90 decreased during the early stages of submergence compared to CK, whereas the total chlorophyll and proline contents, and activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) increased. The contents of endogenous hormones, including abscisic acid (ABA), gibberellin (GA), and indole-3-acetic acid (IAA), decreased during the change in leaf shape; the decline in ABA was more obvious. The leaf primordium generated transition leaves and submerged leaves to resolve the "carbon starvation" of plants. The maximum values of non-structural carbohydrates (NSC) in the leaves of W50 and W90 occurred at day 30, reaching 14.0 mg g- 1and 10.5 mg g- 1, respectively. The contents of SP and starch, activities of SOD and CAT of the roots in submerged treatments increased, while SS and proline content decreased at day 7. These results demonstrated that developing heterophyllous leaves, increasing chlorophyll content, and regulating plant carbon allocation and consumption were important mechanisms of R. rotundifolia to adapt to underwater habitats.

Red light alleviates Cd toxicity in Egeria densa by modifying carbon-nitrogen metabolism and boosting energy metabolism

Among the various pollutants detected in aquatic ecosystems, cadmium (Cd) is considered as one of the most hazardous. Freshwater macrophytes have been recognized as possible candidates for eliminating Cd from environment. Nevertheless, the impact of light quality on their ability to tolerate Cd toxicity remains unclear, and the underlying mechanisms have yet to be fully elucidated. In this study, we utilized physiological testing and metabolomics to explore the potential mechanisms by which light quality influences the ability of Egeria densa, a significant Cd hyperaccumulator, to withstand Cd toxicity. The study demonstrated that following Cd treatment, E. densa grown under red light exhibited superior photosynthetic efficiency compared to those grown under blue light, as evidenced by significantly increased photosynthetic rate, higher starch content, and greater activity of photosynthetic enzymes. Moreover, metabolomic analyses revealed that under Cd stress, E. densa grown under red light exhibited an enhanced glycolysis for increased energy production. Sucrose metabolism was also improved to generate sufficient sugar including glucose, fructose and mannose for osmotic adjustment. Moreover, under red light, the heightened production of α-ketoglutarate via tricarboxylic acid (TCA) cycle redirected nitrogen flow towards the synthesis of resilient substances such as γ-Aminobutyric Acid (GABA) and methionine. The production of these substances was ∼2.0 and 1.3 times greater than that of treatment with Cd under blue light, thereby improving E. densa's capacity to withstand Cd stress. This study represents the initial investigation into the possible mechanisms by which light quality influences the ability of E. densa to withstand Cd toxicity through regulating CN metabolism. Furthermore, these findings have the potential to improve phytoremediation strategies aimed at reducing Cd pollution.

The CAM lineages of planet Earth

BACKGROUND AND SCOPE

The growth of experimental studies of crassulacean acid metabolism (CAM) in diverse plant clades, coupled with recent advances in molecular systematics, presents an opportunity to re-assess the phylogenetic distribution and diversity of species capable of CAM. It has been more than two decades since the last comprehensive lists of CAM taxa were published, and an updated survey of the occurrence and distribution of CAM taxa is needed to facilitate and guide future CAM research. We aimed to survey the phylogenetic distribution of these taxa, their diverse morphology, physiology and ecology, and the likely number of evolutionary origins of CAM based on currently known lineages.

RESULTS AND CONCLUSIONS

We found direct evidence (in the form of experimental or field observations of gas exchange, day-night fluctuations in organic acids, carbon isotope ratios and enzymatic activity) for CAM in 370 genera of vascular plants, representing 38 families. Further assumptions about the frequency of CAM species in CAM clades and the distribution of CAM in the Cactaceae and Crassulaceae bring the currently estimated number of CAM-capable species to nearly 7 % of all vascular plants. The phylogenetic distribution of these taxa suggests a minimum of 66 independent origins of CAM in vascular plants, possibly with dozens more. To achieve further insight into CAM origins, there is a need for more extensive and systematic surveys of previously unstudied lineages, particularly in living material to identify low-level CAM activity, and for denser sampling to increase phylogenetic resolution in CAM-evolving clades. This should allow further progress in understanding the functional significance of this pathway by integration with studies on the evolution and genomics of CAM in its many forms.

The diverse diaspora of CAM: a pole-to-pole sketch

BACKGROUND

Crassulacean acid metabolism (CAM) photosynthesis is a successful adaptation that has evolved often in angiosperms, gymnosperms, ferns and lycophytes. Present in ~5 % of vascular plants, the CAM diaspora includes all continents apart from Antarctica. Species with CAM inhabit most landscapes colonized by vascular plants, from the Arctic Circle to Tierra del Fuego, from below sea level to 4800 m a.s.l., from rainforests to deserts. They have colonized terrestrial, epiphytic, lithophytic, palustrine and aquatic systems, developing perennial, annual or geophyte strategies that can be structurally arborescent, shrub, forb, cladode, epiphyte, vine or leafless with photosynthetic roots. CAM can enhance survival by conserving water, trapping carbon, reducing carbon loss and/or via photoprotection.

SCOPE

This review assesses the phylogenetic diversity and historical biogeography of selected lineages with CAM, i.e. ferns, gymnosperms and eumagnoliids, Orchidaceae, Bromeliaceae, Crassulaceae, Euphorbiaceae, Aizoaceae, Portulacineae (Montiaceae, Basellaceae, Halophytaceae, Didiereaceae, Talinaceae, Portulacaceae, Anacampserotaceae and Cactaceae) and aquatics.

CONCLUSIONS

Most extant CAM lineages diversified after the Oligocene/Miocene, as the planet dried and CO2 concentrations dropped. Radiations exploited changing ecological landscapes, including Andean emergence, Panamanian Isthmus closure, Sundaland emergence and submergence, changing climates and desertification. Evidence remains sparse for or against theories that CAM biochemistry tends to evolve before pronounced changes in anatomy and that CAM tends to be a culminating xerophytic trait. In perennial taxa, any form of CAM can occur depending upon the lineage and the habitat, although facultative CAM appears uncommon in epiphytes. CAM annuals lack strong CAM. In CAM annuals, C3 + CAM predominates, and inducible or facultative CAM is common.

