Localization of Nitrogen-Assimilating Enzymes in the Chloroplast of Chlamydomonas reinhardtii

Lighting the way to sustainable development: Physiological response and light control strategy in microalgae-based wastewater treatment under illumination

The Sustainable Development Goals link pollutant control with carbon dioxide reduction. Toward the goal of pollutant and carbon reduction, microalgae-based wastewater treatment (MBWT), which can simultaneously remove pollutants and convert carbon dioxide into biomass with value-added metabolites, has attracted considerable attention. The photosynthetic organism microalgae and the photobioreactor are the functional body and the operational carrier of the MBWT system, respectively; thus, light conditions profoundly influence its performance. Therefore, this review takes the general rules of how light influences the performance of MBWT systems as a starting point to elaborate the light-influenced mechanisms in microalgae and the light control strategies for photobioreactors from the inside out. Wavelength, light intensity and photoperiod solely or interactively affect biomass accumulation, pollutant removal, and value-added metabolite production in MBWT. Physiological processes, including photosynthesis, photooxidative damage, light-regulated gene expression, and nutrient uptake, essentially explain the performance influence of MBWT and are instructive for specific microalgal strain improvement strategies. In addition, light causes unique reactions in MBWT systems as it interacts with components such as photooxidative damage enhancers present in types of wastewater. In order to provide guidance for photobioreactor design and light control in a large-scale MBWT system, wavelength transformation, light transmission, light source distribution, and light-dark cycle should be considered in addition to adjusting the light source characteristics. Finally, based on current research vacancies and challenges, future research orientation should focus on the improvement of microalgae and photobioreactor, as well as the integration of both.

Glutamate dehydrogenase in "Liverworld"-A study in selected species to explore a key enzyme of plant primary metabolism in Marchantiophyta

In plants, glutamate dehydrogenase (GDH) is an ubiquitous enzyme that catalyzes the reversible amination of 2-oxoglutarate in glutamate. It contributes to both the amino acid homeostasis and the management of intracellular ammonium, and it is regarded as a key player at the junction of carbon and nitrogen assimilation pathways. To date, information about the GDH of terrestrial plants refers to a very few species only. We focused on selected species belonging to the division Marchantiophyta, providing the first panoramic overview of biochemical and functional features of GDH in liverworts. Native electrophoretic analyses showed an isoenzymatic profile less complex than what was reported for Arabidposis thaliana and other angiosperms: the presence of a single isoform corresponding to an α-homohexamer, differently prone to thermal inactivation on a species- and organ-basis, was found. Sequence analysis conducted on amino acid sequences confirmed a high similarity of GDH in modern liverworts with the GDH2 protein of A. thaliana, strengthening the hypothesis that the duplication event that gave origin to GDH1-homolog gene from GDH2 occurred after the evolutionary bifurcation that separated bryophytes and tracheophytes. Experiments conducted on Marchantia polymorpha and Calypogeia fissa grown in vitro and compared to A. thaliana demonstrated through in gel activity detection and monodimensional Western Blot that the aminating activity of GDH resulted in strongly enhanced responses to ammonium excess in liverworts as well, even if at a different extent compared to Arabidopsis and other vascular species. The comparative analysis by bi-dimensional Western Blot suggested that the regulation of the enzyme could be, at least partially, untied from the protein post-translational pattern. Finally, immuno-electron microscopy revealed that the GDH enzyme localizes at the subcellular level in both mitochondria and chloroplasts of parenchyma and is specifically associated to the endomembrane system in liverworts.

Development of a stable semi-continuous lipid production system of an oleaginous Chlamydomonas sp. mutant using multi-omics profiling

Background

Microalgal lipid production has attracted global attention in next-generation biofuel research. Nitrogen starvation, which drastically suppresses cell growth, is a common and strong trigger for lipid accumulation in microalgae. We previously developed a mutant Chlamydomonas sp. KAC1801, which can accumulate lipids irrespective of the presence or absence of nitrates. This study aimed to develop a feasible strategy for stable and continuous lipid production through semi-continuous culture of KAC1801.

