The role of irradiance in controlling coralline algal calcification

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 (CF) 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 CF; and 2) a higher daytime pH in the diffusion boundary layer that raises pHCF. 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 internal magnesium to calcium ratio [Mg]CF/[Ca]CF. This study adds to our mechanistic understanding of calcification in coralline and fills in much needed knowledge about the role of light in controlling their physiology.