Seawater carbonate chemistry and physiological response of the Mediterranean crustose coralline alga Lithophyllum cabiochae to elevated pCO2 and temperature

The response of respiration, photosynthesis, and calcification to elevated pCO2 and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near-ambient conditions of irradiance, at ambient or elevated temperature (+3 °C), and at ambient (ca. 400 µatm) or elevated pCO2 (ca. 700 µatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO2 but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO2. Conversely, photosynthesis was strongly affected by pCO2 with a decline under elevated pCO2 in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO2. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO2 but decreased with rising temperature under elevated pCO2. Surprisingly, the highest rates in summer were found under elevated pCO2 and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO2. The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO2. However, there is likely a trade-off with other physiological processes. For example, photosynthesis declines in response to increased pCO2 under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.

Adaptive variability to low-pH river discharges in Acartia tonsa and stress responses to high PCO2 conditions

Environmental transitions leading to spatial physical-chemical gradients are of ecological and evolutionary interest because they are able to induce variations in phenotypic plasticity. Thus, the adaptive variability to low-pH river discharges may drive divergent stress responses [ingestion rates (IR) and expression of stress-related genes such as Heat shock protein 70 (Hsp70) and Ferritin] in the neritic copepod Acartia tonsa facing changes in the marine chemistry associated to ocean acidification (OA). These responses were tested in copepod populations inhabiting two environments with contrasting carbonate system parameters (an estuarine versus coastal area) in the Southern Pacific Ocean, and assessing an in situ and 96-h experimental incubation under conditions of high pressure of CO2 (PCO2 1200 ppm). Adaptive variability was a determining factor in driving variability of copepods' responses. Thus, the food-rich but colder and corrosive estuary induced a traits trade-off expressed as depressed IR under in situ conditions. However, this experience allowed these copepods to tolerate further exposure to high PCO2 levels better, as their IRs were on average 43% higher than those of the coastal individuals. Indeed, expression of both the Hsp70 and Ferritin genes in coastal copepods was significantly higher after acclimation to high PCO2 conditions. Along with other recent evidence, our findings confirm that adaptation to local fluctuations in seawater pH seems to play a significant role in the response of planktonic populations to OA-associated conditions. Facing the environmental threat represented by the inter-play between multiple drivers of climate change, this biological feature should be examined in detail as a potential tool for risk mitigation policies in coastal management arrangements.