Structural and physiological responses of Halodule wrightii to ocean acidification

Coastal areas face high variability of seawater pH. Ocean acidification (OA) and local stressors are enhancing this variability, which poses a threat to marine life. However, these organisms present potential phenotypic plasticity that can offer physiological and structural tools to survive in these extreme conditions. In this study, we evaluated the effects of elevated CO₂ levels and consequent pH reduction on the physiology, anatomy and ultrastructure of the seagrass Halodule wrightii. A mesocosm study was conducted in an open system during a 30-day experiment, where different concentrations of CO₂ were simulated following the natural variability observed in coastal reef systems. This resulted in four experimental conditions simulating the (i) environmental pH (control condition, without CO₂ addition) and (ii) reduced pH by ? 0.3 units, (iii) ? 0.6 units and (iv) ? 0.9 units, in relation to the field condition. The evaluated population only suffered reduced optimum quantum yield (Y(II)), leaf width and cross-section area under the lowest CO₂ addition (? 0.3 pH units) after 30 days of experiment. This fitness commitment should be related to carbon concentration mechanisms present in the evaluated species. For the highest CO₂ level, H. wrightii demonstrated a capacity to compensate any negative effect of the lowest pH. Our results suggest that the physiological behaviour of this primary producer is driven by the interactions among OA and environmental factors, like irradiance and nutrient availability. The observed behaviour highlights that high-frequency pH variability and multifactorial approaches should be applied, and when investigating the impact of OA, factors like irradiance, nutrient availability and temperature must be considered as well.