Results

KAC1801 continuously accumulated > 20% lipid throughout the subculture (five generations) when inoculated with a dry cell weight of 0.8–0.9 g L⁻¹ and cultured in a medium containing 18.7 mM nitrate, whereas the parent strain KOR1 accumulated only 9% lipid. Under these conditions, KAC1801 continuously produced biomass and consumed nitrates. Lipid productivity of 116.9 mg L⁻¹ day⁻¹ was achieved by semi-continuous cultivation of KAC1801, which was 2.3-fold higher than that of KOR1 (50.5 mg L⁻¹ day⁻¹). Metabolome and transcriptome analyses revealed a depression in photosynthesis and activation of nitrogen assimilation in KAC1801, which are the typical phenotypes of microalgae under nitrogen starvation.

Conclusions

By optimizing nitrate supply and cell density, a one-step cultivation system for Chlamydomonas sp. KAC1801 under nitrate-replete conditions was successfully developed. KAC1801 achieved a lipid productivity comparable to previously reported levels under nitrogen-limiting conditions. In the culture system of this study, metabolome and transcriptome analyses revealed a nitrogen starvation-like response in KAC1801.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02196-w.

Revisiting carbon, nitrogen, and phosphorus metabolisms in microalgae for wastewater treatment

Threats posed to humans - including environmental pollution, water scarcity, food shortages, and resource crises drive a new concept to think about wastewater and its treatment. Wastewater is not only a waste but also a source of energy, renewable and/or non-renewable resources, including water itself. The nutrient in wastewater should not only be removed but also need to be upcycled. Microalgae based wastewater treatment has attracted considerable interests because algae have the potential to efficiently redirect nutrients from wastewater to the accumulated algal biomass. Additionally, microalgae are commercialized in human consumption and animal feed owing to their high content of essential amino and fatty acids, vitamins, and pigments. The whole process establishes a circular economy, totally relying on the ability of microalgae to uptake and store nutrients in wastewater, such as carbon (C), nitrogen (N), and phosphorus (P). It makes the study of the mechanisms underlying the uptake and storage of nutrients in microalgae of great interest. This review specifically aims to summarize C, N, and P metabolisms in microalgae for a better understanding of the microalgae-based wastewater treatment from the nutrient uptake pathway, and examine the key physiological factors or the operating conditions related to nutrient metabolisms that may affect the treatment efficiency. At last, I discuss the potential approaches to enhance the overall treatment performance by adjusting the critical parameters for C, N, and P metabolisms.

Application of nitrogen sufficiency conversion strategy for microalgae-based ammonium-rich wastewater treatment

Ammonium ([Formula: see text]-N)-rich wastewater, a main cause for eutrophication, can serve as a promising medium for fast microalgae cultivation with efficient [Formula: see text]-N removal. To achieve this goal, a well-controlled three-stage treatment process was developed. Two trophic modes (mixotrophy and heterotrophy) in Stage 1 and Stage 2, with two nitrogen availability conditions (N sufficient and N deprived) in Stage 2, and different [Formula: see text]-N concentrations in Stage 3 were compared to investigate the effects of nitrogen sufficiency conversion on indigenous strain UMN266 for [Formula: see text]-N removal. Results showed that mixotrophic cultures in the first two stages with N deprivation in Stage 2 was the optimum treatment strategy, and higher [Formula: see text]-N concentration in Stage 3 facilitated both microalgal growth and [Formula: see text]-N removal, with average and maximum biomass productivity of 55.3 and 161.0 mg L(-1) d(-1), and corresponding removal rates of 4.2 and 15.0 mg L(-1) d(-1), respectively, superior to previously published results. Observations of intracellular compositions confirmed the optimum treatment strategy, discovering excellent starch accumulating property of strain UMN266 as well. Combination of bioethanol production with the proposed three-stage process using various real wastewater streams at corresponding stages was suggested for future application.

Kinetic flux profiling dissects nitrogen utilization pathways in the oleaginous green alga Chlorella protothecoides

As a promising candidate for biodiesel production, the green alga Chlorella protothecoides can efficiently produce oleaginous biomass and the lipid biosynthesis is greatly influenced by the availability of nitrogen source and corresponding nitrogen assimilation pathways. Based on isotope-assisted kinetic flux profiling (KFP), the fluxes through the nitrogen utilization pathway were quantitatively analyzed. We found that autotrophic C. protothecoides cells absorbed ammonium mainly through glutamate dehydrogenase (GDH), and partially through glutamine synthetase (GS), which was the rate-limiting enzyme of nitrogen assimilation process with rare metabolic activity of glutamine oxoglutarate aminotransferase (GOGAT, also known as glutamate synthase); whereas under heterotrophic conditions, the cells adapted to GS-GOGAT cycle for nitrogen assimilation in which GS reaction rate was associated with GOGAT activity. The fact that C. protothecoides chooses the adenosine triphosphate-free and less ammonium-affinity GDH pathway, or alternatively the energy-consuming GS-GOGAT cycle with high ammonium affinity for nitrogen assimilation, highlights the metabolic adaptability of C. protothecoides exposed to altered nitrogen conditions.

