Ocean warming and acidification affect the transitional element and macromolecular accumulation in harmful raphidophyte Heterosigma akashiwo

Abstract Despite ocean warming and acidification being expected to increase the harmful algal species worldwide, the raphidophyte Heterosigma akashiwo is reported to have decreased. However, it is unknown on the transitional scale how this species physically and metabolically modifies its elements, and macromolecular accumulation leads to such condition. With 1st,10th, and 20th culture generation under present (21℃; pCO2 400ppm [LTLC]) and projected (25℃; pCO2 1000ppm [HTHC]) ocean conditions we examined these elemental and macromolecular changes along with transcriptome sequencing. Results showed that compared to HTHC (1st generation), the (20th generation) cells showed large decreases in carbon (QC:40%), nitrogen (QN:36%), and phosphorus-quotas (QP:43%), reflected in their reduction of overall DNA and RNA quantity. Decreased metabolic pathways in photosynthesis, carbon fixation, and lipid accumulation were coincident with changes in photosynthetic efficiency, carbon, and lipid quantity with long-term (20th generation) exposure to HTHC conditions. We observed that these variations of internal metabolic changes are caused by external changes in temperature by activating the (Ca+) signaling pathway and external changes in pCO2 by altering the (proton exchange) pathways. Our results suggest that H. akashiwo in the future ocean will undergo severe changes in its elemental and macromolecular properties, leading to programmed cell death.