Nitrogen Metabolism of the Marine Microalga Chlorella autotrophica

High-affinity nitrate/nitrite transporter genes (Nrt2) in Tisochrysis lutea: identification and expression analyses reveal some interesting specificities of Haptophyta microalgae

Microalgae have a diversity of industrial applications such as feed, food ingredients, depuration processes and energy. However, microalgal production costs could be substantially improved by controlling nutrient intake. Accordingly, a better understanding of microalgal nitrogen metabolism is essential. Using in silico analysis from transcriptomic data concerning the microalgae Tisochrysis lutea, four genes encoding putative high-affinity nitrate/nitrite transporters (TlNrt2) were identified. Unlike most of the land plants and microalgae, cloning of genomic sequences and their alignment with complementary DNA (cDNA) sequences did not reveal the presence of introns in all TlNrt2 genes. The deduced TlNRT2 protein sequences showed similarities to NRT2 proteins of other phyla such as land plants and green algae. However, some interesting specificities only known among Haptophyta were also revealed, especially an additional sequence of 100 amino acids forming an atypical extracellular loop located between transmembrane domains 9 and 10 and the function of which remains to be elucidated. Analyses of individual TlNrt2 gene expression with different nitrogen sources and concentrations were performed. TlNrt2.1 and TlNrt2.3 were strongly induced by low NO3 (-) concentration and repressed by NH4 (+) substrate and were classified as inducible genes. TlNrt2.2 was characterized by a constitutive pattern whatever the substrate. Finally, TlNrt2.4 displayed an atypical response that was not reported earlier in literature. Interestingly, expression of TlNrt2.4 was rather related to internal nitrogen quota level than external nitrogen concentration. This first study on nitrogen metabolism of T. lutea opens avenues for future investigations on the function of these genes and their implication for industrial applications.

Acclimation and toxicity of high ammonium concentrations to unicellular algae

A literature review on the effects of high ammonium concentrations on the growth of 6 classes of microalgae suggests the following rankings. Mean optimal ammonium concentrations were 7600, 2500, 1400, 340, 260, 100 μM for Chlorophyceae, Cyanophyceae, Prymnesiophyceae, Diatomophyceae, Raphidophyceae, and Dinophyceae respectively and their tolerance to high toxic ammonium levels was 39,000, 13,000, 2300, 3600, 2500, 1200 μM respectively. Field ammonium concentrations <100 μM would not likely reduce the growth rate of most microalgae. Chlorophytes were significantly more tolerant to high ammonium than diatoms, prymnesiophytes, dinoflagellates, and raphidophytes. Cyanophytes were significantly more tolerant than dinoflagellates which were the least tolerant. A smaller but more complete data set was used to estimate ammonium EC₅₀ values, and the ranking was: Chlorophyceae>Cyanophyceae, Dinophyceae, Diatomophyceae, and Raphidophyceae. Ammonia toxicity is mainly attributed to NH₃ at pHs >9 and at pHs <8, toxicity is likely associated with the ammonium ion rather than ammonia.

Molecular evolution of glutamine synthetase II: Phylogenetic evidence of a non-endosymbiotic gene transfer event early in plant evolution

BACKGROUND

Glutamine synthetase (GS) is essential for ammonium assimilation and the biosynthesis of glutamine. The three GS gene families (GSI, GSII, and GSIII) are represented in both prokaryotic and eukaryotic organisms. In this study, we examined the evolutionary relationship of GSII from eubacterial and eukaryotic lineages and present robust phylogenetic evidence that GSII was transferred from gamma-Proteobacteria (Eubacteria) to the Chloroplastida.

