Coralline algae are an essential element of benthic ecosystems throughout the ocean’s photic zone. Yet, the role of light in shaping the physiology of coralline algae from cold-water, low-light habitats is poorly understood. Here, we assess the calcification physiology of five cool-temperate coralline algae in response to different irradiances over three months. We show that in contrast to current models focused on warmer water species, previously observed enhancement of calcification rates by photosynthesis is largely limited to lower irradiances, and that the removal of CO₂ from the calcifying fluid is not the underlying mechanism of this enhancement. Instead, this likely occurs via two processes: 1) increased ion pumping rates to elevate the calcium carbonate saturation state in the calcifying fluid; and 2) a higher daytime pH in the diffusion boundary layer that raises calcifying fluid pH. However, as irradiance increases, ion pumping becomes increasingly saturated limiting further enhancements. Our results also suggest the existence of two calcification strategies in coralline algae and indicate that magnesium incorporation is determined by the magnesium-to-calcium ratio in the calcifying fluid ([Mg]_CF/[Ca]_CF). This study adds to our mechanistic understanding of calcification in coralline algae and fills in much-needed knowledge about the role of light in controlling their physiology.

• Chlorophyll flourometry was used to obtain Fv/Fm (variable fluorescence [Fv] normalised to maximal fluorescence [Fm]), rETRmax (maximum relative elctron transport rate), Ek (minimum saturation intensity) and alpha (light use efficiency).
• Skeletal magnesium content in mol% and Full-Width-Half-Maximum were measured using confocal Raman spectroscopy.
• Chlorophyll a, phycoerything and phycocyanin content was calculated from absorbance readings (UV-vis spectrophotometer).
• Calcification rates were calculated from changes in weight (buoyant weight) which were normalised for surface area (cm^-2) and time (d^-1).
• Short-term calcification rates in the dark and the light were calculated from changes in total alkalinity which were normalised for surface area (cm-2) and time (h-1).
• Photosynthetic rates (gross) were calculated from changes in oxygen (O₂) during the day (net photosynthesis) and the night (dark respiration) which were normalised for surface area (cm^-2) and time (h^-1).
• Organic tissue carbon isotope values were calculated from the ratio between ¹³C and ¹²C