Large-scale RNA profiling revealed that high irradiance differentially regulated 577 out of 1,439 non-redundant genes of the Antarctic marine diatom Chaetoceros neogracile, represented on a custom cDNA chip, during 6 h of treatment. Among genes that were up- or down-regulated more than twofold within 30 min of treatment (310/1,439), about half displayed an acclimatory response during 6 h under high light. Expression of the remaining non-acclimatory genes also rapidly returned to initial levels within 30 min following a shift to low irradiance. High light altered expression of most of the photosynthesis genes (48/70), in contrast to genes in other functional categories. In addition, opposite response patterns were provoked in genes encoding fucoxanthin chlorophyll a/c binding protein (FCP), the main component of the diatom light-harvesting complex; high irradiance caused a decrease in expression of most FCP genes, but drove the rapid and specific up-regulation of ten others. C. neogracile responded very promptly to a change in light intensity by rapidly adjusting the transcript levels of FCP genes up-regulated by high light, and these dynamic adjustments coincided well with diatoxanthin (Dtx) levels formed by the xanthophyll cycle under the same conditions. The observation that the non-photochemical quenching (NPQ) capacity of this polar diatom was highly dependent on Dtx, which could bind to FCP and trigger NPQ, suggests that the up-regulated FCP gene products may participate in a photoprotective process as Dtx-binding proteins.

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The unicellular green alga Dunaliella salina is an attractive model organism for studying photoacclimation responses and the photosystem II (PSII) damage and repair process in the photosynthetic apparatus. Irradiance during cell growth defines both the photoacclimation and the PSII repair status of the cells. To identify genes specific to these processes, a cDNA library was created from irradiance-stressed D. salina. From the cDNA library, 1112 randomly selected expressed sequence tags (ESTs) were analyzed. Because ESTs constitute the expressed part of the genome, the strategy of randomly sequencing cDNA clones at their 5'-ends allowed us to obtain information about the transcript level of numerous genes in light-stressed D. salina. The results of a BLASTX search performed on the obtained total set of ESTs showed that approximately 1% of the ESTs could be assigned to genes coding for proteins that are known to be up-regulated in response to high-light stress. Specifically, after 48 h of high-light exposure of the cells, an increase in the expression level of antioxidant genes, such as Fe-SOD and APX, was observed, as well as elevated levels of the Cbr transcript, a light-harvesting Chl-protein homolog. Further, the ATP-dependent Clp protease gene was also up-regulated in D. salina cells after 48 h of exposure to high light. The results provide initial insight into the global gene regulation process in response to irradiance.

The development of highly inducible promoters is critical for designing effective transformation systems for transgenic analyses. In this study, we investigated the promoter of the light-inducible protein gene (LIP) of the marine alga Dunaliella sp. LIPs are homologs of the early light-induced proteins (ELIPs) of Arabidopsis thaliana. DNA sequence analysis revealed that the LIP promoter contains several light-responsive motifs. Constructs containing progressive truncations of the LIP promoter fused with a Renilla luciferase gene were introduced into Chlamydomonas reinhardtii to identify the light-responsive region in the promoter. Transcription from the LIP promoter was stimulated by high light (HL) in a light intensity-dependent manner. In contrast, oxidative stress induced by chemicals had little effect on the LIP promoter, which implies that the LIP promoter is exclusively induced by high light. Truncation of the promoter to a -100 base pair (bp) region abrogated light inducibility, which suggests the presence of a negative cis-regulatory element upstream of the -100 bp fragment. The LIP promoter can be utilized in transgenic research to specifically select and propagate transgenic microalgae under high-light conditions.

Dunaliella salina and Dunaliella bardawil are well known for carotenogenesis, the overproduction of carotenoids, under stress conditions. The effect of high light (HL) and low light (LL) on the growth, morphology, photosynthetic efficiency, and the β-carotene and zeaxanthin production of D. salina CCAP 19/18 and D. bardawil was investigated and compared. Both strains showed similar growth kinetics under LL growth condition, but D. salina CCAP 19/18 was faster. As the light intensity increased, D. salina CCAP 19/18 cells were elongated and D. bardawil cells became larger. Both strains showed decrease of the maximum quantum yield of PSII (F v /F m ) and election transport rate (ETR) under HL growth condition and D. salina CCAP 19/18 was less liable to the light stress. Both strains had about 1.8 and 5 times difference in the O 2 evolution rate at LL and HL conditions, respectively. The β-carotene and zeaxanthin production were increased as the light intensity increased in both strains. D. bardawil was more sensitive to light intensity than D. salina CCAP 19/18. The possible application of D. salina CCAP 19/18 as a carotenogenic strain will be discussed.