Physiological Responses of Two Key Tropical Macroalgae to Variable Environmental Conditions

With escalating effects of global climate change, it is important to understand responses of key organisms, such as macroalgae, as they play several critical roles on maintaining key ecological processes on coastal ecosystems. Evidence from empirical studies have shown that elevated temperatures and CO₂ concentrations (e.g. ocean acidification, OA), can enhance macroalgal growth and abundance. However, the response of macroalgae to OA and temperature can be species-specific due to the physiological mechanisms by which macroalgae utilize CO₂ or HCO₃^- from seawater, otherwise known as CO₂ concentrating mechanisms (CCMs). This study examined the effects of elevated pCO₂ and temperature on the physiology of two common tropical fleshy macroalgae (Turbinaria ornata and Sargassum pacificum). Photosynthetic rates, growth, and CCM activity were measured over a 10-day incubation period at ambient 400 µatm/26 °C and elevated 1000 µatm/30 °C conditions for each species. Physiological responses differed between the two species. Photosynthetic rates of T. ornata were consistent regardless of elevated pCO₂ and temperature, while growth rate decreased significantly with elevated CO₂ supply. S. pacificum on the other hand exhibited no significant difference in growth rates under elevated pCO₂ and temperature, but significantly lower photosynthetic rates with increasing temperature. These results suggest species specific differences in the response of macroalgal communities to environmental change so overall community-wide responses may be dependent on the species composition of fleshy macroalgae. Understanding the various physiological responses of macroalgal communities can help predict how these responses will affect vital coral reef ecosystem services in a future warmer and more acidic ocean.