Cold-water corals (CWC) face an uncertain future under climate change. They seem to grow successfully under low pH conditions but physiological mechanisms and the role of energy efficiency in sustaining metabolic rates are largely unknown. The solitary, pseudo-colonial CWC Desmophyllum dianthus thrives in Comau Fjord (Northern Patagonia, Chile) despite low levels of aragonite saturation (Ωarag). To examine the seasonal growth and metabolism in relation with food availability and the physico-chemical environment of the fjord, we carried out an in situ reciprocal transplantation-experiment between (1) fjord mouth (20 m: Ωarag > 1, high seasonality) and fjord head (20 m: Ωarag > 1 in winter, Ωarag < 1 in summer, high seasonality) and (2) two depths at the middle of the fjord (20 m: Ωarag > 1, high seasonality; 300 m: Ωarag < 1, low seasonality). D. dianthus showed highest calcification and respiration rates in 300 m depth with maximum growth rates in winter. This applied to in situ control corals and those transplanted from 20 m to 300 m. The lower plankton availability at depth and in winter suggests seasonal differences in energy gain and allocation among growth, basal metabolism and other processes (e.g. reproduction). Despite Ωarag < 1 the lack of seasonal variation in the physico-chemical environment in deep waters may be beneficial for growth in contrast to seasonal fluctuations in shallow waters which may require a recurrent energy expenditure of acclimation. In 20 m depth, calcification and respiration rates at the fjord mouth were similar between seasons and always higher than at the fjord head. No significant differences were detected between control and transplanted corals from the fjord head to the mouth. A high aragonite saturation (Ωarag > 1) and plankton supply during summer combined with a well-functioning redistribution of energy reserves within D. dianthus in winter can explain this picture. At the fjord head growth changed seasonally in control and transplanted corals with higher rates during summer indicating good growth conditions despite Ωarag < 1 due to sufficient food availability but a less effective energy allocation during low food supply in winter. The present results emphasize the effect of seasonal environmental changes on CWC to conceive both the extent of natural variability where these corals live in and their acclimation potential to deal with it.