Transcriptome sequencing of the marine microalga, Chlorella pyrenoidosa (Chlorophyta), and analysis of carbonic anhydrase expression under salt stress

In this study, the transcriptome of the marine microalga, Chlorella pyrenoidosa 820, was sequenced by high-throughput RNA-seq using Illumina HiSeq 2000. Because the genome sequence of this species is unknown, the resulting transcriptome was assembled de novo and annotated. This resulted in 4.71G clean nucleotides with a 56.91% GC content and yielded a total of 36,826 unigenes with a mean length of 1089 nt. Among these, 23,015 unigenes were annotated in the NCBI-NR, NCBI-NT, Swiss-Prot, KEGG, COG, and GO databases with a cutoff E-value of 10-5. In the annotated sequences, 21 unigenes were identified as carbonic anhydrase (CA), which is an important enzyme in the CO2-concentrating mechanism (CCM), and were homologous to nine alpha-, eight beta-, and four gamma-CAs. Interestingly, the expression of three CA subtypes, including one alpha-, one beta-, and one gamma-CA, analyzed by real-time quantitative PCR, showed induction by high salinity. These results will enrich the CA gene information in the database and will help understand CCM in the genus Chlorella.