Anaerobic metabolism of bivalve molluscs during exposure to air

The anaerobic metabolism of Mytilus edulis, Cardium edule, Scrobicularia plana and Macoma balthica was investigated. On exposure to the atmosphere, all of these species were found to be able to utilise ¹⁴CO₂. This suggests that these species gape during exposure to the atmosphere. A comparative study on the pattern of ¹⁴C incorporation suggests that there is a similarity between M. edulis and S. plana in the extent of the utilization of the succinate pathway during exposure to air. However C. edule and M. balthica were also similar in the extent of utilizing the succinate pathway, even though there were significant species differences between the similar animals. It is suggested that exposure in S. plana represents a stressful situation and that this species might react to this stress by utilizing the succinate pathway. A higher incorporation of radiocarbon into alanine by M. balthica could be due to high activity of the enzymes that control the reactions leading to production of radioactive pyruvate.     

Seasonal variations in intermediatory metabolism of the bivalves Cerastoderma edule and Scrobicularia plana during exposure to air

Seasonal changes in the operation of the succinate pathway by different tissues of two species of bivalve molluscs (Cerastoderma edule (L.) and Scrobicularia plana) were investigated. No significant changes were observed in tissues of C. edule. However, there were significant seasonal changes in the operation of the pathway by tissues of S. plana. The succinate pathway operates in S. plana at a higher rate in summer and autumn than in witer. The results are discussed in relation to food availability, temperature and the reproductive cycle.     

The aerobic metabolism of 14C-sugars and 14CO2 by the daughter sporocysts of Microphallus similis (JÄG) and Macrophallus pygmaeus (Levinsen) (Digenea: Microphallidae)

The sporocysts of Microphallus similis and M. pygmaeus can aerobically utilise radiosugars and ¹⁴ CO₂in vitro. Both have an EMP pathway, TCA cycle, pentose-phosphate shunt, are able to undergo transamination reactions and can synthesise some essential amino acids. Carbon dioxide fixation involves the carboxylation of phosphoenolpyruvate to form oxaloacetate.     

Anaerobic formation of succinate from glucose and bicarbonate in resting cells of Leishmania donovani

The cell suspension of Leishmania donovani incorporates ¹⁴CO₂ resulting in the formation of [¹⁴C]-succinic acid under anaerobic conditions. The results showed that the [¹⁴C]-succinate formation from [1-¹⁴C]-glucose is much greater than that from [6-¹⁴C]-glucose. [¹⁴C-pyruvate] takes part in the production of succinic acid under anaerobic conditions without decarboxylation. The anaerobic formation of succinate appears to involve the production of malate, which is then converted to succinate via the reduction of fumarate by the reversal of the tricarboxylic acid cycle. Evidence indicated that the active species in this carboxylation reaction was CO₂ although HCO₃ was active to some extent.     

14CO2 dark fixation in the halophytic species mesembryanthemum crystallinum

The time course of ¹⁴CO₂ dark fixation was studied in leaves of the facultatively halophytic plant species Mesembryanthemum crystallinum cultivated with and without 400 mM NaCl in the nutrient medium. It is generally known from the literature that plants grown under saline conditions incorporate ¹⁴C predominately into amino acids. By contrast in leaves of M. crystallinum grown on NaCl and exposed to ¹⁴CO₂ in the dark, relatively more radioactivity is incorporated in the organic acids (especially malate) than in amino acids. The data obtained are discussed in relation to the NaCl induced Crassulacean acid metabolism in M. crystallinum reported earlier.     

Fatty acids’ profiles as indicators of stress induced by of a common herbicide on two marine bivalves species: Cerastoderma edule (Linnaeus, 1758) and Scrobicularia plana (da Costa, 1778)

