Alexandrium pacificum is a toxic dinoflagellate that induces harmful algal blooms (HABs), but the molecular mechanisms of its rapid growth leading to red tides is still unknown. To effectively prevent and control the occurrence of HABs, it is necessary to conduct in-depth research on its growth mechanism. Organism growth and metabolic processes are affected by histone lysine methylation, an important mode of epigenetic regulation. Histones are expressed in trace amounts in dinoflagellates and have little effect on genome packaging. It is speculated that the key role of histones lies in histone modifications. In this study, we first determined that the algal histone does have multiple histone modifications by methods such as WB detection and mass spectrometry using multiple modification antibodies. Secondly, 179 ApSDGs (SET domain genes) and 30 ApDOT1 (DOT1 domain genes) encoding histone lysine methyltransferase members were identified based on the transcriptome data, and the sequence characteristics and gene expression were analyzed. Subsequently, the functions of ApDOT1.9 were verified by in vitro methylation assay. Then, Western Blot was used to detect the abundance of several modification sites in A. pacificum at different stages and growth conditions. The results showed that the abundance of various histone modifications could respond to changes in high nitrogen or high light intensity conditions that cause red tide outbreaks. Finally, the growth effect genes downstream of H3K4me3 and H3K79me modifications were explored using ChIP-seq technology, respectively. The research results found that H3K4me3 is involved in the rapid growth of A. pacificum under HL and HN conditions, including acting on genes related to nitrogen metabolism, endocytosis, and vitamin metabolism. H3K79me participates in the rapid growth of A. pacificum under HL conditions through photosynthesis. This study investigated the rapid growth regulation mechanism of A. pacificum at the level of histone modification and provided epigenetic data and new insights to explain the mechanism of HABs' occurrence.

harmful algal blooms, Alexandrium pacificum, histone lysine methylation, ChIP-seq, rapid growth