The way in which organisms use space defines and structures their communities, many of those being characterized by a low number of dominant taxa that determine the physical structure of the habitat (e.g., tropical forests, grasslands, and coral reefs; Menge & Sutherland, 1987;Angelini et al., 2011). In sedimentary intertidal environments, the availability of substrate for sessile species is limited and mytilids most likely play a key role by providing a suitable substrate for opportunist colonizers to settle, including non-native species colonizing new habitats (

Over the last two decades, an increasing interest has arisen in the responses of primary producers to global-change drivers and, more recently, in the need to consider how those various drivers may interact. To understand how Corallina officinalis (hereafter Corallina) can be affected by future changing conditions, we investigated the short-term direct effects of co-occurring increased nutrient loads, solar radiation, and lower pH, assessing how these clustered drivers affected Corallina's overall physiological performance in a harsh Patagonian coastal environment. To describe the seasonal trend of the physiological parameters in the field, we sampled subtidal Corallina to determine their net oxygen production (NOP), pigments, and carbonate content (CC). Furthermore, we conducted seasonal 10-days experiments, simulating the conditions predicted for the year 2100 by the IPCC (RCP 8.5) —manipulating pH, nutrients, and irradiance—along with the current conditions. The pigments and carotenoids/chlorophyll-a ratio were, in general, constant in the field over the seasons; but the NOP and CC dropped in spring, when the carotenoids peaked. After the experiment, the highest carotenoid/chlorophyll-a ratio was registered in summer under both the currentand the predictedconditions and in winter under the predictedcondition. This lower physiological status was also reflected in almost all other variables. Thus, Corallina may display an acclimatation strategy to cope with high ultraviolet-radiation levels by adjusting its pigment composition to avoid photoinhibition. An understanding of how Corallina, as a habitat-forming species, will respond to future global-change may provide clues about the extent of effects on the ecosystem functions and services.