Responses of marine macroalgae to climate change drivers

Climate changes are progressively altering global ocean environments, leading to ocean acidification and warming, marine heatwaves, deoxygenation, and enhanced exposure of UV radiations within upper mixing layers. Marine macroalgae are affected by these environmental changes in coastal waters, where changing magnitudes of these drivers are usually larger than in open oceans. While macroalgae have developed physiological mechanisms to cope with these stressors, their responses to or tolerances to these stressors are species-specific and spatiotemporally variable. Fleshy macroalgal species are commonly capable of tolerating moderate decline of pH and diel fluctuations of pH, and their growth and photosynthesis can be enhanced by elevated CO2 concentrations in seawater and in the air during emersion at low tides. However, macroalgal calcifiers are especially sensitive to ocean acidification, with their calcification being reduced, which exacerbates the harm of solar UV radiation due to thinned protective calcareous layers. Marine warming and heatwaves, however, may endanger most macroalgal species as their seasonality of life cycle is temperature-dependent. Macroalgae either distributed in upper or lower intertidal zones are susceptible to UV radiation, which may have negative, neutral, or beneficial effects on them, depending on the levels of UV and other factors. UV-A (315–400 nm) can stimulate the photosynthesis of macroalgae under low to moderate levels of solar radiation; however, UV-B (280–315 nm) mainly causes negative effects. While the combined effects of elevated temperature, CO2, and UV radiation have rarely been documented, exposures to marine heatwaves and high levels of UV can be fatal to microscopic stages of macroalgae. Apart from the species found in estuaries, the physiology and community structure of macroalgae can be influenced by reduced salinity and pH associated with rainfall and/or terrestrial runoffs. Nevertheless, reduced O2 availability associated with ocean deoxygenation and/or hypoxia, promoted by eutrophication and ocean warming, may favor macroalgal carbon fixation because of suppressed photorespiration due to reduced O2 vs. CO2 ratios, although little documentation exists to support this possibility. While macroscopic stages of macroalgae are resilient or even benefit from some of the drivers, their microscopic stages and/or juveniles are susceptible to ocean climate changes, and the sustainability of their life cycles is endangered. In this chapter, we review and analyze the responses of different macroalgal groups and different life cycle stages to climate change drivers individually and/or jointly based on the literature surveyed, along with perspectives for future studies on the multifaceted effects of ocean climate changes.

Ji Y. & Gao K., 2023. Responses of marine macroalgae to climate change drivers. In: Kennish M. J., Paerl H. W. & Crosswell J. R. (Eds.), Climate change and estuaries, pp. 335-354. Boca Raton: CRC Press. Book chapter (restricted access).

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