De novo transcriptomic profiling of Dunaliella salina reveals concordant flows of glycerol metabolic pathways upon reciprocal salinity changes

Regulation of glycerol contents is vital for adaptation to salinity changes in Dunaliella salina. However, transcriptional regulation of genes encoding enzymes involved in glycerol metabolisms upon salinity changes remains controversial. To address this issue, we de novo assembled D. salina transcriptome by using Illumina PE90 strategy with an average 700-fold coverage. Transcriptomic profiling analysis of D. salina cells in response to reciprocal salinity changes within the range of optimal growth conditions reveals 330 and 553 differentially transcribed ESTs out of a total of 6700 annotated ones upon salinity increase and decrease, respectively. We found 130 common differentially-transcribed ESTs under both conditions, nearly all of which display salinity-correlated transcriptional response. Based on 33 enzymes involved in metabolisms of glycerol and its potential carbon sources, 8 out of 10 common differentially-transcribed ESTs appear to encode enzymes at key sites. Cluster analysis indicated that 28 out of 33 enzymes exhibit salinity-correlated transcriptional profiles in response to reciprocal salinity changes. Transcriptional profiles of the enzymes are in full agreement with the flow of glycerol metabolisms upon salinity changes. This result is consistent with the observation that glycerol and starch contents are positively and negatively correlated with salinity. Furthermore, it indicates that photosynthetic sugar preferentially avails upon salinity decrease, suggesting that photosynthetic sugar is a preferential carbon source for starch accumulation but not glycerol synthesis. Taken together, our analyses demonstrate that transcriptional profiling in response to reciprocal salinity alterations within the range of optimal growth conditions greatly enriches for salinity-specific responsive ESTs in D. salina. Under these conditions, we are able to show that transcriptional regulation plays a clear role in controlling enzymatic activities involved in metabolisms of glycerol and its potential carbon sources in D. salina. We propose that transcriptional regulation of osmobalancing is likely to be evolutionarily conserved in other halotolerant unicellular organisms.