Effects of the ammonium stress on photosynthesis and ammonium assimilation in submerged leaves of Ottelia cordata - an endangered aquatic plant

Although ammonium (NH₄⁺-N) is an important nutrient for plants, increases in soil nitrogen (N) input and atmospheric deposition have made ammonium toxicity a serious ecological problem. In this study, we explored the effects of NH₄⁺-N stress on the ultrastructure, photosynthesis, and NH₄⁺-N assimilation of Ottelia cordata (Wallich) Dandy, an endangered heteroblastic plant native to China. Results showed that 15 and 50 mg L^-1 NH₄⁺-N damaged leaf ultrastructure and decreased the values of maximal quantum yield (F_v/F_m), maximal fluorescence (F_m), and relative electron transport rate (rETR) in the submerged leaves of O. cordata. Furthermore, when NH₄⁺-N was ≥ 2 mg L^-1, phosphoenolpyruvate carboxylase activity (PEPC) and soluble sugar and starch contents decreased significantly. The content of dissolved oxygen in the culture water also decreased significantly. The activity of the NH₄⁺-N assimilation enzyme glutamine synthetase (GS) significantly increased when NH₄⁺-N was ≥ 10 mg L^-1 and NADH-glutamate synthase (NADH-GOGAT) and Fd-glutamate synthase (Fd-GOGAT) increased when NH₄⁺-N was at 50 mg L^-1. However, the activity of nicotinamide adenine dinucleotide-dependent glutamate dehydrogenase (NADH-GDH) and nicotinamide adenine dinucleotide phosphate-dependent glutamate dehydrogenase (NADPH-GDH) did not change, indicating that GS/GOGAT cycle may play an important role in NH₄⁺-N assimilation in the submerged leaves of O. cordata. These results show that short-term exposure to a high concentration of NH₄⁺-N is toxic to O. cordata.

Different photosynthetic inorganic carbon utilization strategies in the heteroblastic leaves of an aquatic plant Ottelia ovalifolia

The leaves of the heteroblastic aquatic plant Ottelia ovalifolia faces submerged and aerial environments during its life history. However, the acclimation of the submerged leaves and floating leaves to these two environments in morphology, physiology, and biochemistry remain unclear. In the present study, we investigated the acclimation of the CO₂-concentrating mechanisms in these two types of leaves. We found that the submerged leaves were longer, narrower, and thinner than the floating leaves, which increased the specific surface area of the leaves and lead to better absorption of the inorganic carbon underwater. Meanwhile, the floating leaves absorbed atmospheric CO₂ directly through the stomata to acclimate to the aerial environment. Both the leaf types had the ability to use HCO 3 - , but the capacity in submerged leaves was stronger than that in floating leaves. The extracellular carbonic anhydrase and anion exchanger were responsible for the HCO 3 - use in both types of leaves. The higher ratio of chlorophyll a/b and content of anthocyanin in floating leaves than that in submerged leaves indicated that the acclimation of aerial and submerged photosynthesis depended on changes in the photosynthetic pigments. Based on the stable carbon isotope ratio, key enzyme activities of the C4 pathway indicated that submerged leaves might have the ability to perform C4 metabolism while floating leaves only performed C3 metabolism. In summary, O. ovalifolia acclimates to submerged and aerial environments through changes in morphology, physiology, and biochemistry during different growth stages.

Inorganic carbon utilization: A target of silver nanoparticle toxicity on a submerged macrophyte

Submerged macrophytes play an important role in the global carbon cycle through diversified pathways of inorganic carbon (Ci) utilization distinct from terrestrial plants. However, the effects of silver nanoparticles (AgNPs), an emerging contaminant, were unknown on the Ci utilization of submerged macrophytes. In Ottelia alismoides, the only known submerged macrophyte with three pathways of Ci utilization, before absorption, AgNPs inhibited the external carbonic anhydrase activity thus reducing the capacity of the plant to use HCO₃^-. After entering the plant, AgNPs mainly aggregated at the cell wall and in the chloroplast. The internalized AgNPs inhibited ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) activity blocking CO₂ fixation and disturbed C4 and crassulacean acid metabolism (CAM) by inhibiting phosphoenolpyruvate carboxylase (PEPC), pyruvate phosphate dikinase (PPDK), and NAD-dependent malic enzyme (NAD-ME) activities to alter intracellular malate biosynthesis and decarboxylation. Overall, our findings indicate that the Ci utilization of the submerged macrophyte is a target of AgNPs toxicity that might affect the carbon cycle in aquatic systems.

Carbon and nitrogen metabolic regulation in freshwater plant Ottelia alismoides in response to carbon limitation: A metabolite perspective

Carbon and nitrogen metabolism are basic, but pivotal metabolic pathways in plants and are tightly coupled. Maintaining the balance of carbon and nitrogen metabolism is critical for plant survival. Comprehensively revealing the metabolic balance of carbon-nitrogen interactions is important and helpful for understanding the adaptation of freshwater plants to CO₂ limited aqueous environment. A comprehensive metabolomics analysis combined with physiological measurement was performed in the freshwater plant Ottelia alismoides acclimated to high and low CO₂, respectively, for a better understanding of how the carbon and nitrogen metabolic adjustment in freshwater plants respond to carbon limitation. The present results showed that low CO₂ acclimated O. alismoides exhibited significant diurnal titratable acidity and malate fluctuations, as well as an opposite diel pattern of starch change and high enzymatic activities required for crassulacean acid metabolism (CAM) photosynthesis, which indicates that CAM was induced under low CO₂. Moreover, the metabolomic analysis showed that most intermediates of glycolysis, pentose phosphate pathway (PPP) and tricarboxylic acid (TCA) cycle, were increased under low CO₂, indicative of active respiration in low-CO₂-treated O. alismoides. Meanwhile, the majority of amino acids involved in pathways of glutamate and arginine metabolism, aspartate metabolism, and the branched-chain amino acids (BCAAs) metabolism were significantly increased under low CO₂. Notably, γ-aminobutyric acid (GABA) level was significantly higher in low CO₂ conditions, indicating a typical response with GABA shunt compensated for energy deprivation at low CO₂. Taken together, we conclude that in low-CO₂-stressed O. alismoides, CAM photosynthesis was induced, leading to higher carbon and nitrogen as well as energy requirements. Correspondingly, the respiration was greatly fueled via numerous starch degradation to ensure CO₂ fixation in dark, while accompanied by linked promoted N metabolism, presumably to produce energy and alternative carbon sources and nitrogenous substances for supporting the operation of CAM and enhancing tolerance for carbon limitation. This study not only helps to elucidate the regulating interaction between C and N metabolism to adapt to different CO₂ but also provides novel insights into the effects of CO₂ variation on the metabolic profiling of O. alismoides.