RESULTS

GSII sequences were isolated from four species of green algae (Trebouxiophyceae), and additional green algal (Chlorophyceae and Prasinophytae) and streptophyte (Charales, Desmidiales, Bryophyta, Marchantiophyta, Lycopodiophyta and Tracheophyta) sequences were obtained from public databases. In Bayesian and maximum likelihood analyses, eubacterial (GSIIB) and eukaryotic (GSIIE) GSII sequences formed distinct clades. Both GSIIB and GSIIE were found in chlorophytes and early-diverging streptophytes. The GSIIB enzymes from these groups formed a well-supported sister clade with the gamma-Proteobacteria, providing evidence that GSIIB in the Chloroplastida arose by horizontal gene transfer (HGT). Bayesian relaxed molecular clock analyses suggest that GSIIB and GSIIE coexisted for an extended period of time but it is unclear whether the proposed HGT happened prior to or after the divergence of the primary endosymbiotic lineages (the Archaeplastida). However, GSIIB genes have not been identified in glaucophytes or red algae, favoring the hypothesis that GSIIB was gained after the divergence of the primary endosymbiotic lineages. Duplicate copies of the GSIIB gene were present in Chlamydomonas reinhardtii, Volvox carteri f. nagariensis, and Physcomitrella patens. Both GSIIB proteins in C. reinhardtii and V. carteri f. nagariensis had N-terminal transit sequences, indicating they are targeted to the chloroplast or mitochondrion. In contrast, GSIIB proteins of P. patens lacked transit sequences, suggesting a cytosolic function. GSIIB sequences were absent in vascular plants where the duplication of GSIIE replaced the function of GSIIB.

CONCLUSIONS

Phylogenetic evidence suggests GSIIB in Chloroplastida evolved by HGT, possibly after the divergence of the primary endosymbiotic lineages. Thus while multiple GS isoenzymes are common among members of the Chloroplastida, the isoenzymes may have evolved via different evolutionary processes. The acquisition of essential enzymes by HGT may provide rapid changes in biochemical capacity and therefore be favored by natural selection.

Heterologous expression of cDNAs encoding Chlorella sorokiniana NADP-specific glutamate dehydrogenase wild-type and mutant subunits in Escherichia coli cells and comparison of kinetic and thermal stability properties of their homohexamers

Full-length cDNAs encoding the alpha- and beta-subunits and a truncated mutant subunit of the Chlorella sorokiniana NADP-GDH isozymes were constructed and expressed in Escherichia coli cells. The kinetic and thermal stability properties of the resultant homohexamers were examined. The electrophoretic mobility of the recombinant alpha- and beta-subunits was identical to that of the native subunits as determined by immunoblotting. The homohexamers were purified by anion-exchange and gel-filtration chromatography. The alpha- and beta-homohexamers that were synthesized in the bacterial cells were shown to have similar Michaelis constants for their substrates as previously shown after synthesis in C. sorokiniana cells (Bascomb and Schmidt, 1987). The alpha homohexamer synthesized in the bacterium was allosteric with respect to NADPH but to a lesser degree than when isolated from the alga. The mutant homohexamer was composed of subunits that were truncated by 40 amino acids at their N-termini. This mutant isozyme was kinetically similar to the larger, anabolic alpha-homohexamer, but it did not display the allosteric response to NADPH shown by the alpha-homohexamer. The three isozymes had significant thermal tolerance and were stable at 50 degrees C. The temperature optimum for catalytic activity for the alpha- and beta-homohexamers was 60 degrees C, and 65 degrees C for the delta40N homohexamer. This study demonstrated that most of the kinetic properties of the Chlorella sorokiniana NADP-GDH isozymes were retained after their synthesis in a heterologous system, and that the distinctive N-terminal domains of these isozymes have dramatic effects on their biochemical characteristics.

Inorganic nitrogen uptake by symbiotic marine cnidarians: a critical review

The physiological data support host involvement in net ammonium uptake by intact symbioses. The evidence for nitrate assimilation by intact symbioses is equivocal. The depletion-diffusion model can account for net ammonium uptake by intact symbioses, but is inadequate to account for phosphate or nitrate uptake by symbioses. There is no evidence for nitrogen limitation as the means by which the host regulates algal growth in symbiosis; phosphorus limitation appears to be more likely.