Trophic mode conversion and nitrogen deprivation of microalgae for high ammonium removal from synthetic wastewater

In this study, a well-controlled three-stage process was proposed for high ammonium removal from synthetic wastewater using selected promising microalgal strain UMN266. Three trophic modes (photoautotrophy, heterotrophy, and mixotrophy), two N sufficiency conditions (N sufficient and N deprived), two inoculum modes (photoautotrophic and heterotrophic), and different NH4(+)-N concentrations were compared to investigate the effect of trophic mode conversion and N deprivation on high NH4(+)-N removal by UMN266. Results showed that photoautotrophic inoculum with trophic mode conversion from heterotrophy to photoautotrophy and N deprivation in Stage 2 turned was the optimum plan for NH4(+)-N removal, and average removal rates were 12.4 and 19.1mg/L/d with initial NH4(+)-N of 80 and 160mg/L in Stage 3. Mechanism investigations based on algal biomass carbon (C) and N content, cellular composition, and starch content confirmed the above optimum plan and potential of UMN266 as bioethanol feedstock.

Understanding nitrate assimilation and its regulation in microalgae

Nitrate assimilation is a key process for nitrogen (N) acquisition in green microalgae. Among Chlorophyte algae, Chlamydomonas reinhardtii has resulted to be a good model system to unravel important facts of this process, and has provided important insights for agriculturally relevant plants. In this work, the recent findings on nitrate transport, nitrate reduction and the regulation of nitrate assimilation are presented in this and several other algae. Latest data have shown nitric oxide (NO) as an important signal molecule in the transcriptional and posttranslational regulation of nitrate reductase and inorganic N transport. Participation of regulatory genes and proteins in positive and negative signaling of the pathway and the mechanisms involved in the regulation of nitrate assimilation, as well as those involved in Molybdenum cofactor synthesis required to nitrate assimilation, are critically reviewed.

Whole-Genome Resequencing Reveals Extensive Natural Variation in the Model Green Alga Chlamydomonas reinhardtii

We performed whole-genome resequencing of 12 field isolates and eight commonly studied laboratory strains of the model organism Chlamydomonas reinhardtii to characterize genomic diversity and provide a resource for studies of natural variation. Our data support previous observations that Chlamydomonas is among the most diverse eukaryotic species. Nucleotide diversity is ∼3% and is geographically structured in North America with some evidence of admixture among sampling locales. Examination of predicted loss-of-function mutations in field isolates indicates conservation of genes associated with core cellular functions, while genes in large gene families and poorly characterized genes show a greater incidence of major effect mutations. De novo assembly of unmapped reads recovered genes in the field isolates that are absent from the CC-503 assembly. The laboratory reference strains show a genomic pattern of polymorphism consistent with their origin as the recombinant progeny of a diploid zygospore. Large duplications or amplifications are a prominent feature of laboratory strains and appear to have originated under laboratory culture. Extensive natural variation offers a new source of genetic diversity for studies of Chlamydomonas, including naturally occurring alleles that may prove useful in studies of gene function and the dissection of quantitative genetic traits.

Glutamine Assimilation and Feedback Regulation of L-acetyl-N-glutamate Kinase Activity in Chlorella variabilis NC64A Results in Changes in Arginine Pools

Glutamine is a metabolite of central importance in nitrogen metabolism of microorganisms and plants. The Chlorella PII signaling protein controls, in a glutamine-dependent manner, the key enzyme of the ornithine/arginine biosynthesis pathway, N-acetyl-L-glutamate kinase (NAGK) that leads to arginine formation. We provide evidence that glutamine promotes effective growth of C. variabilis strain NC64A. The present study shows that externally supplied glutamine directly influences the internal pool of arginine in NC64A. Glutamine synthetase (GS) catalyzes the ATP-dependent conversion of glutamate and ammonium to glutamine. The results of this study demonstrate that glutamine acts as a negative effector of GS activity. These data emphasize the importance of glutamine-dependent coupling of metabolism and signaling as components of an efficient pathway allowing the maintenance of metabolic homeostasis and sustaining growth of Chlorella.