Evolution of an intermediate C4 photosynthesis in the non-foliar tissues of the Poaceae

Carbon concentrating mechanisms (CCMs) in plants are abaptive features that have evolved to sustain plant growth in unfavorable environments, especially at low atmospheric carbon levels and high temperatures. Uptake of CO₂ and its storage in the aerenchyma tissues of Lycopsids and diurnal acidity fluctuation in aquatic plants during the Palaeozoic era (ca. 300 Ma.) would represent the earliest evolution of a CCM. The CCM parts of the dark reactions of photosynthesis have evolved many times, while the light reactions are conserved across plant lineages. A C₄ type CCM, leaf C₄ photosynthesis is evolved in the PACMAD clade of the Poaceae family. The evolution of C₄ photosynthesis from C₃ photosynthesis was an abaptation. Photosynthesis in reproductive tissues of sorghum and maize (PACMAD clade) has been shown to be of a weaker C₄ type (high CO₂ compensation point, low carbon isotope discrimination, and lack of Rubisco compartmentalization, when compared to the normal C₄ types) than that in the leaves (normal C₄ type). However, this does not fit well with the character polarity concept from an evolutionary perspective. In a recent model proposed for CCM evolution, the development of a rudimentary CCM prior to the evolution of a more efficient CCM (features contrasting to a weaker C₄ type, leading to greater biomass production rate) has been suggested. An intermediate crassulacean acid metabolism (CAM) type of CCM (rudimentary) was reported in the genera, Brassia, Coryanthes, Eriopsis, Peristeria, of the orchids (well-known group of plants that display the CAM pathway). Similarly, we propose here the evolution of a rudimentary CCM (C₄-like type pathway) in the non-foliar tissues of the Poaceae, prior to the evolution of the C₄ pathway as identified in the leaves of the C₄ species of the PACMAD clade.

Mitochondrial phosphoenolpyruvate carboxylase contributes to carbon fixation in the diatom Phaeodactylum tricornutum at low inorganic carbon concentrations

Photosynthetic carbon fixation is often limited by CO₂ availability, which led to the evolution of CO₂ concentrating mechanisms (CCMs). Some diatoms possess CCMs that employ biochemical fixation of bicarbonate, similar to C₄ plants, but whether biochemical CCMs are commonly found in diatoms is a subject of debate. In the diatom Phaeodactylum tricornutum, phosphoenolpyruvate carboxylase (PEPC) is present in two isoforms, PEPC1 in the plastids and PEPC2 in the mitochondria. We used real-time quantitative polymerase chain reaction, Western blots, and enzymatic assays to examine PEPC expression and PEPC activity, under low and high concentrations of dissolved inorganic carbon (DIC). We generated and analyzed individual knockout cell lines of PEPC1 and PEPC2, as well as a PEPC1/2 double-knockout strain. While we could not detect an altered phenotype in the PEPC1 knockout strains at ambient, low or high DIC concentrations, PEPC2 and the double-knockout strains grown under ambient air or lower DIC availability conditions showed reduced growth and photosynthetic affinity for DIC while behaving similarly to wild-type (WT) cells at high DIC concentrations. These mutants furthermore exhibited significantly lower ¹³ C/¹² C ratios compared to the WT. Our data imply that in P. tricornutum at least parts of the CCM rely on biochemical bicarbonate fixation catalyzed by the mitochondrial PEPC2.

Antidote or Trojan horse for submerged macrophytes: Role of microplastics in copper toxicity in aquatic environments

Due to their unique surface structures and physicochemical properties, microplastics (MPs) can adsorb other contaminants, thus impacting their toxicity and fate in aquatic ecosystems. In the present study, the adsorption and transportation of copper ions (Cu²⁺) in polyethylene (PE, 5 and 150 μm) and their combined effects on four submerged macrophyte species were assessed. Results demonstrated that the addition of PE reduced the Cu²⁺ concentration in copper sulfate (CuSO₄) solution and the adsorption of Cu²⁺ in PE (10 mg/L) increased with CuSO₄ concentration (100-600 μmol/L). PE alone exhibited no inhibitory effects on macrophytes, while Cu²⁺ showed fatal toxicity toward the macrophytes. However, the combination of PE and Cu²⁺ showed lower inhibitory effects on macrophytes and the toxicity attenuation varied among species. Additionally, PE may act as a carrier (like a Trojan horse) for the environmental transfer of Cu²⁺, thereby hosting Cu²⁺ toxicity against macrophytes in the imported environment. Our findings indicate that PE acts as both an antidote to and carrier of Cu²⁺ toxicity in macrophytes. This study should help in clarifying the combined effects and risk assessments of MPs and heavy metals in future studies.

Transition From Proto-Kranz-Type Photosynthesis to HCO3 - Use Photosynthesis in the Amphibious Plant Hygrophila polysperma

Hygrophila polysperma is a heterophyllous amphibious plant. The growth of H. polysperma in submerged conditions is challenging due to the low CO2 environment, increased resistance to gas diffusion, and bicarbonate ion (HCO3 -) being the dominant dissolved inorganic carbon source. The submerged leaves of H. polysperma have significantly higher rates of underwater photosynthesis compared with the terrestrial leaves. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonate (DIDS), an anion exchanger protein inhibitor, and ethoxyzolamide (EZ), an inhibitor of internal carbonic anhydrase, repressed underwater photosynthesis by the submerged leaves. These results suggested that H. polysperma acclimates to the submerged condition by using HCO3 - for photosynthesis. H. polysperma transports HCO3 - into the leaf by a DIDS-sensitive HCO3 - transporter and converted to CO2 by carbonic anhydrase. Additionally, proteome analysis revealed that submerged leaves accumulated fewer proteins associated with C4 photosynthesis compared with terrestrial leaves. This finding suggested that H. polysperma is capable of C4 and C3 photosynthesis in the terrestrial and submerged leaves, respectively. The ratio of phosphoenol pyruvate carboxylase to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the submerged leaves was less than that in the terrestrial leaves. Upon anatomical observation, the terrestrial leaves exhibited a phenotype similar to the Kranz anatomy found among C4 plants; however, chloroplasts in the bundle sheath cells were not located adjacent to the vascular bundles, and the typical Kranz anatomy was absent in submerged leaves. These results suggest that H. polysperma performs proto-Kranz type photosynthesis in a terrestrial environment and shifts from a proto-Kranz type in terrestrial leaves to a HCO3 - use photosynthesis in the submerged environments.

