Seasonal effects of sun exposure and emersion on intertidal seaweed physiology: Fluctuations in antioxidant contents, photosynthetic pigments and photosynthetic efficiency in the red alga Porphyra umbilicalis Kützing (Rhodophyta, Bangiales)

Abstract

There is a great deal of speculation regarding the physiological and biochemical mechanisms that give certain seaweed species the ability to colonize the intertidal zone. Frequent exposure to ambient temperatures and high irradiance levels in addition to dehydration during tidal emersion generates acute physiological stress. The ability of seaweeds like Porphya to overcome these challenges and survive in such a harsh environment has been linked to elevated reactive oxygen metabolism. The current study focused on measuring seasonal changes in antioxidant enzymes plus alterations in pigment contents and photosynthetic efficiency of P. umbilicalis plants found growing in the uppermost intertidal zone. Our results suggest that P. umbilicalis exhibits increased antioxidant metabolism, which could contribute to its success in colonizing such a stressful habitat. Elevated levels of glutathione reductase GTR, catalase and carotenoid contents during emersion suggested heightened protection against reactive oxygen species ROS damage is a necessary attribute for species in the upper intertidal regions. This hypothesis was further strengthened by the finding that the greatest antioxidant increases were observed during summer months when irradiance levels and temperatures were at their peak. Winter emersion did not elicit the same physiological response, as antioxidant levels were similar in submersed and emersed plants. For the most part, photosynthetic pigments were largely affected by sun exposure and less by emersion stress. Shaded blades maintained higher concentrations of photosynthetic pigments compared to sun exposed thalli concurring with established research. Photosynthetic efficiency measurements indicated emersion and not sun exposure was the greater facilitator of photoinhibitory damage and ROS generation at PSII. The findings of this field study strengthen previous assertions that protection via elevated antioxidant metabolism and increased PSII repair are involved in providing relief from the acute environmental stresses in the intertidal zone.