In Europe, mainly the Mediterranean region, intensive use of fertilizers and pesticides has been recorded over the past 30 years, exceeding, in some cases, the limits of contamination authorized by the European Union. The intensive use of pollutants in fields near ecological coastal wetlands has led to implementation of pesticide monitoring programs to recover aquatic systems such as the Mondego estuary (Figueira da Foz, Portugal). According to information from the agricultural cooperatives of the Mondego valley, Primextra^® Gold TZ is the most-used herbicide in corn crop fields. Biomarkers, such fatty acids (FAs), proved to be new and potentially powerful tools to detect, illustrate, and evaluate exposure to and the effects of contamination hazards. They play important roles in establishing neural levels in organisms’ biochemical and physiological responses and are considered good bio-indicators of stress and potential indicators of ecosystem health. Bivalves are currently used in ecotoxicological bioassays because of their ecological importance, wide geographic distribution, ease of handling in the laboratory and in the field, and their ability to filter and ingest large volumes of water and sediment particles. Thus, the main goal of this work was to determine the toxic and biochemical (namely fatty acid profiles) responses of two size classes (small and big) of the two marine bivalve species Cerastoderma edule and Scrobicularia plana to the herbicide Primextra^® Gold. Furthermore, we aimed to compare the fatty acid contents, and thus the nutritive values, of both species and size classes collected in the field with those under laboratory conditions. Results show S. plana is more sensitive to the herbicide than C. edule. In general, among the larger-sized specimens in the field, S. plana is more nutritive than C. edule, but among the smaller-sized specimens, the opposite tendency is seen, where C. edule presents a greater abundance of FAs.     

Food supply and seawater pCO2 impact calcification and internal shell dissolution in the blue mussel Mytilus edulis

Progressive ocean acidification due to anthropogenic CO₂ emissions will alter marine ecosytem processes. Calcifying organisms might be particularly vulnerable to these alterations in the speciation of the marine carbonate system. While previous research efforts have mainly focused on external dissolution of shells in seawater under saturated with respect to calcium carbonate, the internal shell interface might be more vulnerable to acidification. In the case of the blue mussel Mytilus edulis, high body fluid pCO₂ causes low pH and low carbonate concentrations in the extrapallial fluid, which is in direct contact with the inner shell surface. In order to test whether elevated seawater pCO₂ impacts calcification and inner shell surface integrity we exposed Baltic M. edulis to four different seawater pCO₂ (39, 142, 240, 405 Pa) and two food algae (310–350 cells mL⁻¹ vs. 1600–2000 cells mL⁻¹) concentrations for a period of seven weeks during winter (5°C). We found that low food algae concentrations and high pCO₂ values each significantly decreased shell length growth. Internal shell surface corrosion of nacreous ( = aragonite) layers was documented via stereomicroscopy and SEM at the two highest pCO₂ treatments in the high food group, while it was found in all treatments in the low food group. Both factors, food and pCO₂, significantly influenced the magnitude of inner shell surface dissolution. Our findings illustrate for the first time that integrity of inner shell surfaces is tightly coupled to the animals'' energy budget under conditions of CO₂ stress. It is likely that under food limited conditions, energy is allocated to more vital processes (e.g. somatic mass maintenance) instead of shell conservation. It is evident from our results that mussels exert significant biological control over the structural integrity of their inner shell surfaces.     

Rates and properties of endogenous cyclic photophosphorylation of isolated intact chloroplasts measured by CO2 fixation in the presence of dihydroxyacetone phosphate

1. Dihydroxyacetone phosphate in concentrations ? 2.5 mM completely inhibits CO₂-dependent O₂ evolution in isolated intact spinach chloroplasts. This inhibition is reversed by the addition of equimolar concentrations of P_i, but not by addition of 3-phosphoglycerate. In the absence of P_i, 3-phosphoglycerate and dihydroxyacetone phosphate, only about 20% of the ¹⁴C-labelled intermediates are found in the supernatant, whereas in the presence of each of these substances the percentage of labelled intermediates in the supernatant is increased up to 70–95%. Based on these results the mechanism of the inhibition of O₂ evolution by dihydroxyacetone phosphate is discussed with respect to the function of the known phosphate translocator in the envelope of intact chloroplasts.2. Although O₂ evolution is completely suppressed by dihydroxyacetone phosphate, CO₂ fixation takes place in air with rates of up to 65μ mol · mg^?1 chlorophyll · h^?1. As non-cyclic electron transport apparently does not occur under these conditions, these rates must be due to endogenous pseudocyclic and/or cyclic photophosphorylation.3. Under anaerobic conditions, the rates of CO₂ fixation in presence of dihydroxyacetone phosphate are low (2.5–7 μmol · mg^?1 chlorophyll · h^?1), but they are strongly stimulated by addition of dichlorophenyl-dimethylurea (e.g. 2 · 10^?7 M) reaching values of up to 60 μmol · mg^?1 chlorophyll · h^?1. As under these conditions the ATP necessary for CO₂ fixation can be formed by an endogenous cyclic photophosphorylation, the capacity of this process seems to be relatively high, so it might contribute significantly to the energy supply of the chloroplast. As dichlorophenyl-dimethylurea stimulates CO₂ fixation in presence of dihydroxyacetone phosphate under anaerobic but not under aerobic conditions, it is concluded that only under anaerobic conditions an “overreduction” of the cyclic electron transport system takes place, which is removed by dichlorophenyl-dimethylurea in suitable concentrations. At concentrations above 5 · 10^?7 M dichlorophenyl-dimethylurea inhibits dihydroxyacetone phosphate-dependent CO₂ fixation under anaerobic as well as under aerobic conditions in a similar way as normal CO₂ fixation. Therefore, we assume that a properly poised redox state of the electron transport chain is necessary for an optimal occurrence of endogenous cyclic photophosphorylation.4. The inhibition of dichlorophenyl-dimethylurea-stimulated CO₂ fixation in presence of dihydroxyacetone phosphate by dibromothymoquinone under anaerobic conditions indicates that plastoquinone is an indispensible component of the endogenous cyclic electron pathway.     