Low-level CAM photosynthesis in a succulent-leaved member of the Urticaceae, Pilea peperomioides

Pilea peperomioides Diels (Urticaceae) is a semi-succulent herbaceous species native to south-western China that has become popular in cultivation as an ornamental plant. To investigate whether this species possesses the capacity for CAM photosynthesis, measurements were made of CO2 gas exchange and titratable acidity in plants under both well-watered and water-deficit conditions. Plants were found to assimilate CO2 almost exclusively in the light via C3 photosynthesis. However, distinct transient reductions in the rate of net nocturnal CO2 release were consistently observed during the course of the dark period, and under water-deficit conditions one plant exhibited a brief period of net nocturnal CO2 uptake, providing unequivocal evidence of CAM activity. Furthermore, nocturnal increases in titratable acidity in both leaf laminas and petioles were observed in all plants exposed to wet-dry-wet cycles. This is the first report of CAM in the family Urticaceae. The results are discussed in relation to the phylogenetic position of Pilea and the partially shaded montane habitats in which this species is typically found. An updated list of all plant families currently known to contain species with CAM is presented.

External α-carbonic anhydrase and solute carrier 4 are required for bicarbonate uptake in a freshwater angiosperm

The freshwater monocot Ottelia alismoides is the only known species to operate three CO2-concentrating mechanisms (CCMs): constitutive bicarbonate (HCO3-) use, C4 photosynthesis, and facultative Crassulacean acid metabolism, but the mechanism of HCO3- use is unknown. We found that the inhibitor of an anion exchange protein, 4,4'-diisothio-cyanatostilbene-2,2'-disulfonate (DIDS), prevented HCO3- use but also had a small effect on CO2 uptake. An inhibitor of external carbonic anhydrase (CA), acetazolamide (AZ), reduced the affinity for CO2 uptake but also prevented HCO3- use via an effect on the anion exchange protein. Analysis of mRNA transcripts identified a homologue of solute carrier 4 (SLC4) responsible for HCO3- transport, likely to be the target of DIDS, and a periplasmic α-carbonic anhydrase 1 (α-CA1). A model to quantify the contribution of the three different pathways involved in inorganic carbon uptake showed that passive CO2 diffusion dominates inorganic carbon uptake at high CO2 concentrations. However, as CO2 concentrations fall, two other pathways become predominant: conversion of HCO3- to CO2 at the plasmalemma by α-CA1 and transport of HCO3- across the plasmalemma by SLC4. These mechanisms allow access to a much larger proportion of the inorganic carbon pool and continued photosynthesis during periods of strong carbon depletion in productive ecosystems.

Responses of CO2-concentrating mechanisms and photosynthetic characteristics in aquatic plant Ottelia alismoides following cadmium stress under low CO2

The effects of cadmium (Cd) have been investigated in an aquatic plant Ottelia alismoides grown under low CO₂. Under low CO₂, no Cd treated O. alismoides operated three carbon dioxide-concentrating mechanisms (CCMs) efficiently, including HCO₃^- acquisition, C4 and CAM photosynthesis. After 4 days of treatment with 200 μM and 2000 μM Cd, O. alismoides exhibited an elevated Cd accumulation along with the increasing Cd concentration. Both Cd treatments induced appreciable phytotoxicities in O. alismoides. The leaves showed chlorosis symptoms and the anatomy as well as chloroplast ultrastructure were obviously damaged. Significant decreases in the content of pigments, chlorophyll fluorescence (Fv/Fm and Yield of PS II) and carbon isotope ratio (δ¹³C) were measured in leaf extracts of O. alismoides grown with both concentrations of Cd. In addition, the pH-drift technique showed that both Cd-treated O. alismoides plants could not uptake HCO₃^-. The maximum and minimum acidity in Cd-exposed O. alismoides were greatly decreased and the diurnal change of acidity was absent in both Cd treated plants. Furthermore, significant decreases in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), pyruvate phosphate dikinase (PPDK) and phosphoenolpyruvate carboxylase (PEPC) activities were also found at Cd treated O. alismoides plants, indicating the disturbance within C4 cycle. The alterations in the functionality of CCMs in O. alismoides induced by Cd might be related with the inhibition of the enzymes such as carbonic anhydrase (CA) and PEPC involved in inorganic carbon fixation, and the destruction of chloroplasts, as well as the re-allocation of energy and nutrients involved in CCMs and Cd detoxification.

Responses of Leaf Anatomy and CO2 Concentrating Mechanisms of the Aquatic Plant Ottelia cordata to Variable CO2

Acclimation to variable CO₂ was studied in floating leaves of the freshwater monocot Ottelia cordata grown in either low or high CO₂. The most striking anatomical variations responding to high CO₂ included the enlarged upper epidermal cells and the decreased area of epidermal chloroplasts. Stomata that distributed on the upper surface, and the stomatic chamber area, showed no significant response to high CO₂. pH-drift experiments indicated that floating leaves of O. cordata were able to use bicarbonate regardless of CO₂ concentrations. Photosynthetic enzyme activities and patterns of organic acids fluctuation confirmed that floating leaves of O. cordata can operate CAM only at low CO₂, and perform C₄-like metabolism at both high and low CO₂. Overall, the present results imply that the floating leaves of O. cordata does not just rely on the atmospheric CO₂ for its inorganic carbon, but is also dependent on CO₂ and bicarbonate in the water. By showing these effects of CO₂ variation, we highlight the need for further experimental studies on the regulatory mechanisms in O. cordata floating leaves, that prevent futile cycling among the three CO₂ concentrating mechanisms (bicarbonate use, C₄, and CAM metabolism) and the strategy for exploiting atmospheric CO₂, as well as studies on the detailed biochemical pathway for C₄ and CAM metabolism in this species.

