Cold stress tolerance of the intertidal red alga Neoporphyra haitanensis.

Background: Red algae Porphyra sensu lato grow naturally in the unfavorable intertidal environment, in which they are exposed to substantial temperature fluctuations. The strategies of Porphyra to tolerate cold stress are poorly understood.

Results: Herein, investigations revealed that chilling and freezing induced alterations in the physiological properties, gene transcriptional profiles and metabolite levels in the economically important red algae species, Neoporphyra haitanensis. Control samples (kept at 20 °C) were compared to chilled thalli (10 and 4 °C) and to thalli under - 4 °C conditions. Chilling stress did not affect the health or photosynthetic efficiency of gametophytes, but freezing conditions resulted in the arrest of growth, death of some cells and a decrease in photosynthetic activity as calculated by Fv/Fm. Transcriptome sequencing analysis revealed that the photosynthetic system was down-regulated along with genes associated with carbon fixation and primary metabolic biosynthesis. Adaptive mechanisms included an increase in unsaturated fatty acids levels to improve membrane fluidity, an increase in floridoside and isofloridoside content to enhance osmotic resistance, and an elevation in levels of some resistance-associated phytohormones (abscisic acid, salicylic acid, and methyl jasmonic acid). These physiochemical alterations occurred together with the upregulation of ribosome biogenesis.

Conclusions: N. haitanensis adopts multiple protective mechanisms to maintain homeostasis of cellular physiology in tolerance to cold stress.