The influence of irradiance and interspecific differences on δ¹¹B, δ¹³C and elemental ratios in four coralline algae complexes

Abstract. Coralline algae are a cosmopolitan group of important foundational species. The calcium carbonate they produce is increasingly being investigated as paleoenvironmental archives, as well as used to trace physiological responses of these important macroalgae to environmental change. Here we address the impact of light (irradiance) on 4 species complexes of coralline red algae including two morphologies; geniculate (branching) and non-geniculate (encrusting). The four complexes up-regulated their δ¹¹B derived pH_CF relative to seawater by 0.6 to 0.8 pH unit. δ¹¹B was not measurably affected by varing irradiance despite evidence of increasing photosynthesis constrained by measurements of photophysiological parameters and δ¹³C_mineral. All complexes were able to maintain and elevate their pH_CF relative to seawater for all treatments. Non-geniculate and geniculate complexes had distinct geochemical signatures of δ¹¹B, δ¹³C_mineral and trace elements. These differences in geochemical signatures indicate a variety of calcification mechanisms exist within coralline algae.

We propose that different sources of dissolved inorganic carbon (DIC) are necessary to explain the observed δ¹³C_mineral. As geniculate species have higher photosynthetic activity (i.e. gross photosynthesis), the DIC sources allocated to calcification might be limited due to greater CO₂ drawdown. This is supported by B/Ca and U/Ca ratios suggesting modulation of carbonate chemistry and especially lower DIC_CF in geniculate relative to non-geniculate complexes. DIC sources might come from direct CO₂ diffusion or better recycling of metabolic CO₂ which would explain the depleted δ¹³C_mineral. This strategy likely arises from the different energy needs of the organisms, with non-geniculate using relatively more energy to support calcification. We suggest the different calcification mechanisms between morphologies are linked to distinct photosynthetic activity strategies. While photosynthesis can provide energy to geniculate complexes to maintain their metabolic needs, their calcification may be limited by DIC. In contrast, non-geniculate forms, may benefit from more limited DIC drawdown due to lower photosynthetic activity, therefore maintaining higher internal DIC concentrations ultimately supporting faster calcification.