Effects of elevated CO2, temperatures and nitrogen availabilities on the physiological responses of the aragnonite-calcified species Dichotomaria marginata (Rhodophyta, Nemaliales)

Studies reporting the effects of elevated CO₂, temperatures and nitrogen availabilities on calcified macroalgae have generally focused on the impact on marine calcifiers with skeletons of high‑magnesium calcite, but little is known about the physiology and metabolism of aragonite-calcified species, such as Dichotomaria marginata (J.Ellis & Solander) Lamarck. Therefore, our study aimed to assess the combined effects of three temperatures (low, 21 °C; intermediate, 25 °C; and high, 30 °C), three CO₂ concentrations (WC - without CO₂ addition; ambient air, addition of 380 ppm; and high, addition of 1000 ppm) and three nitrogen availabilities in seawater (without addition of nitrate, WN; intermediate nitrate, 125; and high nitrate, 500 μM; or without addition of ammonium, WA; intermediate ammonium, 50; and high ammonium, 100 μM) on growth and physiological responses (contents of total protein and pigments, thallus contents of C, H and N, calcification and photosynthetic performance) of D. marginata cultured in laboratory-controlled conditions. In general, optimal values of variables were observed in 125 and 500 μM of nitrate, or 50 and 100 μM of ammonium, and WC and 380 ppm of CO₂ at all temperatures. However, the highest CO₂ (1000 ppm) and temperature (30 °C) negatively affected the growth rate, photosynthetic performance and calcification of D. marginata, but maintained its metabolism by nitrogen assimilation (phycobiliproteins and total soluble proteins), and stored carbon and hydrogen. The decrease in calcification, due to aragonite dissolution, may have caused a decrease in the growth rate since the fresh biomass reduced, even though the thallus remained alive and pigmented. The present study is the first to evaluate the physiological and biochemical responses of D. marginata, an aragonite-calcified red alga, exposed to environmental stressors related to the global climate changes.