The effect of glufosinate on nitrogen assimilation at the physiological, biochemical and molecular levels in Phaeodactylum tricornutum

This study investigated the effects of glufosinate, a widely used herbicide, on the marine diatom Phaeodactylum tricornutum through short-term toxicity tests at the physiological and gene transcriptional levels. Glufosinate (4 mg L(-1)) decreased the amount of pigments but increased reactive oxygen species (ROS) and malondialdehyde levels. As a glutamine synthetase (GS) inhibitor, glufosinate affected the transcripts and activities of key enzymes related to nitrogen assimilation. Transcript levels of GS and nitrate reductase (NR) in P. tricornutum decreased to only 57 and 26 % of the control. However, transcript levels of nitrate transporter (NRT) and the small subunit of glutamate synthase (GltD) were 1.79 and 1.76 times higher than that of the control. The activities of NRT, GS and GOGAT were consistent with gene expression except for NR, which was regulated mainly by post-translational modification. Furthermore, the results of electron microscopy showed that chloroplast structure was disrupted in response to glufosinate exposure. These results demonstrated that glufosinate first disturbed nitrogen metabolism and caused a ROS burst, which disrupted chloroplast ultrastructure. Ultimately, the growth of P. tricornutum was greatly inhibited by glufosinate.

Nitrogen-Sparing Mechanisms in Chlamydomonas Affect the Transcriptome, the Proteome, and Photosynthetic Metabolism

Nitrogen (N) is a key nutrient that limits global primary productivity; hence, N-use efficiency is of compelling interest in agriculture and aquaculture. We used Chlamydomonas reinhardtii as a reference organism for a multicomponent analysis of the N starvation response. In the presence of acetate, respiratory metabolism is prioritized over photosynthesis; consequently, the N-sparing response targets proteins, pigments, and RNAs involved in photosynthesis and chloroplast function over those involved in respiration. Transcripts and proteins of the Calvin-Benson cycle are reduced in N-deficient cells, resulting in the accumulation of cycle metabolic intermediates. Both cytosolic and chloroplast ribosomes are reduced, but via different mechanisms, reflected by rapid changes in abundance of RNAs encoding chloroplast ribosomal proteins but not cytosolic ones. RNAs encoding transporters and enzymes for metabolizing alternative N sources increase in abundance, as is appropriate for the soil environmental niche of C. reinhardtii. Comparison of the N-replete versus N-deplete proteome indicated that abundant proteins with a high N content are reduced in N-starved cells, while the proteins that are increased have lower than average N contents. This sparing mechanism contributes to a lower cellular N/C ratio and suggests an approach for engineering increased N-use efficiency.

Amino Acid transporter inventory of the selaginella genome

Amino acids play fundamental roles in a multitude of functions including protein synthesis, hormone metabolism, nerve transmission, cell growth, production of metabolic energy, nucleobase synthesis, nitrogen metabolism, and urea biosynthesis. Selaginella as a member of the lycophytes is part of an ancient lineage of vascular plants that had arisen ∼400 million years ago. In angiosperms, which have attracted most of the attention for nutrient transport so far, we have been able to identify many of the key transporters for nitrogen. Their role is not always fully clear, thus an analysis of Selaginella as a representative of an ancient vascular plant may help shed light on the evolution and function of these diverse transporters. Here we annotated and analyzed the genes encoding putative transporters involved in cellular uptake of amino acids present in the Selaginella genome.

Proteomic analysis of hydrogen photoproduction in sulfur-deprived Chlamydomonas cells

The green alga Chlamydomonas reinhardtii is a model organism to study H(2) metabolism in photosynthetic eukaryotes. To understand the molecular mechanism of H(2) metabolism, we used 2-DE coupled with MALDI-TOF and MALDI-TOF/TOF-MS to investigate proteomic changes of Chlamydomonas cells that undergo sulfur-depleted H(2) photoproduction process. In this report, we obtained 2-D PAGE soluble protein profiles of Chlamydomonas at three time points representing different phases leading to H(2) production. We found over 105 Coomassie-stained protein spots, corresponding to 82 unique gene products, changed in abundance throughout the process. Major changes included photosynthetic machinery, protein biosynthetic apparatus, molecular chaperones, and 20S proteasomal components. A number of proteins related to sulfate, nitrogen and acetate assimilation, and antioxidative reactions were also changed significantly. Other proteins showing alteration during the sulfur-depleted H(2) photoproduction process were proteins involved in cell wall and flagella metabolisms. In addition, among these differentially expressed proteins, 11 were found to be predicted proteins without functional annotation in the Chlamydomonas genome database. The results of this proteomic analysis provide new insight into molecular basis of H(2) photoproduction in Chlamydomonas under sulfur depletion.