Effect of Oxygen on the Light-enhanced Dark Carbon Dioxide Fixation in Chlorella Cells

With Chlorella ellipsoidea cells, the effect of oxygen was investigated on the products of enhanced dark ¹⁴CO₂ fixation immediately following preillumination in the absence of CO₂. When the reaction mixture was made aerobic by bubbling air (CO₂-free) throughout preillumination and the following dark ¹⁴CO₂ fixation periods, the initial fixation product was mainly 3-phosphoglyceric acid. When nitrogen gas had been used instead of air, only about one-half of the total radioactivity in the initial fixation products was in 3-phosphoglyceric acid and the rest in aspartic, phosphoenolpyruvic, and malic acids. The percentage distribution of radioactivity incorporated in these initial products rapidly decreased during the rest of the dark period. Concurrent with the decrease in the initial ¹⁴CO₂ fixation products, some increase was observed in the radioactivities of the sugar phosphates. The maximal radioactivity incorporated in sugar mono- and diphosphates accounted for only 10% of total ¹⁴C, under either the aerobic or anaerobic conditions. Under anaerobic conditions most of the ¹⁴C incorporated was eventually transferred to alanine, whereas the main end products under aerobic conditions were aspartate and glutamate. The pattern of ¹⁴CO₂ fixation products was unaffected by the atmospheric condition during the period of preillumination. The preferential flow of the fixed carbon atom to alanine or aspartate depended on the presence or absence of oxygen during the period of dark CO₂ fixation.     

Isotope Labeling and Microautoradiography of Active Heterotrophic Bacteria on the Basis of Assimilation of 14CO2

Most heterotrophic bacteria assimilate CO₂ in various carboxylation reactions during biosynthesis. In this study, assimilation of ¹⁴CO₂ by heterotrophic bacteria was used for isotope labeling of active microorganisms in pure cultures and environmental samples. Labeled cells were visualized by microautoradiography (MAR) combined with fluorescence in situ hybridization (FISH) to obtain simultaneous information about activity and identity. Cultures of Escherichia coli and Pseudomonas putida assimilated sufficient ¹⁴CO₂ during growth on various organic substrates to obtain positive MAR signals. The MAR signals were comparable with the traditional MAR approach based on uptake of ¹⁴C-labeled organic substrates. Experiments with E. coli showed that ¹⁴CO₂ was assimilated during both fermentation and aerobic and anaerobic respiration. The new MAR approach, HetCO₂-MAR, was evaluated by targeting metabolic active filamentous bacteria, including “Candidatus Microthrix parvicella” in activated sludge. “Ca. Microthrix parvicella” was able to take up oleic acid under anaerobic conditions, as shown by the traditional MAR approach with [¹⁴C]oleic acid. However, the new HetCO₂-MAR approach indicated that “Ca. Microthrix parvicella,” did not significantly grow on oleic acid under anaerobic conditions with or without addition of NO₂⁻, whereas the addition of O₂ or NO₃⁻ initiated growth, as indicated by detectable ¹⁴CO₂ assimilation. This is a metabolic feature that has not been described previously for filamentous bacteria. Such information could not have been derived by using the traditional MAR procedure, whereas the new HetCO₂-MAR approach differentiates better between substrate uptake and substrate metabolism that result in growth. The HetCO₂-MAR results were supported by stable isotope analysis of ¹³C-labeled phospholipid fatty acids from activated sludge incubated under aerobic and anaerobic conditions in the presence of ¹³CO₂. In conclusion, the novel HetCO₂-MAR approach expands the possibility for studies of the ecophysiology of uncultivated microorganisms.     

