Comparative transcriptome and cross-section analyses provide insights into the mechanisms underlying Sargassum fusiforme responses to the harmful algal bloom-forming dinoflagellate Karenia mikimotoi

Abstract The impacts of harmful algal blooms (HABs) on macrophytes have rarely been studied, although HABs are currently considered one of the major worldwide marine environmental threats. In this study, we incorporated photophysiology experimental data with paraffin-embedded section and transcriptome analysis of S. fusiforme to explore the effects of dense K. mikimotoi culture on the growth, photosynthetic activities, and leaf microstructure of S. fusiforme, as well as the expression of the genes related to its photosynthesis. The results showed that the fresh weight, Chl a contents, apparent photosynthetic efficiency (α) and relative electron transfer rate (rETRmax) of S. fusiforme seedlings cultivated for ten days with K. mikimotoi decreased by 53.60%, 13.48%, 6.30% and 36.51%, as compared with those cultivated without K. mikimotoi. The leaves of the S. fusiforme seedlings began to rot and detach from the stems after a week of cultivation with K. mikimotoi, and additional epidermic cells decomposed with prolonged exposure to the dinoflagellate. Transcripts of the genes related to photosystem reaction center subunits and photosynthesis antenna proteins were expressed differentially during the cultivation with K. mikimotoi, consistent with changes in the photosynthesis of S. fusiforme seedlings. These combined results indicate that the main mechanism for resistance to HABs is the formation of epidermis by a layer of tightly arranged cylindrical cells combined with differential expression of the photosynthesis-related genes.