Plasticity of the mitoproteome to nitrogen sources (nitrate and ammonium) in Chlamydomonas reinhardtii: the logic of Aox1 gene localization

Nitrate and ammonium constitute primary inorganic nitrogen sources that can be incorporated into carbon skeletons in photosynthetic eukaryotes. In Chlamydomonas, previous studies and the present one showed that the mitochondrial AOX is up-regulated in nitrate-grown cells in comparison with ammonium-grown cells. In this work, we have performed a comparative proteomic analysis of the soluble mitochondrial proteome of Chlamydomonas cells growth either on nitrate or ammonium. Our results highlight important proteomics modifications mostly related to primary metabolism in cells grown on nitrate. We could note an up-regulation of some TCA cycle enzymes and a down-regulation of cytochrome c1 together with an up-regulation of l-arginine and purine catabolism enzymes and of ROS scavenging systems. Hence, in nitrate-grown cells, AOX may play a dual role: (1) lowering the ubiquinone pool reduction level and (2) permitting the export of mitochondrial reducing power under the form of malate for nitrate and nitrite reduction. This role of AOX in the mitochondrial plasticity makes logical the localization of Aox1 in a nitrate assimilation gene cluster.

Cultivation of green algae Chlorella sp. in different wastewaters from municipal wastewater treatment plant

The objective of this study was to evaluate the growth of green algae Chlorella sp. on wastewaters sampled from four different points of the treatment process flow of a local municipal wastewater treatment plant (MWTP) and how well the algal growth removed nitrogen, phosphorus, chemical oxygen demand (COD), and metal ions from the wastewaters. The four wastewaters were wastewater before primary settling (#1 wastewater), wastewater after primary settling (#2 wastewater), wastewater after activated sludge tank (#3 wastewater), and centrate (#4 wastewater), which is the wastewater generated in sludge centrifuge. The average specific growth rates in the exponential period were 0.412, 0.429, 0.343, and 0.948 day(-1) for wastewaters #1, #2, #3, and #4, respectively. The removal rates of NH4-N were 82.4%, 74.7%, and 78.3% for wastewaters #1, #2, and #4, respectively. For #3 wastewater, 62.5% of NO3-N, the major inorganic nitrogen form, was removed with 6.3-fold of NO2-N generated. From wastewaters #1, #2, and #4, 83.2%, 90.6%, and 85.6% phosphorus and 50.9%, 56.5%, and 83.0% COD were removed, respectively. Only 4.7% was removed in #3 wastewater and the COD in #3 wastewater increased slightly after algal growth, probably due to the excretion of small photosynthetic organic molecules by algae. Metal ions, especially Al, Ca, Fe, Mg, and Mn in centrate, were found to be removed very efficiently. The results of this study suggest that growing algae in nutrient-rich centrate offers a new option of applying algal process in MWTP to manage the nutrient load for the aeration tank to which the centrate is returned, serving the dual roles of nutrient reduction and valuable biofuel feedstock production.

MACRONUTRIENT UTILIZATION BY PHOTOSYNTHETIC EUKARYOTES AND THE FABRIC OF INTERACTIONS

Organisms acclimate to a continually fluctuating nutrient environment. Acclimation involves responses specific for the limiting nutrient as well as responses that are more general and occur when an organism experiences different stress conditions. Specific responses enable organisms to efficiently scavenge the limiting nutrient and may involve the induction of high-affinity transport systems and the synthesis of hydrolytic enzymes that facilitate the release of the nutrient from extracellular organic molecules or from internal reserves. General responses include changes in cell division rates and global alterations in metabolic activities. In photosynthetic organisms there must be precise regulation of photosynthetic activity since when severe nutrient limitation prevents continued cell growth, excitation of photosynthetic pigments could result in the formation of reactive oxygen species, which can severely damage structural and functional features of the cell. This review focuses on ways that photosynthetic eukaryotes assimilate the macronutrients nitrogen, sulfur, and phosphorus, and the mechanisms that govern assimilatory activities. Also discussed are molecular responses to macronutrient limitation and the elicitation of those responses through integration of environmental and cellular cues.