Purification and properties of three NAD(P)+ isozymes of L-glutamate dehydrogenase of Chlamydomonas reinhardtii

Three isozymes of glutamate dehydrogenase (GDH) of Chlamydomonas reinhardtii, induced under different trophic and stress conditions, have been purified about 800-1000-fold to electrophoretic homogeneity. They are hexamers of Mr 266,000-269,000 as deduced from gel filtration and sedimentation coefficient data. GDH1 consisted of six identical subunits of 44 kDa each, whereas both GDH2 and GDH3 consisted of six similar-sized monomers (4 of 44 kDa and 2 of 46 kDa). Optimum pH for the three activities with each pyridine nucleotide was identical (8.5 with NADH; 7.7 with NADPH; and 9.0 with NAD+). The isozymes exhibited similar high optimum temperature values (60-62 degrees C) and isoelectric points (7.9-8.1). Activity was enhanced in vitro by Ca2+ ions and strongly inhibited by pyridoxal 5'-phosphate, KCN, o-phenanthroline and EDTA, and to a lesser extent by pHMB and methylacetimidate. In the aminating reaction the three isozymes were inhibited in a concentration-dependent process by both NADH and NADPH, with apparent Km values for NH4+ ranging from 13-53 mM; 0.36-1.85 mM for 2-oxoglutarate and 0.07-0.78 mM for NADH and NADPH. In the deaminating reaction apparent Km values ranged from 0.64-3.52 mM for L-glutamate and 0.20-0.32 for NAD+. In addition, the three isozymes exhibited a non-hyperbolic kinetics for NAD+ with negative cooperativity (n = 0.8).

Nitrogen metabolism in the ectomycorrhizal fungus Laccaria bicolor (R. Mre.) Orton

Blended colonies of the ectomycorrhizal fungus, Laccaria bicolor (R. Mre.) Orton, grew axenically as a suspension of fine hyphae in a defined buffered medium with glucose (doubling time 1-2 d) but not acetate as the carbon source, and either ammonium or nitrate as nitrogen sources. A number of amino acids were found to be excellent nitrogen sources for this basidiomycete, but were less effective as sources of carbon. During post-exponential growth in medium containing inorganic nitrogen, the fungal symbiont released amino acids. L. bicolor has the enzymatic potential to assimilate ammonium by the activities of glutamine synthetase, NADH-glutamate dehydrogenase and NADPH-glutamate dehydrogenase. It also contains highly active aspartate and alanine aminotransferases. The activities of glutamine synthetase, NADPH-glutamate dehydrogenase and aspartate aminotransferase were greater in the presence of nitrate than in the presence of ammonium and declined as the culture aged, suggesting a biosynthetic role for these enzymes. In contrast, the activities of NADH-glutamate dehydrogenase and alanine aminotransferase increased during post-exponential growth, and also in cultures growing on amino acids as a carbon source, suggesting a catabolic role for these enzymes.

Regulation of Chloroplast Development by Nitrogen Source and Growth Conditions in a Chlorella protothecoides Strain

Chlorella strain (UTEX 27) maintains optimal photosynthetic capacity when growing photoautotrophically in the presence of ammonium. Nitrate-grown photoautotrophic cells, however, show a drastic loss of chlorophyll content and ribulose-1,6-bisphosphate carboxylase/oxygenase activity, resulting in a greater than 10-fold decrease in photosynthetic capacity and growth rate. Nitrate-grown cells are not deficient in protein content, and under mixotrophic and heterotrophic conditions, the alga can utilize nitrate as well as it does ammonium. The alga metabolizes both glucose and acetate in the dark with a doubling time of 5 to 6 hours. However, its growth on acetate is inhibited by light. Ribulose-1,6-biphosphate carboxylase/oxygenase activity correlates well with photosynthetic capacity, and glucose 6-phosphate dehydrogenase and hexokinase activities are altered in a manner consistent with the availability of glucose in growing cells. The alga appears to assimilate ammonium under photoautotrophic conditions primarily via the glutamine synthetase pathway, and shows an induction of both NADH and NADPH dependent glutamate dehydrogenase pathways under mixotrophic and heterotrophic conditions. Multiple isoforms are present only for hexokinase and glucose 6-phosphate dehydrogenase. Etiolated nitrate-grown cells resume greening and increase their photosynthetic capacity after about 6 hours of incubation in the presence of ammonium under photoautotrophic conditions. Similarly, the loss of photosynthetic capacity in ammonium-grown photoautotrophic cells commence about 9 hours after their transfer to heterotrophic nitrate containing media.