Structural basis for C4 photosynthesis without Kranz anatomy in leaves of the submerged freshwater plant Ottelia alismoides

BACKGROUND AND AIMS

Ottelia alismoides (Hydrocharitaceae) is a freshwater macrophyte that, unusually, possesses three different CO2-concentrating mechanisms. Here we describe its leaf anatomy and chloroplast ultrastructure, how these are altered by CO2 concentration and how they may underlie C4 photosynthesis.

METHODS

Light and transmission electron microscopy were used to study the anatomy of mature leaves of O. alismoides grown at high and low CO2 concentrations. Diel acid change and the activity of phosphoenolpyruvate carboxylase were measured to confirm that CAM activity and C4 photosynthesis were present.

KEY RESULTS

When O. alismoides was grown at low CO2, the leaves performed both C4 and CAM photosynthesis whereas at high CO2 leaves used C4 photosynthesis. The leaf comprised an upper and lower layer of epidermal cells separated by a large air space occupying about 22 % of the leaf transverse-section area, and by mesophyll cells connecting the two epidermal layers. Kranz anatomy was absent. At low CO2, chloroplasts in the mesophyll cells were filled with starch even at the start of the photoperiod, while epidermal chloroplasts contained small starch grains. The number of chloroplasts in the epidermis was greater than in the mesophyll cells. At high CO2, the structure was unchanged but the thicknesses of the two epidermal layers, the air space, mesophyll and the transverse-section area of cells and air space were greater.

CONCLUSIONS

Leaves of O. alismoides have epidermal and mesophyll cells that contain chloroplasts and large air spaces but lack Kranz anatomy. The high starch content of mesophyll cells suggests they may benefit from an internal source of CO2, for example via C4 metabolism, and are also sites of starch storage. The air spaces may help in the recycling of decarboxylated or respired CO2. The structural similarity of leaves at low and high CO2 is consistent with the constitutive nature of bicarbonate and C4 photosynthesis. There is sufficient structural diversity within the leaf of O. alismoides to support dual-cell C4 photosynthesis even though Kranz anatomy is absent.

Insights on the Functions and Ecophysiological Relevance of the Diverse Carbonic Anhydrases in Microalgae

Carbonic anhydrases (CAs) exist in all kingdoms of life. They are metalloenzymes, often containing zinc, that catalyze the interconversion of bicarbonate and carbon dioxide-a ubiquitous reaction involved in a variety of cellular processes. So far, eight classes of apparently evolutionary unrelated CAs that are present in a large diversity of living organisms have been described. In this review, we focus on the diversity of CAs and their roles in photosynthetic microalgae. We describe their essential role in carbon dioxide-concentrating mechanisms and photosynthesis, their regulation, as well as their less studied roles in non-photosynthetic processes. We also discuss the presence in some microalgae, especially diatoms, of cambialistic CAs (i.e., CAs that can replace Zn by Co, Cd, or Fe) and, more recently, a CA that uses Mn as a metal cofactor, with potential ecological relevance in aquatic environments where trace metal concentrations are low. There has been a recent explosion of knowledge about this well-known enzyme with exciting future opportunities to answer outstanding questions using a range of different approaches.

Catchment properties and the photosynthetic trait composition of freshwater plant communities

Unlike in land plants, photosynthesis in many aquatic plants relies on bicarbonate in addition to carbon dioxide (CO2) to compensate for the low diffusivity and potential depletion of CO2 in water. Concentrations of bicarbonate and CO2 vary greatly with catchment geology. In this study, we investigate whether there is a link between these concentrations and the frequency of freshwater plants possessing the bicarbonate use trait. We show, globally, that the frequency of plant species with this trait increases with bicarbonate concentration. Regionally, however, the frequency of bicarbonate use is reduced at sites where the CO2 concentration is substantially above the air equilibrium, consistent with this trait being an adaptation to carbon limitation. Future anthropogenic changes of bicarbonate and CO2 concentrations may alter the species compositions of freshwater plant communities.

Species-Specific Responses of Submerged Macrophytes to Simulated Extreme Precipitation: A Mesocosm Study

More frequent extreme climate events (e.g., extreme precipitation) are to be expected in the future, and such events may potentially have significant effects on freshwater ecosystems. In the present mesocosm study, the effects of simulated extreme precipitation on submerged macrophytes were evaluated for three different macrophyte community (MC) treatments (MC1, MC2 and MC3). MC1 consisted of only Vallisneria denseserrulata, while MC2 and MC3 included three and six species of various growth forms. Two treatments of extreme precipitation (EP) were simulated—an extreme treatment (E) simulating a sudden increase of water level from 75 cm to 150 cm within one day and a gradual treatment (G) simulating an increase to the same water level within 3 months, combined with two control treatments. Total macrophyte community biomass was resilient to the EP and MC treatments, while species-specific variations in responses, in terms of biomass, maximum height, and sexual reproduction, were found. For instance, E led to earlier flowering of Potamogeton lucens and production of more flowers, while it had adverse effects on the flowering of Ottelia alismoides. We conclude that freshwater ecosystems with high coverage of submerged macrophytes may be overall resilient to extreme precipitation under nutrient-limited conditions, especially communities with diverse growth forms.

Nitrogen fertilizers promote plant growth and assist in manganese (Mn) accumulation by Polygonum pubescens Blume cultured in Mn tailings soil

This study examined how different nitrogen (N) forms and application levels promote plant growth and assist in manganese (Mn) remediation of Polygonum pubescens Blume (P. pubescens) cultured in soil with a high Mn level. The effects of ammonium chloride (a) and urea (u), at three application levels (10, 20, and 30 mg L^-1 N) and control (no N addition, CK) on the growth, Mn accumulation, and enzymatic anti-oxidative defenses of P. pubescens were examined. In general, both ammonium-N and urea-N promoted the plant mass and height of P. pubescens. The total Mn amount of roots, stems, and leaves in N treatments were higher (p < 0.05) than that of CK. The ammonium-N treatments showed greater plant biomass and Mn accumulation compared to the urea-N ones. In general, the accumulations of Mn, Cr, Zn, and Cu were significantly lower (p < 0.05) in the N fertilizer treatment than those in the control; while the accumulations of Pb were higher (p < 0.05) in P. pubescens across all N fertilizer treatments than those in the control. The N addition decreased the contents of O₂^- and H₂O₂ in the leaves of P. pubescens, while increasing the activities of enzymatic anti-oxidative defenses.

