Abstract
The haptophyte Phaeocystis globosa is a well-known species for its pivotal role in global carbon and sulfur cycles and for its capability of forming harmful algal blooms (HABs) with serious ecological consequences. Its mitochondrial genome (mtDNA) sequence has been reported in 2014 but it remains incomplete due to its long repeat sequences. In this study, we constructed the first full-length mtDNA of P. globosa, which was a circular genome with a size of 43,585 bp by applying the PacBio single molecular sequencing method. The mtDNA of this P. globosa strain (CNS00066), which was isolated from the Beibu Gulf, China, encoded 19 protein-coding genes (PCGs), 25 tRNA genes, and two rRNA genes. It contained two large repeat regions of 6.7 kb and ∼14.0 kb in length, respectively. The combined length of these two repeat regions, which were missing from the previous mtDNA assembly, accounted for almost half of the entire mtDNA and represented the longest repeat region among all sequenced haptophyte mtDNAs. In this study, we tested the hypothesis that repeat unit amplification is a driving force for different mtDNA sizes. Comparative analysis of mtDNAs of five additional P. globosa strains (four strains obtained in this study, and one strain previously published) revealed that all six mtDNAs shared identical numbers of genes but with dramatically different repeat regions. A homologous repeat unit was identified but with hugely different numbers of copies in all P. globosa strains. Thus, repeat amplification may represent an important driving force of mtDNA evolution in P. globosa.
Highlights
The haptophyte Phaeocystis globosa is a cosmopolitan phytoplankton that plays a pivotal role in global carbon and sulfur cycles by releasing substantial quantities of dimethylsulfide propionate (DMSP) (Schoemann et al, 2005)
We successfully constructed the full-length mtDNA of P. globosa for the first time. It was a circular genome with a size of 43,585 bp, encoding 19 protein-coding genes (PCGs), 25 Transfer RNAs (tRNAs) genes and two ribosomal RNAs (rRNAs) genes
The length and the repetitive nature of these two repeat regions made it challenging to construct the complete length of full-length mtDNA of P. globosa
Summary
The haptophyte Phaeocystis globosa is a cosmopolitan phytoplankton that plays a pivotal role in global carbon and sulfur cycles by releasing substantial quantities of dimethylsulfide propionate (DMSP) (Schoemann et al, 2005). It is an important HAB species that causes blooms in various ocean regions including the Arabian sea, the Southern North Sea, and the coastal waters of East Asia and Southeast Asia Small circular bulges were observed in small flagellate cells of China strains, while such characteristics were not observed in European strains (Hu et al, 2019)
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