Chloroplast genome of Sargassum horneri (Sargassaceae, Phaeophyceae): comparative chloroplast genomics of brown algae

Sargassum horneri (Turner) C. Agardh is an important canopy-forming seaweed native to the northwestern Pacific coast growing in the upper sublittoral zone. In this study, the chloroplast genome of S. horneri was fully sequenced and compared with three other brown algal chloroplast genomes. The S. horneri chloroplast genome is 124,068 bp in size with an inverted repeat of 5436 bp and is slightly smaller than the other three brown algal chloroplast DNAs (cpDNAs). It contains 173 genes, including 6 ribosomal RNA (rRNA) genes, 28 transfer RNA (tRNA) genes, and 139 protein-coding genes (PCGs). The coding sequence constitutes 86.32 % of the S. horneri cpDNA. The total spacer size was found to be reduced to 16,967 bp with an average length of 101.0 bp, which makes the S. horneri cpDNA more compact than the other three brown algal cpDNAs. Sixteen small inverted and nine tandem repeats were detected in S. horneri chloroplast genomes. Two introns in trnL2 and trnW genes were identified with the size of 209 and 90 bp, respectively. Its gene content and order are identical to those of Fucus vesiculosus cpDNA, indicating that the chloroplast genome organization at the level of the order Fucales is highly conserved.