Duc Tran

The Dunaliella salina organelle genomes: large sequences, inflated with intronic and intergenic DNA

BackgroundDunaliella salina Teodoresco, a unicellular, halophilic green alga belonging to the Chlorophyceae, is among the most industrially important microalgae. This is because D. salina can produce massive amounts of β-carotene, which can be collected for commercial purposes, and because of its potential as a feedstock for biofuels production. Although the biochemistry and physiology of D. salina have been studied in great detail, virtually nothing is known about the genomes it carries, especially those within its mitochondrion and plastid. This study presents the complete mitochondrial and plastid genome sequences of D. salina and compares them with those of the model green algae Chlamydomonas reinhardtii and Volvox carteri.ResultsThe D. salina organelle genomes are large, circular-mapping molecules with ~60% noncoding DNA, placing them among the most inflated organelle DNAs sampled from the Chlorophyta. In fact, the D. salina plastid genome, at 269 kb, is the largest complete plastid DNA (ptDNA) sequence currently deposited in GenBank, and both the mitochondrial and plastid genomes have unprecedentedly high intron densities for organelle DNA: ~1.5 and ~0.4 introns per gene, respectively. Moreover, what appear to be the relics of genes, introns, and intronic open reading frames are found scattered throughout the intergenic ptDNA regions -- a trait without parallel in other characterized organelle genomes and one that gives insight into the mechanisms and modes of expansion of the D. salina ptDNA.ConclusionsThese findings confirm the notion that chlamydomonadalean algae have some of the most extreme organelle genomes of all eukaryotes. They also suggest that the events giving rise to the expanded ptDNA architecture of D. salina and other Chlamydomonadales may have occurred early in the evolution of this lineage. Although interesting from a genome evolution standpoint, the D. salina organelle DNA sequences will aid in the development of a viable plastid transformation system for this model alga, and they will complement the forthcoming D. salina nuclear genome sequence, placing D. salina in a group of a select few photosynthetic eukaryotes for which complete genome sequences from all three genetic compartments are available.

An update on carotenoid biosynthesis in algae: phylogenetic evidence for the existence of two classes of phytoene synthase

Carotenoids play crucial roles in structure and function of the photosynthetic apparatus of bacteria, algae, and higher plants. The entry-step reaction to carotenoid biosynthesis is catalyzed by the phytoene synthase (PSY), which is structurally and functionally related in all organisms. A comparative genomic analysis regarding the PSY revealed that the green algae Ostreococcus and Micromonas possess two orthologous copies of the PSY genes, indicating an ancient gene duplication event that produced two classes of PSY in algae. However, some other green algae (Chlamydomonas reinhardtii, Chlorella vulgaris, and Volvox carteri), red algae (Cyanidioschyzon merolae), diatoms (Thalassiosira pseudonana and Phaeodactylum tricornutum), and higher plants retained only one class of the PSY gene whereas the other gene copy was lost in these species. Further, similar to the situation in higher plants recent gene duplications of PSY have occurred for example in the green alga Dunaliella salina/bardawil. As members of the PSY gene families in some higher plants are differentially regulated during development or stress, the discovery of two classes of PSY gene families in some algae suggests that carotenoid biosynthesis in these algae is differentially regulated in response to development and environmental stress as well.

Draft Nuclear Genome Sequence of the Halophilic and Beta-Carotene-Accumulating Green Alga Dunaliella salina Strain CCAP19/18

ABSTRACT The halotolerant alga Dunaliella salina is a model for stress tolerance and is used commercially for production of beta-carotene (=pro-vitamin A). The presented draft genome of the genuine strain CCAP19/18 will allow investigations into metabolic processes involved in regulation of stress responses, including carotenogenesis and adaptations to life in high-salinity environments.

High Light Stress Regimen on Dunaliella Salina Strains For Carotenoids Induction

The microalgae Dunaliella salina is the richest source of commercial β-carotene known to man. This natural compound has been proven invaluable in medicine, industry and other fields of science, due to its provitamin A activity and potential disease suppression, as well as usage as a supplement for food and animal feed including as additive to food and cosmetics. However, β-carotene content in Dunaliella cells depends heavily on growth conditions and nutrient parameters. A set of experiments was conducted to determine the optimum high light stress regimen for Dunaliella salina to achieve the highest carotenoids induction. Three D. salina strains (D. salina CCAP 19/18, D. salina A9 and D. bardawil) were cultured in MD4 1.5M medium under stress condition at different regimens for a period of 26 days. Following the first phase of exponential growth, 3 different growth cycles were tested: a cycle of three-day at 800 μmol.photons/m2/s and one day at 50 μmol.photons/ m2/s, a cycle of one day at 800 μmol.photons/m2/s and three-day at 50 μmol.photons/m2/s and finally an all-time stress at 800 μmol.photons/m2/s. Total carotenoids were analyzed over the experimental period, including the antioxidant capacities and total phenolic contents of the algal carotenoid extract were simultaneously evaluated. Result revealed that all three D. salina strains produced the highest concentration of total carotene under the all-time stress regimen of 800 μmol.photons/ m2/s, and D. salina CCAP had higher total carotenoid content than D. salina A9 and D. bardawil in all stress conditions. This study could surely serve as the basis for scaling up this process to industrial-level applications, which will undoubtedly require further investigation and evaluation of the extraction and testing procedures. Introduction Currently there are 26 known or reported species of Dunaliella. These include among others Dunaliella salina, Dunaliella bardawil, Dunaliella tertiolecta [1-5]. Dunaliella salina is a type of unicellular and halophilic green biflagellate microalga without a rigid cell wall structure which can grow at very high salinities and levels of irradiance [6-9]. Dunaliella salina accumulates massive amounts of β-carotene in electrodense globules located within the inter-thylakoid spaces in the chloroplast. Various stress factors are known to interrupt the physiological balance of a normal Dunaliella salina cell. Therefore, in order to protect itself and continue to grow, Dunaliella cell generates additional β-carotene restoring its physiology balance under stress conditions [10]. Carotenoids are organic pigments that are known to be crucial for normal vision and have been associated with reducing the risk of several degenerative diseases, including cancer [11,12]. β-carotene is perhaps the most important carotenoid from over 600 types of carotenoids found in nature. It is highly valuable due to its nutritional benefit as a precursor of vitamin A and for its properties such as a color additive, antioxidant, anticancer, antiaging and immunomodulatory [13,14]. Recently, the rate and extent of carotenoid accumulation in Dunaliella salina has been researched under various stress conditions such as high salinity [15,16], high temperature [17,18] and these types of relationship are well established. However, according to scientific community, there are no reports on carotenoids induction in D. salina cultured under different high light stress regimens. Therefore, the Correspondence to: Duc Tran, Algae2Omega Holdings, 7625 17th Street, Vero Beach, FL 32968, USA, E-mail: tnducminh@yahoo.com