The Analysis of Leaf Traits of Eight Ottelia Populations and Their Potential Ecosystem Functions in Karst Freshwaters in China

Submerged macrophytes play a structuring role in the shallow freshwater ecosystem by increasing the heterogeneous state in freshwaters. The macrophytes in genus Ottelia were featured for their broad leaves, which might consequently produce specialized functions that differed from other submerged species. To explore the potential ecological role of Ottelia, a field investigation was conducted on leaf traits in eight populations of Ottelia ranging from the southwestern Yunnan-Guizhou plateau to the southern Hainan island in China covering a distance of >1,700 km. The eight populations included all the extant Ottelia species and varieties in China except the well-documented O. alismoides. Carbon-related traits [bicarbonate usage, photosynthetic characteristics, capability of Crassulacean acid metabolism (CAM)], pigment content and parameters of chlorophyll fluorescence, morphology and mass of the leaves were determined. The different populations showed distinct functional traits of mature leaves; O. acuminata var. songmingensis had the thickest and longest leaf with CaCO3 precipitation on the both sides of the leaf, and O. cordata showed putative CAM activity with the highest diel acidity changes 12.5 μequiv g-1 FW. Our results indicated an important role of Ottelia populations in carbon cycling as the dominant species in karst freshwaters in China.

Different CO2 acclimation strategies in juvenile and mature leaves of Ottelia alismoides

The freshwater macrophyte, Ottelia alismoides, is a bicarbonate user performing C4 photosynthesis in the light, and crassulacean acid metabolism (CAM) when acclimated to low CO₂. The regulation of the three mechanisms by CO₂ concentration was studied in juvenile and mature leaves. For mature leaves, the ratios of phosphoenolpyruvate carboxylase (PEPC) to ribulose-bisphosphate carboxylase/oxygenase (Rubisco) are in the range of that of C4 plants regardless of CO₂ concentration (1.5-2.5 at low CO₂, 1.8-3.4 at high CO₂). In contrast, results for juvenile leaves suggest that C4 is facultative and only present under low CO₂. pH-drift experiments showed that both juvenile and mature leaves can use bicarbonate irrespective of CO₂ concentration, but mature leaves have a significantly greater carbon-extracting ability than juvenile leaves at low CO₂. At high CO₂, neither juvenile nor mature leaves perform CAM as indicated by lack of diurnal acid fluctuation. However, CAM was present at low CO₂, though the fluctuation of titratable acidity in juvenile leaves (15-17 µequiv g^-1 FW) was slightly but significantly lower than in mature leaves (19-25 µequiv g^-1 FW), implying that the capacity to perform CAM increases as leaves mature. The increased CAM activity is associated with elevated PEPC activity and large diel changes in starch content. These results show that in O. alismoides, carbon-dioxide concentrating mechanisms are more effective in mature compared to juvenile leaves, and C4 is facultative in juvenile leaves but constitutive in mature leaves.

The intracellular distribution of inorganic carbon fixing enzymes does not support the presence of a C4 pathway in the diatom Phaeodactylum tricornutum

Diatoms are unicellular algae and important primary producers. The process of carbon fixation in diatoms is very efficient even though the availability of dissolved CO₂ in sea water is very low. The operation of a carbon concentrating mechanism (CCM) also makes the more abundant bicarbonate accessible for photosynthetic carbon fixation. Diatoms possess carbonic anhydrases as well as metabolic enzymes potentially involved in C4 pathways; however, the question as to whether a C4 pathway plays a general role in diatoms is not yet solved. While genome analyses indicate that the diatom Phaeodactylum tricornutum possesses all the enzymes required to operate a C4 pathway, silencing of the pyruvate orthophosphate dikinase (PPDK) in a genetically transformed cell line does not lead to reduced photosynthetic carbon fixation. In this study, we have determined the intracellular location of all enzymes potentially involved in C4-like carbon fixing pathways in P. tricornutum by expression of the respective proteins fused to green fluorescent protein (GFP), followed by fluorescence microscopy. Furthermore, we compared the results to known pathways and locations of enzymes in higher plants performing C3 or C4 photosynthesis. This approach revealed that the intracellular distribution of the investigated enzymes is quite different from the one observed in higher plants. In particular, the apparent lack of a plastidic decarboxylase in P. tricornutum indicates that this diatom does not perform a C4-like CCM.

The MADS-Box Gene SlMBP21 Regulates Sepal Size Mediated by Ethylene and Auxin in Tomato

Normal organ size is achieved by successful co-ordination of cell proliferation and cell expansion, which are modulated by multiple factors such as ethylene and auxin. In our work, SlMBP21-RNAi (RNA interference) tomato exhibited longer sepals and improved fruit set. Histological analysis indicated that longer sepals were attributed to cell expansion. To explore how SlMBP21 regulates sepal size, we compared the transcriptomes of sepals between SlMBP21-RNAi and the wild type by RNA sequencing and found that the differentially expressed genes were dominantly related to cell expansion, ethylene and auxin, and photosynthesis. Down-regulation of SlMBP21 affected ethylene production and the free IAA and IAA-Val intensity in sepals. Hormone treatment further indicated that SlMBP21 was involved in the ethylene and auxin pathways. As reported, ethylene and auxin were important factors for cell expansion. Hence, SlMBP21 negatively regulated cell expansion to control sepal size, and ethylene and auxin may mediate this process. Additionally, the contents of Chl and the activity of ribulose-1, 5-bisphosphate carboxylase/oxygenase, the key photosynthetic enzyme, were both increased in SlMBP21-RNAi sepals, which indicated that photosynthesis might be enhanced in transgenic longer sepals. Therefore, the longer sepal, with better protection and enhanced photosynthesis, may contribute to improve fruit set. Altogether, these results suggested that SlMBP21 was a novel factor involved in organ size control. Moreover, our study provided potential application value for improving fruit set.

