Abstract

Sargassum is one of the most important genera of brown algae, playing a significant role in the development of marine food products, marine pharmaceuticals, and industrial raw materials. Currently, chloroplast genomes have been extensively applied in various fields, including molecular markers, species identification, and phylogenetics. In this study, the chloroplast DNA (cpDNA) sequences of different Sargassum species— S. fusiforme , S. horneri ( Floating type ), and S. thunbergii —were sequenced and analyzed. The chloroplast genomes of the three Sargassum species were annotated with a total of 173 genes, including 139 mRNAs, 28 tRNAs, and 6 rRNAs. The lengths of the cp genomes for the three species were 124,286 bp, 124,093 bp, and 124,574 bp, respectively. Each genome was divided by two inverted repeat (IR) regions (5,450 bp, 5,436 bp, and 5,446 bp), which were separated by the large single-copy (LSC) regions (73,342 bp, 73,342 bp, and 73,659 bp) and small single-copy (SSC) regions (39,960 bp, 39,879 bp, and 40,023 bp). The GC content of the three genomes was 30.43%, 30.58%, and 30.41%, respectively. In the genomes of the three Sargassum species, a total of 24, 29, and 34 long repeat sequences were detected, respectively. These included 2, 4, and 3 forward repeats; 31, 22, and 18 palindromic repeats; and 1, 3, and 3 reverse repeats. Additionally, a rich distribution of simple sequence repeats (SSRs) was identified, with 255, 230, and 221 SSRs detected in the three species, respectively. The SSR repeat types consisted of mononucleotide repeats (92, 92, and 89), dinucleotide repeats (12, 11, and 16), trinucleotide repeats (113, 116, and 110), and tetranucleotide repeats (7, 9, and 3); Both S. fusiforme and S. horneri contain one hexanucleotide repeat sequence, while S. horneri additionally includes one pentanucleotide repeat sequence. Phylogenetic analysis revealed that S. fusiforme , S. horneri , and S. thunbergii all belong to the genus Sargassum within the phylum Ochrophyta. Furthermore, S. fusiforme exhibits a closer phylogenetic relationship with S. thunbergii compared to S. horneri . In t his study , the structural and differential characteristics of the complete chloroplast genomes of S. fusiforme , S. horneri , and S. thunbergii were systematically analyzed and compared in their wild habitats. The findings provide scientific data support for a deeper understanding of the evolutionary traits, environmental adaptation characteristics, and gene functional features of chloroplasts within typical species of the Sargassum genus.