The Relationship between Inorganic Nitrogen Metabolism and Proline Accumulation in Osmoregulatory Responses of Two Euryhaline Microalgae

Chlorella autotrophica, a euryhaline marine alga, and Stichococcus bacillaris, a salt-tolerant soil alga, grow in the presence of methionine sulfoximine (MSX), an inhibitor of glutamine synthetase, by maintaining high levels of NADPH-glutamate dehydrogenase. Nitrate reductase showed no change in MSX-adapted cells. For both species, MSX-adapted cells retained their capacity to accumulate proline in response to salinity, and in S. bacillaris no major shift was observed in the presence of MSX toward the accumulation of sorbitol. Following transfer from 33 to 150% artificial seawater (ASW), both algae exhibited increases in organic solute levels without a lag. Within 6 h of this sudden increase in salinity, the levels of proline in C. autotrophica and of proline and sorbitol in S. bacillaris were similar to those found in steady state 150% ASW cultures. Following transfer from 33 to 150% ASW, S. bacillaris continued [(14)C] bicarbonate photoassimilation at a normal rate and maintained active enzymes of nitrogen assimilation. The incorporation of [(14)C]phenylalanine into proteins was inhibited for about 30 minutes in MSX-free cells and 90 minutes in MSX-adapted cells following transfer from 33 to 150% ASW; the recovery after these lag periods was almost complete.

Glutamine Synthetase Isoenzymes in the Green Soil Alga Stichococcus bacillaris Naeg

Two forms of glutamine synthetase (GS(1) and GS(2)) have been separated from cells of Stichococcus bacillaris by fast protein liquid chromatography. The activities of the two isoenzymes were influenced by the composition of the media employed; thiol reagents were essential for stabilizing GS(2) but they suppressed GS(1) activity. The activity of each isoenzyme was, therefore, determined following separate purification procedures. Growth conditions influenced both isoenzymes; GS(2) showed maximum activity under photoautotrophic conditions, whereas GS(1) showed maximum activity under heterotrophic conditions.

Different Rates of Synthesis and Degradation of Two Chloroplastic Ammonium-Inducible NADP-Specific Glutamate Dehydrogenase Isoenzymes during Induction and Deinduction in Chlorella sorokiniana Cells

The kinetics of accumulation (per milliliter of culture) of the alpha- and beta- subunits, associated with chloroplast-localized ammonium inducible nicotinamide adenine dinucleotide phosphate-specific glutamate dehydrogenase (NADP-GDH) isoenzymes, were measured during a 3 hour induction of synchronized daughter cells of Chlorella sorokiniana in 29 millimolar ammonium medium under photoautotrophic conditions. The beta-subunit holoenzyme(s) accumulated in a linear manner for 3 hours without an apparent induction lag. A 40 minute induction lag preceded the accumulation of the alpha-subunit holoenzyme(s). After 120 minutes, the alpha-subunit ceased accumulating and thereafter remained at a constant level (i.e. steady state between synthesis and degradation). From pulsechase experiments, using (35)SO(4) and immunochemical procedures, the rate of synthesis of the alpha-subunit was shown to be greater than the beta-subunit during the first 80 minutes of induction. The alpha- and beta-subunits had different rates of degradation during the induction period (t((1/2)) = 50 versus 150 minutes, respectively) and during the deinduction period (t((1/2)) = 5 versus 13.5 minutes) after removal of ammonium from the culture. During deinduction, total NADP-GDH activity decreased with a half-time of 9 minutes. Cycloheximide completely inhibited the synthesis and degradation of both subunits. A model for regulation of expression of the NADP-GDH gene was proposed.