Anaerobic Carbon Metabolism by the Tricarboxylic Acid Cycle : Evidence for Partial Oxidative and Reductive Pathways during Dark Ammonium Assimilation

Nitrogen-limited cells of Selenastrum minutum (Naeg.) Collins are able to assimilate NH₄⁺ in the dark under anaerobic conditions. Addition of NH₄⁺ to anaerobic cells results in a threefold increase in tricarboxylic acid cycle (TCAC) CO₂ efflux and an eightfold increase in the rate of anaplerotic carbon fixation via phosphoenolpyruvate carboxylase. Both of these observations are consistent with increased TCAC carbon flow to supply intermediates for amino acid biosynthesis. Addition of H¹⁴CO₃⁻ to anaerobic cells assimilating NH₄⁺ results in the incorporation of radiolabel into the α-carboxyl carbon of glutamic acid. Incorporation of radiolabel into glutamic acid is not simply a short-term phenomenon following NH₄⁺ addition as the specific activity of glutamic acid increases over time. This indicates that this alga is able to maintain partial oxidative TCAC carbon flow while under anoxia to supply α-ketoglutarate for glutamate production. During dark aerobic NH₄⁺ assimilation, no radiolabel appears in fumarate or succinate and only a small amount occurs in malate. During anaerobic NH₄⁺ assimilation, these metabolites contain a large proportion of the total radiolabel and radiolabel accumulates in succinate over time. Also, the ratio of dark carbon fixation to NH₄⁺ assimilation is much higher under anaerobic than aerobic conditions. These observations suggest the operation of a partial reductive TCAC from oxaloacetic acid to malate, fumarate, and succinate. Such a pathway might contribute to redox balance in an anaerobic cell maintaining partial oxidative TCAC activity.     

Effect of oxygen on photosynthesis by spinach leaf protoplasts

The photosynthetic CO₂ fixation by spinach leaf (Spinacia oleracea L. var. Kyoho) protoplasts was inhibited by substituting an atmosphere of N₂ with one of either air (21% O₂) or 100% O₂. The inhibitory effect of 100% O₂ was greater than that of air. The mode of inhibition by 100% O₂ and air was competitive with respect to CO₂; Ki(O₂) value was 0.32 mM at pH 7 and 0.28 mM at pH 8.5 The labeling patterns of compounds in protoplasts exposed to ¹⁴CO₂ in light after transferring them from N₂ to O₂ atmospheres were examined. There was no detectable ¹⁴CO₂ incorporation into glycolate under anaerobic and O₂ atmospheres; a more marked labeling of glycine occurred under an oxidative environment compared to that under the anaerobic condition, presumably because of a rapid transformation of glycolate to glycine in the protoplasts.     

The impact of O(2) on the Fe-S cluster biogenesis requirements of Escherichia coli FNR

In this study, the functions of two established Fe-S cluster biogenesis pathways, Isc (iron-sulfur cluster) and Suf (sulfur mobilization), under aerobic and anaerobic growth conditions were compared by measuring the activity of the Escherichia coli global anaerobic regulator FNR. A [4Fe-4S] cluster is required for FNR activity under anaerobic conditions. An assay of the expression of FNR-dependent promoters in strains containing various deletions of the iscSUAhscBAfdx operon revealed that, under anaerobic conditions, FNR activity was reduced by 60% in the absence of the Isc pathway. In contrast, a mutant lacking the entire Suf pathway had normal FNR activity, although overexpression of the suf operon fully rescued the anaerobic defect in FNR activity in strains lacking the Isc pathway. Expression of the sufA promoter and levels of SufD protein were upregulated by twofold to threefold in Isc ⁻ strains under anaerobic conditions, suggesting that increased expression of the Suf pathway may be partially responsible for the FNR activity remaining in strains lacking the Isc pathway. In contrast, use of the O₂-stable [4Fe-4S] cluster FNR variant FNR-L28H showed that overexpression of the suf operon did not restore FNR activity to strains lacking the Isc pathway under aerobic conditions. In addition, FNR-L28H activity was more impaired under aerobic conditions than under anaerobic conditions. The greater requirement for the Isc pathway under aerobic conditions was not due to a change in the rate of Fe-S cluster acquisition by FNR-L28H under aerobic and anaerobic conditions, as shown by ⁵⁵Fe-labeling experiments. Using [³⁵S]methionine pulse-chase assays, we observed that the Isc pathway, but not the Suf pathway, is the major pathway required for conversion of O₂-inactivated apo-FNR into [4Fe-4S]FNR upon the onset of anaerobic growth conditions. Taken together, these findings indicate a major role for the Isc pathway in FNR Fe-S cluster biogenesis under both aerobic and anaerobic conditions.     

