Isolation of the plasma-membrane of the halotolerant alga Dunaliella salina using sulforhodamine B as a probe

Toxicity evaluation of two typical surfactants to Dunaliella bardawil, an environmentally tolerant alga

Sodium dodecyl benzene sulfonate (SDBS) and cetyl trimethyl ammonium chloride (CTAC) are two kinds of surfactants widely applied in various industries. The tremendous direct discharge of these surfactants into natural waters has posed a significant threat to ecosystems. Dunaliella bardawil was employed in the present research to test the toxic effects of SDBS, CTAC, and their mixture on cell growth, cellular morphology, β-carotene accumulation, and enzymatic activities of superoxide dismutase (SOD) and catalase (CAT). The results showed that SDBS at 200, 550, 900, 1,350, 1,800, and 2,400 mg/L and CTAC at 0.4, 0.7, 1.0, 1.3, 2.8, and 3.5 mg/L inhibited algal growth and β-carotene accumulation, both of which declined and then increased. In particular, CTAC (median inhibitory concentration at 10 days [IC50](10 d)  = 2.8 ± 1.49 mg/L) was more hazardous than SDBS (IC50(10 d)  = 2,044 ± 637.3 mg/L). The additive index (AI) calculated from carotene content data was (-4.10, -1.67) < 0, indicating an antagonistic effect between SDBS and CTAC. Algae cultivated at level 6 of the binary system showed hormesis due to the mitigated toxicity; SDBS at 2,400 mg/L, CTAC at 3.5 mg/L, and combined surfactants at level 6 exerted lethal effects on D. bardawil. Both SOD and CAT activities showed similar associations with varied concentrations of surfactants: SOD was significantly promoted by 550 to 1,800 mg/L SDBS, 0.7 to 1.3 mg/L CTAC, and mixtures at levels 2 to 4; CAT was clearly promoted by 900 mg/L SDBS, 0.4 to 1.3 mg/L CTAC, and mixtures at levels 2 to 4.

Toxic effects of chemical pesticides (trichlorfon and dimehypo) on Dunaliella salina

Dunaliella salina, a unicellular green alga of environmental tolerance, was employed as test organism to investigate the toxicity effects of trichlorfon and dimehypo widely used in agriculture and veterinary as pesticides. The influences of trichlorfon and dimehypo on cell growth, β-carotene level, cell morphology changes, and activities of superoxide dismutase (Sod) and catalase (Cat) were investigated. At the concentrations less than 0.050 g L(-1) trichlorfon or 0.0005 g L(-1) dimehypo, cell responses were similar to control. When treated with 0.075-0.100 g L(-1) trichlorfon or 0.001-0.004 g L(-1) dimehypo, cell growth and β-carotene levels declined at first and then revived. When concentrations were higher than 0.125 g L(-1) trichlorfon or 0.005 g L(-1) dimehypo, both cell growth and β-carotene levels decreased until they were undetectable. The 10-d IC50 of trichlorfon and dimehypo on D. salina were 0.179 g L(-1) and 0.032 g L(-1). Both pollutants could stimulate the increase of Cat activity at a low concentration. Tolerance of D. salina to trichlorfon was obviously higher than that of dimehypo.

Effects of salinity changes on the growth of Dunaliella salina and its isozyme activities of glycerol-3-phosphate dehydrogenase

Dunaliella salina could survive in media containing a wide range of NaCl concentrations ranging from about 0.05 M to saturation (around 5.5 M). Glycerol is an important osmolyte when Dunaliella survive in various salt environments, and G3pdh is a key enzyme in glycerol metabolism. The osmotic response of D. salina was investigated by studying its cell growth, glycerol content change, and isozyme activity of glycerol-3-phosphate dehydrogenase (G3pdh) in different salinities. Results showed that 2.0 M NaCl was the optimal salinity for the growth of D. salina, in which condition the highest glycerol content of 64.02 +/- 3.21 (mean +/- SD) microg/mL was detected. D. salina could rapidly increase or decrease glycerol contents to adapt to hypoosmotic or hyperosmotic environments. The glycerol content declined 52.05% when salinity was changed from 2.0 to 0.5 M NaCl, and the glycerol content increased 43.61% when salinity was increased from 2.0 to 5.0 M NaCl. In the isozyme electrophoresis assay two kinds of isozymes, G3pdh and superoxide dismutase (Sod), were detected synchronously. Interestingly, it was first found that there are five isozymes of G3pdh in D. salina. G3pdh-2 mainly takes effect in moderate to high salinities, whereas the other four isozymes take effect in low salinities, which may provide an important clue for future research on osmoregulation mechanisms.