Purification and Partial Kinetic and Physical Characterization of Two Chloroplast-Localized NADP-Specific Glutamate Dehydrogenase Isoenzymes and Their Preferential Accumulation in Chlorella sorokiniana Cells Cultured at Low or High Ammonium Levels

Two ammonium-inducible, chloroplast-localized NADP-specific glutamate dehydrogenase isoenzymes were purified to homogeneity from Chlorella sorokiniana. These isoenzymes were homopolymers of either alpha- or beta-subunits with molecular weights of 55,500 or 53,000, respectively. The alpha-isoenzyme was preferentially induced at low ammonium concentrations (2 millimolar or lower), whereas only the beta-isoenzyme accumulated after cells were fully induced (120 minutes) at high ammonium concentrations (29 millimolar). Purification of isoenzymes was achieved by (NH(4))(2)SO(4) fractionation, gel-filtration, anion-exchange fast protein liquid chromatography, and affinity chromatography. The alpha- and beta-isoenzymes were separated by their differential binding to Type 4 nicotinamide adenine dinucleotide phosphate-Sepharose. Both isoenzymes bound to an antibody affinity column to which purified antibody (prepared against beta-isoenzyme) was covalently attached. Peptide mapping of the subunits showed them to have a high degree of sequence homology. Both subunits were synthesized in vitro from precursor protein(s) with a molecular weight of 58,500. Although the subunits have similar chemical, physical, and antigenic properties, their holoenzymes have strikingly different ammonium K(m) values. The ammonium K(m) of the beta-isoenzyme remained constant at approximately 75 millimolar, whereas this K(m) of the alpha-isoenzyme ranged from 0.02 to 3.5 millimolar, depending upon nicotinamide adenine dinucleotide phosphate concentration.

The Role of Glycerol and Inorganic Ions in Osmoregulatory Responses of the Euryhaline Flagellate Chlamydomonas pulsatilla Wollenweber

The green euryhaline flagellate Chlamydomonas pulsatilla Wollenweber, isolated from a coastal marine environment, was grown exponentially over the salinity range of 10 to 200% artificial seawater (ASW). The cellular volume and aqueous space of the alga, measured by [(14)C] mannitol and (3)H(2)O tracer analyses of centrifuged cell pellets, ranged between 2.3 and 3.1 picoliters and between 1.5 and 2.1 picoliters, respectively. The nonaqueous space determined in those analyses (28-35%) was consistent with the cell composition of the alga. The glycerol content of the alga increased almost linearly with increasing salinity; its contribution to intracellular osmolality at 200% ASW was about 57%. The contribution of amino acids and soluble carbohydrates to the cell osmotic balance was small. Intracellular ion concentrations determined by analyzing centrifuged cell pellets of known [(14)C]mannitol space by atomic absorption spectrophotometry, and by neutron activation analyses of washed cells were similar. At 10% ASW, potassium and magnesium were the major cations, and chloride and phosphate were the major anions. The sodium and chloride content of the alga increased with increasing salinity; at 200% ASW the intracellular concentration of both sodium and chloride was about 400 millimolar. The intracellular osmolality (pi(int)) matched closely the external osmolality (pi(ext)) over the entire salinity range except at 10% ASW where pi(int) exceeded pi(ext) by 120 to 270 milliosmoles per kilogram H(2)O.

Transport and Assimilation of Nitrogen by Stichococcus bacillaris Grown in the Presence of Methionine Sulfoximine

Stichococcus bacillaris Naeg., a green soil alga, can grow in the presence of methionine sulfoximine (MSX), an inhibitor of glutamine synthetase, by maintaining a high level of NADPH-glutamate dehydrogenase activity. MSX-grown cells can utilize both NH(4) (+) and NO(3) (-) as nitrogen source for growth. [(14)C]Methylammonium is not metabolized by S. bacillaris, and is transported by a carrier system that obeys Michaelis Menten kinetics, and is insensitive to MSX.