Acclimation of Chlamydomonas reinhardtii to its nutrient environment

To cope with low nutrient availability in nature, organisms have evolved inducible systems that enable them to scavenge and efficiently utilize the limiting nutrient. Furthermore, organisms must have the capacity to adjust their rate of metabolism and make specific alterations in metabolic pathways that favor survival when the potential for cell growth and division is reduced. In this article I will focus on the acclimation of Chlamydomonas reinhardtii, a unicellular, eukaryotic green alga to conditions of nitrogen, sulfur and phosphorus deprivation. This organism has a distinguished history as a model for classical genetic analyses, but it has recently been developed for exploitation using an array of molecular and genomic tools. The application of these tools to the analyses of nutrient limitation responses (and other biological processes) is revealing mechanisms that enable Chlamydomonas to survive harsh environmental conditions and establishing relationships between the responses of this morphologically simple, photosynthetic eukaryote and those of both nonphotosynthetic organisms and vascular plants.

Isolation and characterization of glutamine synthetase genes in Chlamydomonas reinhardtii

To elucidate the role of glutamine synthetase (GS) in nitrogen assimilation in the green alga Chlamydomonas reinhardtii we used maize GS1 (the cytosolic form) and GS2 (the chloroplastic form) cDNAs as hybridization probes to isolate C. reinhardtii cDNA clones. The amino acid sequences derived from the C. reinhardtii clones have extensive homology with GS enzymes from higher plants. A putative amino-terminal transit peptide encoded by the GS2 cDNA suggests that the protein localizes to the chloroplast. Genomic DNA blot analysis indicated that GS1 is encoded by a single gene, whereas two genomic fragments hybridized to the GS2 cDNA probe. All GS2 cDNA clones corresponded to only one of the two GS2 genomic sequences. We provide evidence that ammonium, nitrate, and light regulate GS transcript accumulation in green algae. Our results indicate that the level of GS1 transcripts is repressed by ammonium but induced by nitrate. The level of GS2 transcripts is not affected by ammonium or nitrate. Expression of both GS1 and GS2 genes is regulated by light, but perhaps through different mechanisms. Unlike in higher plants, no decreased level of GS2 transcripts was detected when cells were grown under conditions that repress photorespiration. Analysis of GS transcript levels in mutants with defects in the nitrate assimilation pathway show that nitrate assimilation and ammonium assimilation are regulated independently.

Toxicity of and mutagenesis by chlorate are independent of nitrate reductase activity in Chlamydomonas reinhardtii

Spontaneous chlorate-resistant (CR) mutants have been isolated from Chlamydomonas reinhardtii wild-type strains. Most of them, 244, were able to grow on nitrate minimal medium, but 23 were not. Genetic and in vivo complementation analyses of this latter group of mutants indicated that they were defective either at the regulatory locus nit-2, or at the nitrate reductase (NR) locus nit-1, or at very closely linked loci. Some of these nit-1 or nit-2 mutants were also defective in pathways not directly related to nitrate assimilation, such as those of amino acids and purines. Chlorate treatment of wild-type cells resulted in both a decrease in cell survival and an increase in mutant cells resistant to a number of different chemicals (chlorate, methylammonium, sulphanilamide, arsenate, and streptomycin). The toxic and mutagenic effects of chlorate in minimal medium were not found when cells were grown either in darkness or in the presence of ammonium, conditions under which nitrate uptake is drastically inhibited. Chlorate was also able to induce reversion of nit- mutants of C. reinhardtii, but failed to produce His+ revertants or Arar mutants in the BA-13 strain of Salmonella typhimurium. In contrast, chlorate treatment induced mutagenesis in strain E1F1 of the phototrophic bacterium Rhodobacter capsulatus. Genetic analyses of nitrate reductase-deficient CR mutants of C. reinhardtii revealed two types of CR, to low (1.5 mM) and high (15 mM) chlorate concentrations. These two traits were recessive in heterozygous diploids and segregated in genetic crosses independently of each other and of the nit-1 and nit-2 loci. Three hcr loci and four lcr loci mediating resistance to high (HC) and low (LC) concentrations of chlorate were identified. Mutations at the nit-2 locus, and deletions of a putative locus for nitrate transport were always epistatic to mutations responsible for resistance to either LC or HC. In both nit+ and nit- chlorate-sensitive (CS) strains, nitrate and nitrite gave protection from the toxic effect of chlorate. Our data indicate that in C. reinhardtii chlorate toxicity is primarily dependent on the nitrate transport system and independent of the existence of an active NR enzyme. At least seven loci unrelated to the nitrate assimilation pathway and mediating CR are thought to control indirectly the efficiency of the nitrate transporter for chlorate transport. In addition, chlorate appears to be a mutagen capable of inducing a wide range of mutations unrelated to the nitrate assimilation pathway.

