Ocean acidification (OA) is a major threat to marine ecosystems, particularly coral reefs which are heavily reliant on calcareous species. OA decreases seawater pH and calcium carbonate saturation state (Ω), and increases the concentration of dissolved inorganic carbon (DIC). Intense scientific effort has attempted to determine the mechanisms via which ocean acidification (OA) influences calcification, led by early hypotheses that calcium carbonate saturation state (Ω) is the main driver. We grew corals and coralline algae for 8-21 weeks, under treatments where the seawater parameters Ω, pH, and DIC were manipulated to examine their differential effects on calcification rates and calcifying fluid chemistry (Ω , pH , and DIC ). Here, using long duration experiments, we provide geochemical evidence that differing physiological controls on carbonate chemistry at the site of calcification, rather than seawater Ω, are the main determinants of calcification. We found that changes in seawater pH and DIC rather than Ω had the greatest effects on calcification and calcifying fluid chemistry, though the effects of seawater carbonate chemistry were limited. Our results demonstrate the capacity of organisms from taxa with vastly different calcification mechanisms to regulate their internal chemistry under extreme chemical conditions. These findings provide an explanation for the resistance of some species to OA, while also demonstrating how changes in seawater DIC and pH under OA influence calcification of key coral reef taxa.

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.

Ocean warming (OW) and marine heatwaves (MHWs) rapidly transform marine ecosystems, especially when they impact keystone or foundation species. Foundation species such as kelps, fucoids and corals are highly sensitive to heat stress, which threatens the future of temperate seaweed forests and tropical reefs. However, functioning and resilience of these systems also relies on the less conspicuous coralline algae, whose thermal tolerances have gone largely untested. Here, we examined the sensitivity of four temperate coralline algal morphotypes from three different species to four realistic present day and future OW and MHW scenarios (ambient [16°C constant]; ambient+MHW [16°C baseline + a symmetric two-week heatwave with a peak intensity of 18.7°C]; future [18.7°C constant]; future+MHW [18.7°C baseline + a symmetric two-week heatwave with a peak intensity of 21.4°C]). Photo-physiology (e.g., Fv/Fm) and calcification physiology (e.g., proxies for calcifying fluid saturation state ΩCF) were generally unaffected by the treatments, implying a high thermo-tolerance of our study species compared to other important marine foundation species. We ascribe this mainly to their photosynthetic apparatus that, unlike in other photoautotrophs, continued to function under heat stress. Experimental evidence presented here and elsewhere implies that coralline algae are likely to continue to play their crucial ecological roles in a warming ocean. Yet, such predictions are fraught with uncertainty due to the substantial gaps in our knowledge. We attempt to highlight some of these gaps and aim to present potential physiological underpinnings of their thermo-tolerance.

Understanding the drivers of net coral reef calcium carbonate production is increasingly important as ocean warming, acidification, and other anthropogenic stressors threaten the maintenance of coral reef structures and the services these ecosystems provide. Despite intense research effort on coral reef calcium carbonate production, the inclusion of a key reef forming/accreting calcifying group, the crustose coralline algae, remains challenging both from a theoretical and practical standpoint. While corals are typically the primary reef builders of contemporary reefs, crustose coralline algae can contribute equally. Here, we combine several sets of data with numerical and theoretical modelling to demonstrate that crustose coralline algae carbonate production can match or even exceed the contribution of corals to reef carbonate production. Despite their importance, crustose coralline algae are often inaccurately recorded in benthic surveys or even entirely missing from coral reef carbonate budgets. We outline several recommendations to improve the inclusion of crustose coralline algae into such carbonate budgets under the ongoing climate crisis.