Modulation of cadmium-induced phytotoxicity in Cabomba caroliniana by urea involves photosynthetic metabolism and antioxidant status

Urea is a widespread organic pollutant, which can be a nitrogen source, playing different roles in the growth of submerged macrophytes depending on concentrations, while high cadmium (Cd) concentrations are often toxic to macrophytes. In order to evaluate the combined effect of urea and Cd on a submerged macrophyte, Cabomba caroliniana, the morphological and physiological responses of C. caroliniana in the presence of urea and Cd were studied. The results showed that high concentrations of urea (400mgL^-1) and Cd (500µmolL^-1) had negative effects on C. caroliniana. There were strong visible symptoms of toxicity after 4 days of exposure under Cd-alone, 400mgL^-1 urea, and Cd+400mgL^-1 urea treatments. In addition, 400mgL^-1 urea and Cd had adverse effects on C. caroliniana's pigment system. Significant losses in chlorophyll fluorescence and photosynthetic rates, as well as Rubisco activity were also observed under Cd-alone, 400mgL^-1 urea, and Cd+400mgL^-1 urea treatments. 400mgL^-1 urea markedly enhanced Cd toxicity in C. caroliniana, reflected by a sharp decrease in photosynthetic activity and more visible toxicity symptoms. The results of thiobarbituric acid reactive substances (TBARS) pointed to extreme oxidative stress in C. caroliniana induced under Cd or 400mgL^-1 urea exposure. Exogenous ascorbate (AsA) protected C. caroliniana from adverse damage in 400mgL^-1 urea, which further corroborated the oxidative stress claim under 400mgL^-1 urea. However, results also demonstrated that lower urea concentration (10mgL^-1) alleviated Cd-induced phytotoxicity by stimulating chlorophyll synthesis and photosynthetic activity, as well as activating the activity of catalase (CAT) and glutathione-S-transferase (GST), which may explain the alleviating effect of urea on C. caroliniana under Cd stress.

Inorganic carbon availability in benthic diatom communities: photosynthesis and migration

Diatom-dominated microphytobenthos (MPB) is the main primary producer of many intertidal and shallow subtidal environments, being therefore of critical importance to estuarine and coastal food webs. Owing to tidal cycles, intertidal MPB diatoms are subjected to environmental conditions far more variable than the ones experienced by pelagic diatoms (e.g. light, temperature, salinity, desiccation and nutrient availability). Nevertheless, benthic diatoms evolved adaptation mechanisms to these harsh conditions, including the capacity to move within steep physical and chemical gradients, allowing them to perform photosynthesis efficiently. In this contribution, we will review present knowledge on the effects of dissolved inorganic carbon (DIC) availability on photosynthesis and productivity of diatom-dominated MPB. We present evidence of carbon limitation of photosynthesis in benthic diatom mats and highly productive MPB natural communities. Furthermore, we hypothesize that active vertical migration of epipelic motile diatoms could overcome local depletion of DIC in the photic layer, providing the cells alternately with light and inorganic carbon supply. The few available longer-term experiments on the effects of inorganic carbon enrichment on the productivity of diatom-dominated MPB have yielded inconsistent results. Therefore, further studies are needed to properly assess the response of MPB communities to increased CO2 and ocean acidification related to climate change.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Responses of Ottelia alismoides, an aquatic plant with three CCMs, to variable CO2 and light

Ottelia alismoides is a constitutive C4 plant and bicarbonate user, and has facultative crassulacean acid metabolism (CAM) at low CO2. Acclimation to a factorial combination of light and CO2 showed that the ratio of phosphoenolpyruvate carboxylase (PEPC) to ribulose-bisphosphate carboxylase/oxygenase (Rubisco) (>5) is in the range of that of C4 plants. This and short-term response experiments showed that the activity of PEPC and pyruvate phosphate dikinase (PPDK) was high even at the end of the night, consistent with night-time acid accumulation and daytime carbon fixation. The diel acidity change was maximal at high light and low CO2 at 17-25 µequiv g-1 FW. Decarboxylation proceeded at ~2-3 µequiv g-1 FW h-1, starting at the beginning of the photoperiod, but did not occur at high CO2; the rate was greater at high, compared with low light. There was an inverse relationship between starch formation and acidity loss. Acidity changes account for up to 21% of starch production and stimulate early morning photosynthesis, but night-time accumulation of acid traps <6% of respiratory carbon release. Ottelia alismoides is the only known species to operate CAM and C4 in the same tissue, and one of only two known aquatic species to operate CAM and bicarbonate use.

Diversity of CO2-concentrating mechanisms and responses to CO2 concentration in marine and freshwater diatoms

The presence of CO2-concentrating mechanisms (CCMs) is believed to be one of the characteristics that allows diatoms to thrive in many environments and to be major contributors to global productivity. Here, the type of CCM and the responses to variable CO2 concentration were studied in marine and freshwater diatoms. At 400 ppm, there was a large diversity in physiological and biochemical mechanisms among the species. While Phaeodactylum tricornutum mainly used HCO3-, Thalassiosira pseudonana mainly used CO2. Carbonic anhydrase was an important component of the CCM in all species and C4 metabolism was absent, even with T. weissflogii. For all species, at 20 000 ppm, the affinity for dissolved inorganic carbon was lower than at 400 ppm CO2 and the reliance on CO2 was higher. Despite the difference in availability of inorganic carbon in marine and fresh waters, there were only small differences in CCMs between species from the two environments, and Navicula pelliculosa behaved similarly when grown in the two environments. The results suggest that species-specific differences are great, and more important than environmental differences in determining the nature and effectiveness of the CCM in diatoms.

