Impacts of Nighttime Hypoxia on the Physiological Performance of Red Sea Macroalgae

Abstract

Marine deoxygenated areas are expanding, and more hypoxic zones emerge globally. Climate change induced warming and stratification can extrapolate the biological oxygen demand, more predominantly at coastal areas and reefs with dense vegetation and high metabolic activity. The diurnal oxygen fluctuation can reach a critically low oxygen level at nighttime, exposing aquatic organisms to severe hypoxia that could interfere with viable ecosystem functions. Little is known about the impact of hypoxia on marine primary producers macroalgae, mainly the physiological adaptation of the Red Sea macroalgae under climate change scenarios is understudied.

Here we investigate hypoxia thresholds at night time for conspicuous Red Sea macroalgae species calcareous Halimeda opuntia and Padina boryana and noncalcifying brown algae Sargassum latifolium. We utilized a computer-based gas system to expose the samples to different oxygen treatments (normoxia, hypoxia, and severe hypoxia) that mimic in situ water chemistry at 32 °C. We monitored algal physiological response during 12 hours of exposure to different oxygen levels in the dark by measuring photochemical efficiency, respiration rates, and cellular viability.

For the duration of our experiments (12h), we did not detect lethal thresholds. In all tested species, severe hypoxia significantly decreased photochemical efficiency, and hypoxia had a limited impact on photochemical efficiency. However, both low oxygen treatments significantly decreased respiration rates and induced changes in cellular activity. We concluded a sublethal O$_2$ thresholds SLC$_{(50)}$ of 1.2 ±0.1, 1.5 ±0.1, and 1.7 ±0.1 mg O$_2$ L$^{−1}$ ±SD for H. opuntia, P. boryana, and S. latifolium responses, respectively. We also found that during 12 hours of treatments, the median time to observe a 50% reduction in photochemical efficiency under severe hypoxia treatment relative to control was 6.3 ±1.4, 3.5 ±1.0, and 0.8 ±1.3 hours ±SD for H. opuntia, P. boryana, and S. latifolium responses, respectively.

This study is the first assessment of Red Sea macroalgae response to hypoxia and the first dark nighttime algal adaptation of its kind for our proposed species. Further investigation is needed to assess daytime recovery, recurring dark hypoxia, and synergic or sequential effects of other environmental stressors on hypoxia thresholds.