Carbohydrate and Amino Acid Fermentation in the Free-Living Primitive Protozoon Hexamita sp.

Hexamita sp. is an amitochondriate free-living diplomonad which inhabits O₂-limited environments, such as the deep waters and sediments of lakes and marine basins. ¹³C nuclear magnetic resonance spectroscopy reveals ethanol, lactate, acetate, and alanine as products of glucose fermentation under microaerobic conditions (23 to 34 μM O₂). Propionic acid and butyric acid were also detected and are believed to be the result of fermentation of alternative substrates. Production of organic acids was greatest under microaerobic conditions (15 μM O₂) and decreased under anaerobic (<0.25 μM O₂) and aerobic (200 to 250 μM O₂) conditions. Microaerobic incubation resulted in the production of high levels of oxidized end products (70% acetate) compared to that produced under anoxic conditions (20% acetate). In addition, data suggest that Hexamita cells contain the arginine dihydrolase pathway, generating energy from the catabolism of arginine to citrulline, ornithine, NH₄⁺, and CO₂. The rate of arginine catabolism was higher under anoxic conditions than under microaerobic conditions. Hexamita cells were able to grow in the absence of a carbohydrate source, albeit with a lower growth rate and yield.     

The biochemical response of two commercial bivalve species to exposure to strong salinity changes illustrated by selected biomarkers

Salinity is a major controlling factor in estuarine systems whose fast change, namely during the occurrence of extreme climatic events, causes drastic alterations on aquatic communities by promoting a physiologically stressful environment. The response of fatty acid (FA) and antioxidant enzymes’ activity (Glutathione S-transferase (GST) and Superoxide dismutase (SOD)) of Cerastoderma edule and Scrobicularia plana were investigated under a wide range of salinity. Species were sampled in Mondego estuary (Portugal). A set of organisms (namely “field”) were stored for biochemical analysis, whereas the remaining organisms collected in the field (namely “lab”) were exposed to a range of salinity concentrations. Organisms were fed daily. In general, results revealed a decrease on enzymatic activity along a set of salinity concentrations with an exception to the GST activity of C. edule where a trend of increase at the activity was observed at almost all treatments. S. plana presented a very low or null activity to both enzymes. Differences in the FA profiles of both groups were also observed, with “lab” organisms not presenting saturated FA of short chain. The diversity on FA and the quantity in unsaturated FA under different salinity concentrations presented the highest values at the extreme salinity treatments. C. edule directly stored from the field presented the highest diversity and quantity in polyunsaturated fatty acids (95.77%) whereas organisms of S. plana from the field showed the highest percentage of highly unsaturated fatty acids (20.93%). Results suggest that, under salinity stress, the consumption of food decreases and the physiological pathways are reduced. Still species can store FA recognized as of high physiological importance to animals, by reducing their activity and energy consumption. Therefore, under an extreme climatic event (e.g. drought or flood) these species may present a higher content of essential FA and, thus, a higher food quality, reducing, in general, the activity of the enzymes SOD and GST.     

Biodegradation of atrazine under denitrifying conditions

Anaerobic biodegradation of atrazine by the bacterial isolate M91-3 was characterized with respect to mineralization, metabolite formation, and denitrification. The ability of the isolate to enhance atrazine biodegradation in anaerobic sediment slurries was also investigated. The organism utilized atrazine as its sole source of carbon and nitrogen under anoxic conditions in fixed-film (glass beads) batch column systems. Results of HPLC and TLC radiochromatography suggested that anaerobic biotransformation of atrazine by microbial isolate M91-3 involved hydroxyatrazine formation. Ring cleavage was demonstrated by ¹⁴CO₂ evolution. Denitrification was confirmed by detection of ¹⁵N₂ in headspace samples of K¹⁵NO₃-amended anaerobic liquid cultures. In aquatic sediments, mineralization of uniformly ring-labeled [¹⁴C]atrazine occurred in both M91-3-inoculated and uninoculated sediment. Inoculation of sediments with M91-3 did not significantly enhance anaerobic mineralization of atrazine as compared to uninoculated sediment, which suggests the presence of indigenous organisms capable of anaerobic atrazine biodegradation. Results of this study suggest that the use of M91-3 in a fixed-film bioreactor may have applications in the anaerobic removal of atrazine and nitrate from aqueous media. Received: 3 September 1997 / Received revision: 4 December 1997 / Accepted: 2 January 1998     

