The influence of irradiance and interspecific differences on <i>δ</i> ¹¹ B, <i>δ</i> ¹³ C and elemental ratios in four coralline algae complexes from Aotearoa, New Zealand

Abstract. Coralline algae are a cosmopolitan group of important foundational species. The calcium carbonate they produce is increasingly being used as paleoenvironmental archives, as well as used to trace physiological responses of these important macroalgae to environmental change. In this context, evaluating the effect of oceanic change and photo-physiological parameters on geochemical proxies is critical, as such gaps may lead to erroneous paleoenvironmental reconstructions, misattributed drivers of calcification responses, and ultimately compromise conservation strategies. Here we address the impact of light (irradiance) on four species complexes of coralline red algae including two morphologies; geniculate (branching) and non-geniculate (encrusting). The four complexes up-regulated their δ11B derived pHCF relative to seawater by 0.6 to 0.8 pH unit. δ11B was not measurably affected by varying irradiance despite evidence of increasing photosynthesis. All complexes were able to maintain and elevate their pHCF relative to seawater for all treatments. Non-geniculate and geniculate complexes had distinct geochemical signatures of δ11B, δ13Cmineral 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 δ13Cmineral. 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 DICCF 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 δ13Cmineral. 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 different interactions between photosynthesis and carbon allocation. 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.