Evidence for Activation of the Oxidative Pentose Phosphate Pathway during Photosynthetic Assimilation of NO(3) but Not NH(4) by a Green Alga

Addition of NO(3) (-) to N-limited Selenastrum minutum during photosynthesis resulted in an immediate drop in the NADPH/NADP ratio and a slower increase of the NADH/NAD ratio. These changes were accompanied by a rapid decrease in glucose-6-phosphate and increase in 6-phosphogluconate, indicating activation of glucose-6-phosphate dehydrogenase and a role for the oxidation pentose phosphate pathway during photosynthetic NO(3) (-) assimilation. In contrast, the short-term changes in pyridine nucleotides and metabolites during photosynthetic assimilation of NH(4) (+) were not consistent with a stimulation of the oxidative pentose phosphate pathway.

Intracellular Localization of Three l-Glutamate Dehydrogenase Isozymes from Chlamydomonas reinhardtii

The intracellular localization of the activity and synthesis of three isozymes of NAD(P)(+)-glutamate dehydrogenase from the unicellular green alga Chlamydomonas reinhardtii cw-92 has been established. Isozyme activities have been located within mitochondria by using differential centrifugation techniques and discontinuous Percoll gradient separations. Experiments with protein synthesis inhibitors cycloheximide, rifampicin, chloramphenicol, and actinomycin D, under dark and carbon starvation conditions, revealed that synthesis of the three isozymes was likely to occur in cytosol as precursor proteins that are then transported and processed inside the mitochondria.

Immunocytochemical localization of nitrite reductase in green algae

The distribution of nitrite reductase (EC 1.7.7.1) in the green algae Chlamydomonas reinhardtii, Monoraphidium braunii, Chlorella fusca, and Scenedesmus obliquus was studied by immunoelectron microscopy. The labeling of ultrathin cryosections was performed with anti-nitrite reductase antibodies followed by gold-labeled goat anti-rabbit antibodies. In C. reinhardtii sections, gold label was mainly associated with the pyrenoid, tonoplast, and plasmalemma. Significant labeling was also detected in the thylakoid region. In all other organisms, label density was lower but distributed in the same locations, except that the plasmalemma of S. obliquus was not significantly labeled. From estimates of the relative volume of different cell regions, we found that approximately 80% of the total enzyme is located in the chloroplastic region (thylakoids plus pyrenoid) of C. reinhardtii, M. braunii, and C. fusca, and 97% in the case of S. obliquus.

Regulation of nitrite uptake and nitrite reductase expression in Chlamydomonas reinhardtii

Expression of nitrite uptake and nitrite reductase activities has been studied in Chlamydomonas reinhardtii under different nutritional conditions. Both activities were expressed at a low level in derepressed cells (with no nitrogen source) and at a high level in induced cells (with nitrate or nitrite). Nitrate was required for both activities to be maximally expressed. Ammonium-grown cells did not show nitrite uptake capability and had a basal nitrite reductase activity. Nitrite uptake but not nitrite reductase levels decreased very significantly in nitrate-induced cells subject to cycloheximide treatment, which suggests that protein(s) involved in the uptake are under a rapid turnover. Nitrite uptake expression was strongly inhibited by the presence of the glutamine synthetase inhibitor L-methionine-D,L-sulfoximine under either derepression or induction conditions, whereas that of nitrite reductase was not affected under the same conditions. Our results indicate that nitrite uptake expression is regulated primarily by ammonium, and that of nitrite reductase by both ammonium and ammonium derivative(s).

