Calcification in free-living coralline algae is strongly influenced by morphology: Implications for susceptibility to ocean acidification

Nadine Schubert,Antonella C Almeida Saá,Celso Peres Fernandes,Dirk De Beer,Rafael Güntzel Arenhart,Laurie C Hofmann,João Silva,Anderson Camargo Moreira,Paulo A Horta

doi:10.1038/s41598-021-90632-6

Abstract

Rhodolith beds built by free-living coralline algae are important ecosystems for marine biodiversity and carbonate production. Yet, our mechanistic understanding regarding rhodolith physiology and its drivers is still limited. Using three rhodolith species with different branching morphologies, we investigated the role of morphology in species’ physiology and the implications for their susceptibility to ocean acidification (OA). For this, we determined the effects of thallus topography on diffusive boundary layer (DBL) thickness, the associated microscale oxygen and pH dynamics and their relationship with species’ metabolic and light and dark calcification rates, as well as species’ responses to short-term OA exposure. Our results show that rhodolith branching creates low-flow microenvironments that exhibit increasing DBL thickness with increasing branch length. This, together with species’ metabolic rates, determined the light-dependent pH dynamics at the algal surface, which in turn dictated species’ calcification rates. While these differences did not translate in species-specific responses to short-term OA exposure, the differences in the magnitude of diurnal pH fluctuations (~ 0.1–1.2 pH units) between species suggest potential differences in phenotypic plasticity to OA that may result in different susceptibilities to long-term OA exposure, supporting the general view that species’ ecomechanical characteristics must be considered for predicting OA responses.

Highlights

Rhodolith beds built by free-living coralline algae are important ecosystems for marine biodiversity and carbonate production
Those were found to exhibit a strong effect on the diffusive boundary layer (DBL), showing an increased DBL thickness in-between protuberances, which is consistent with previous studies in the temperate rhodolith Sporolithon durum[29] and in undulated Macrocystis pyrifera blades[33]
As shown in the latter study, topographic depressions, like those created by protuberances in rhodoliths, create low-flow microhabitats that lead to an increase of DBL thickness

Summary

Introduction

Rhodolith beds built by free-living coralline algae are important ecosystems for marine biodiversity and carbonate production. Marine coastal ecosystems formed by free-living coralline algae that cover 30–100% of the seafloor, so-called rhodolith or maërl beds, are distributed worldwide and have long been known to be important biodiversity hot-spots and major carbonate-producing e cosystems[1,2]. Because of their importance for biodiversity, in several regions of the world they are considered critical habitats for marine conservation and protected by a range of directives, regulations and c onventions[2]. In combination with our limited mechanistic understanding of their calcification mechanism, its regulation and potential species-specific differences, this makes it difficult to anticipate potential future impacts on these organisms and the ecosystems they build—a point widely stressed as an important tool to improve predictive power of OA s tudies[17,18,19,20]

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