Characterization of cDNA of lycopene β-cyclase responsible for a high level of β-carotene accumulation inDunaliella salina

Lycopene β-cyclase (Lyc-B) is the key enzyme in the catalysis of linear lycopene to form cyclic β-carotene, an indispensable part of the photosynthetic apparatus and an important source of vitamin A in human and animal nutrition. Studies showing that the microalga Dunaliella salina can accumulate a high level of β-carotene are lacking. We hypothesize that D. salina is closely involved with the catalytic mechanism of Lyc-B and the molecular regulation of its gene. In this study, we used RT–PCR and RACE–PCR to isolate a 2475 bp cDNA with a 1824 bp open reading frame, encoding a putative Lyc-B, from D. salina. Homology studies showed that the deduced amino acid sequence had a significant overall similarity with sequences of other green algae and higher plants, and that it shared the highest sequence identity, up to 64%, with Lyc-B of Chlamydomonas reinhardtii. Codon analysis showed that synonymous codon usage in the enzyme has a strong bias towards codons ending with adenosine. Two motifs were found in the Lyc-B sequence, one at the N terminus, for binding the hypothetical cofactor FAD, and the other was a substrate carrier motif in oxygenic organisms shared by an earlier carotenogenesis enzyme, phytoene desaturase, and Lyc-B. A tertiary structure prediction suggested that the catalytic or binding site structure within LycB from D. salina is superior to that of both H. pluvialis and C. reinhardtii. The LycB protein from D. salina was quite removed from that of H. pluvialis and C. reinhardtii in the phylogenetic tree. Taken as a whole, this information provides insight into the regulatatory mechanism of Lyc-B at the molecular level and the high level of β-carotene accumulation in the microalga D. salina.

Functional identification of electrogenic Na+-translocating ATPase in the plasma membrane of the halotolerant microalga Dunaliella maritima

The hypothesis that the primary Na+-pump, Na+-ATPase, functions in the plasma membrane (PM) of halotolerant microalga Dunaliella maritima was tested using membrane preparations from this organism enriched with the PM vesicles. The pH profile of ATP hydrolysis catalyzed by the PM fractions exhibited a broad optimum between pH 6 and 9. Hydrolysis in the alkaline range was specifically stimulated by Na+ ions. Maximal sodium dependent ATP hydrolysis was observed at pH 7.5-8.0. On the assumption that the ATP-hydrolysis at alkaline pH values is related to a Na+-ATPase activity, we investigated two ATP-dependent processes, sodium uptake by the PM vesicles and generation of electric potential difference (Deltapsi) across the vesicle membrane. PM vesicles from D. maritima were found to be able to accumulate 22Na+ upon ATP addition, with an optimum at pH 7.5-8.0. The ATP-dependent Na+ accumulation was stimulated by the permeant NO3- anion and the protonophore CCCP, and inhibited by orthovanadate. The sodium accumulation was accompanied by pronounced Deltapsi generation across the vesicle membrane. The data obtained indicate that a primary Na+ pump, an electrogenic Na+-ATPase of the P-type, functions in the PM of marine microalga D. maritima.

Cloning and nucleotide sequence analysis of genes coding for the major chlorophyll-binding protein of the moss Physcomitrella patens and the halotolerant alga Dunaliella salina

Two clones have been isolated from a genomic library of the moss Physcomitrella patens and a cDNA library of the halotolerant green alga Dunaliella salina. The isolates contain genes coding for the major light-harvesting chlorophyll-a/b-binding protein (CAB) in the photosystem II (PSII) light-harvesting complex (LHCII). The 2544-bp insert of the moss genomic clone contains the complete CAB-coding region and 5' and 3' flanking sequences. The coding region contains an intron of 359 bp which is spanned by a pair of 9-bp perfect direct repeats. There are two CCAAT boxes and five enhancer-like elements related to (G)TGGTTTAAA(G) (Weiher et al., 1983) residing in the intron. Comparisons of the moss cab gene with sequences of light-inducible genes of higher plants reveal homologous and repeated sequences similar to the enhancer element in the 5' region upstream from the TATA and CCAAT boxes thought to be responsive to light inducibility. The 1256-bp algal cDNA contains the complete CAB-coding sequence, a 170-bp 5'-nontranslated region, and a 264-bp 3'-nontranslated region. While the overall homology in the nontranslated regions is low between the cab gene of the moss and that of the alga, the 3'-nontranslated regions of the two contain some sequences that are conserved among the cab genes in higher plants. The deduced amino acid sequences of these two clones are highly conserved except for the N-terminal region. Their hydropathic plots are very similar and both possess three hydrophobic segments that are likely alpha-helical transmembrane segments. The proposed CAB transit peptide sequence of the alga is divergent from that of the moss or higher plants, suggesting that they may have evolved from different origins. Southern blot analysis shows that the cab genes in the moss and the alga, as in higher plants, are encoded by a number of homologous genes constituting a multigene family.