Blue Light, a Positive Switch Signal for Nitrate and Nitrite Uptake by the Green Alga Monoraphidium braunii

Blue light was shown to regulate the utilization of oxidized nitrogen sources by green algae, both by activating nitrate reductase and promoting nitrite reductase biosysnthesis (MA Quiñones, PJ Aparicio [1990] Inorganic Nitrogen in Plants and Microorganisms, Springer-Verlag, Berlin, pp 171-177; MA Quiñones, PJ Aparicio [1990] Photochem Photobiol 51: 681-692). The data reported herein show that, when cells of Monoraphidium braunii at pH 8, containing both active nitrate reductase and nitrite reductase, were sparged with CO(2)-free air and irradiated with strong background red light, they took up oxidized nitrogen sources only when PAR comprised blue light. The activation of the transport system(s) of either both nitrate and nitrite was very quick and elicited by low irradiance blue light. In fact, blue light appears to act as a switch signal from the environment, since the uptake of these anions immediately ceased when this radiation was turned off. The requirement of blue light for nitrate uptake was independent of the availability of CO(2) to cells. However, cells under high CO(2) tensions, although they showed an absolute blue light requirement to initially establish the uptake of nitrite, as they gained carbon skeletons to allocate ammonia, gradually increased their nitrite uptake rates in the subsequent red light intervals. Under CO(2)-free atmosphere, cells irradiated with strong background red light of 660 nanometers only evolved oxygen when they were additionally irradiated with low irradiance blue light and either nitrate or nitrite was present in the media to provide electron acceptors for the photosynthetic reaction.

A Plastidial Localization and Origin of l-Glutamate Dehydrogenase in a Soybean Cell Culture

The subcellular distribution of l-glutamate dehydrogenase (GDH, EC 1.4.1.3.) was studied in SB3 soybean (Glycine max) cells using subcellular fractionation techniques. Compounds that inhibit protein synthesis either on 80s or 70s ribosomes were also used to give a preliminary idea of which subcellular fraction is involved in GDH synthesis. It was found that whereas cycloheximide and puromycin considerably reduced the total amount of protein synthesized by the cells, they did not appear to inhibit the synthesis of GDH. In the presence of chloramphenicol, both GDH activity and protein level in the cells were considerably reduced, suggesting that this enzyme was synthesized in organelles and not in the cytosol. Streptomycin, which inhibits plastid protein synthesis, also inhibited synthesis of GDH, indicating that a fraction of GDH activity was plastidial in origin. This is supported by the data on subcellular distribution of the enzyme, which showed that a major fraction of GDH is found in the plastidial fraction, although some activity is found associated with the mitochondrial fraction also. Since a major fraction of GDH activity was found in the plastidial fraction, we studied protein synthesis using isolated plastids and (35)S-methionine. Using antibodies raised against purified GDH, we identified a (35)S-labeled 41-kilodalton polypeptide synthesized by plastids as GDH.

Regulation of Carbon Partitioning to Respiration during Dark Ammonium Assimilation by the Green Alga Selenastrum minutum

The assimilation of NH(4) (+) causes a rapid increase in respiration to provided carbon skeletons for amino acid synthesis. In this study we propose a model for the regulation of carbon partitioning from starch to respiration and N assimilation in the green alga Selenastrum minutum. We provide evidence for both a cytosolic and plastidic fructose-1,6-bisphosphatase. The cytosolic form is inhibited by AMP and fructose-1,6-bisphosphate and the plastidic form is inhibited by phosphate. There is only one ATP dependent phosphofructokinase which, based on immunological cross reactivity, has been identified as being localized in the plastid. It is inhibited by phosphoenolpyruvate and activated by phosphate. No pyrophosphate dependent phosphofructokinase was found. The initiation of dark ammonium assimilation resulted in a transient increase in ADP which releases pyruvate kinase from adenylate control. This activation of pyruvate kinase causes a rapid 80% drop in phosphoenolpyruvate and a 2.7-fold increase in pyruvate. The pyruvate kinase mediated decrease in phosphoenolpyruvate correlates with the activation of the ATP dependent phosphofructokinase increasing carbon flow through the upper half of glycolysis. This increased the concentration of triosephosphate and provided substrate for pyruvate kinase. It is suggested that this increase in triosephosphate coupled with the glutamine synthetase mediated decline in glutamate, serves to maintain pyruvate kinase activation once ADP levels recover. The initiation of NH(4) (+) assimilation causes a transient 60% increase in fructose-2,6-bisphosphate. Given the sensitivity of the cytosolic fructose-1,6-bisphosphatase to this regulator, its increase would serve to inhibit cytosolic gluconeogenesis and direct the triosephosphate exported from the plastid down glycolysis to amino acid biosynthesis.