Ecological imperatives for aquatic CO2-concentrating mechanisms

In aquatic environments, the concentration of inorganic carbon is spatially and temporally variable and CO2 can be substantially oversaturated or depleted. Depletion of CO2 plus low rates of diffusion cause inorganic carbon to be more limiting in aquatic than terrestrial environments, and the frequency of species with a CO2-concentrating mechanism (CCM), and their contribution to productivity, is correspondingly greater. Aquatic photoautotrophs may have biochemical or biophysical CCMs and exploit CO2 from the sediment or the atmosphere. Though partly constrained by phylogeny, CCM activity is related to environmental conditions. CCMs are absent or down-regulated when their increased energy costs, lower CO2 affinity, or altered mineral requirements outweigh their benefits. Aquatic CCMs are most widespread in environments with low CO2, high HCO3-, high pH, and high light. Freshwater species are generally less effective at inorganic carbon removal than marine species, but have a greater range of ability to remove carbon, matching the environmental variability in carbon availability. The diversity of CCMs in seagrasses and marine phytoplankton, and detailed mechanistic studies on larger aquatic photoautotrophs are understudied. Strengthening the links between ecology and CCMs will increase our understanding of the mechanisms underlying ecological success and will place mechanistic studies in a clearer ecological context.

Silver nanoparticles induced reactive oxygen species via photosynthetic energy transport imbalance in an aquatic plant

The rapid growth in silver nanoparticles (AgNPs) commercialization has increased environmental exposure, including aquatic ecosystem. It has been reported that the AgNPs have damaging effects on photosynthesis and induce oxidative stress, but the toxic mechanism of AgNPs is still a matter of debate. In the present study, on the model aquatic higher plant Spirodela polyrhiza, we found that AgNPs affect photosynthesis and significantly inhibit Photosystem II (PSII) maximum quantum yield (F_v/F_m) and effective quantum yield (Φ_PSII). The changes of non-photochemical fluorescence quenching (NPQ), light-induced non-photochemical fluorescence quenching [Y(NPQ)] and non-light-induced non-photochemical fluorescence quenching [Y(NO)] showed that AgNPs inhibit the photo-protective capacity of PSII. AgNPs induce reactive oxygen species (ROS) that are mainly produced in the chloroplast. The activity of ribulose-1, 5-bisphosphate carboxylase-oxygenase (Rubisco) was also very sensitive to AgNPs. The internalized Ag, regardless of whether the exposure was Ag⁺or AgNPs had the same capacity to generate ROS. Our results support the hypothesis that intra-cellular AgNP dissociate into high toxic Ag⁺. Rubisco inhibition leads to slowing down of CO₂ assimilation. Consequently, the solar energy consumption decreases and then the excess excitation energy promotes ROS generation in chloroplast.

The nature of the CO2 -concentrating mechanisms in a marine diatom, Thalassiosira pseudonana

Diatoms are widespread in aquatic ecosystems where they may be limited by the supply of inorganic carbon. Their carbon dioxide-concentrating mechanisms (CCMs) involving transporters and carbonic anhydrases (CAs) are well known, but the contribution of a biochemical CCM involving C4 metabolism is contentious. The CCM(s) present in the marine-centric diatom, Thalassiosira pseudonana, were studied in cells exposed to high or low concentrations of CO2 , using a range of approaches. At low CO2 , cells possessed a CCM based on active uptake of CO2 (70% contribution) and bicarbonate, while at high CO2 , cells were restricted to CO2 . CA was highly and rapidly activated on transfer to low CO2 and played a key role because inhibition of external CA produced uptake kinetics similar to cells grown at high CO2 . The activities of phosphoenolpyruvate (PEP) carboxylase (PEPC) and the PEP-regenerating enzyme, pyruvate phosphate dikinase (PPDK), were lower in cells grown at low than at high CO2 . The ratios of PEPC and PPDK to ribulose bisphosphate carboxylase were substantially lower than 1, even at low CO2 . Our data suggest that the kinetic properties of this species results from a biophysical CCM and not from C4 type metabolism.

Comparing photosynthetic characteristics of Isoetes sinensis Palmer under submerged and terrestrial conditions

Crassulacean acid metabolism (CAM) is widespread in terrestrial and aquatic species, plastic in response to environmental changes. Isoetes L. is one of the earliest basal vascular plants and CAM is popular in this genus. Isoetes sinensis Palmer is an amphibious species, alternating frequently between terrestrial and aquatic environments. Given this, we investigated and compared photosynthetic characteristics over a diurnal cycle under submerged condition (SC) and terrestrial condition (TC). The results suggest that I. sinensis possesses a stronger CAM capacity under SC. Compared with under TC, titratable acidity levels and organic acid concentrations were more enriched under SC, whereas soluble sugar or starch and protein levels were lower under SC. Transcript analyses for nine photosynthetic genes revealed that CAM-associated genes possessed high transcripts under SC, but C₃-related transcripts were highly expressed under TC. In addition, the enzyme activity measurements demonstrated that PEPC activity over a diurnal cycle was slightly higher under SC, whereas Rubisco activity during the daytime was greater under TC. This comprehensive study probably facilitates general understandings about the CAM photosynthetic characteristics of Isoetes in response to the environmental changes.

The biodiversity of carbon assimilation

As all plastids that have been investigated so far can be traced back to endosymbiotic uptake of cyanobacteria by heterotrophic host cells, they accordingly show a high similarity regarding photosynthesis, which includes both the photosystems and the biochemical reactions around the CO2 fixation via the Calvin-Bassham cycle. Major differences between the different algal and plant groups may include the presence or absence of carbon concentrating mechanisms, pyrenoids, Rubisco activases, carbonic anhydrases as well as differences in the regulation of the Calvin-Bassham cycle. This review describes the diversity of primary carbon fixation steps in algae and plants and the respective regulatory mechanisms.

CCM8: the eighth international symposium on inorganic carbon uptake by aquatic photosynthetic organisms

The articles in this special issue of Photosynthesis Research arose from the presentations given at the Eighth International Symposium on Inorganic Carbon Uptake by Aquatic Photosynthetic Organisms held from May 27 to June 1, 2013 in New Orleans, Louisiana USA. The meeting covered all the aspects of CO2 concentrating mechanisms (CCMs) present in photosynthetic bacteria, microalgae and macrophytes, and spanned disciplines from the molecular biology of CCMs to the importance of CCMs in aquatic ecosystems. The publications in this special issue represent our current understanding of CCMs and highlight recent advances in the field. The influences of CCMs on algal biofuel production as well as recent efforts to use the CCM to improve crop plants are also explored.