Behavioral and mortality responses of the bivalves Scrobicularia plana and Cerastoderma edule to temperature,as indicator of climate change's potential impacts

Temperature is one of the most important abiotic factors affected by climate change. It determines physiological processes, ecological patterns and establishes the limits of geographic distribution of species. The induced thermal stress frequently results in physiological and behavioral responses and, in extreme cases, may lead to mortality episodes. Scrobicularia plana and Cerastoderma edule behavioral and mortality responses to temperature were evaluated. Specimens were sampled in the Mondego estuary (Portugal), acclimated and exposed to different temperature treatments (5–35 °C). Individual activity and mortality were registered during 120 h laboratory assays. Both species showed a thermal optimum for their activity (S. plana: 15–23 °C; C. edule: 20–23 °C), and survival was mainly affected by high temperature (S. plana: LC50_120 h = 28.86 °C; C. edule: LC50_120 h = 28.01 °C), with 100% mortality above critical values (≥32 °C). Results further indicated that both species are more affected the higher the temperature and the longer the exposure time. This study indicates that the occurrence of extreme climatic events, especially heat waves, may be particularly impairing for these species.     

Chloroethene Biodegradation in Sediments at 4°C

Microbial reductive dechlorination of [1,2-¹⁴C]trichloroethene to [¹⁴C]cis-dichloroethene and [¹⁴C]vinyl chloride was observed at 4°C in anoxic microcosms prepared with cold temperature-adapted aquifer and river sediments from Alaska. Microbial anaerobic oxidation of [1,2-¹⁴C]cis-dichloroethene and [1,2-¹⁴C]vinyl chloride to ¹⁴CO₂ also was observed under these conditions.     

Alteration in cell permeability as a mechanism of action of certain quinone pesticides

The permeability of the Chlorella pyrenoidosa membrane was studied by following the efflux of ¹⁴C-intracellular material from cells which had been allowed to incorporate ¹⁴CO₂ photosynthetically. It was observed that the efflux increased upon treatment with low concentrations (3-30 μM) of 2, 3-dichloro-1, 4-naphthoquinone (dichlone), 2-amino-3-chloro-1, 4-naphthoquinone (06K-quinone), and 2, 3, 5, 6-tetrachloro-1, 4-benzoquinone (chloranil). Dichlone caused a greater loss of intracellular material than chloranil or 06K-quinone. The rate of loss as well as the total loss of ¹⁴C increased with an increase in the concentration of the quinones. In the dichlone-treated cells, the leakage was observed within 1 minute of the addition of the chemical and the effect on cell permeability was irreversible. Cells exposed to dichlone in the light or under anaerobic conditions released significantly greater amounts of ¹⁴C-material than cells treated in the dark or under aerobic conditions. The aqueous ethanol-soluble fraction of the cell was found to be the source of the released material. The proportion of the ethanol-soluble ¹⁴C that leaked out of the cell varied with the time of ¹⁴C-assimilation prior to treatment with dichlone. In the dichlone-treated cells, practically all the ¹⁴C-sucrose, alanine, glutamine, serine, and glycine leaked out, whereas glutamic, aspartic, succinic, and fumaric acids were lost only partially. Essentially no ¹⁴C-lipids were lost from the cells during dichlone treatment.     

CO2 dark fixation in the halophytic species mesembryanthemum crystallinum

The time course of ¹⁴CO₂ dark fixation was studied in leaves of the facultatively halophytic plant species Mesembryanthemum crystallinum cultivated with and without 400 mM NaCl in the nutrient medium. It is generally known from the literature that plants grown under saline conditions incorporate ¹⁴C predominately into amino acids. By contrast in leaves of M. crystallinum grown on NaCl and exposed to ¹⁴CO₂ in the dark, relatively more radioactivity is incorporated in the organic acids (especially malate) than in amino acids. The data obtained are discussed in relation to the NaCl induced Crassulacean acid metabolism in M. crystallinum reported earlier.