Biotic Control of Surface pH and Evidence of Light-Induced H+ Pumping and Ca2+-H+ Exchange in a Tropical Crustose Coralline Alga.

Laurie C. Hofmann,Dirk de Beer,Marguerite Koch,Wei-Chun Chin

doi:10.1371/journal.pone.0159057

Abstract

Presently, an incomplete mechanistic understanding of tropical reef macroalgae photosynthesis and calcification restricts predictions of how these important autotrophs will respond to global change. Therefore, we investigated the mechanistic link between inorganic carbon uptake pathways, photosynthesis and calcification in a tropical crustose coralline alga (CCA) using microsensors. We measured pH, oxygen (O2), and calcium (Ca2+) dynamics and fluxes at the thallus surface under ambient (8.1) and low (7.8) seawater pH (pHSW) and across a range of irradiances. Acetazolamide (AZ) was used to inhibit extracellular carbonic anhydrase (CAext), which mediates hydrolysis of HCO3-, and 4,4′ diisothiocyanatostilbene-2,2′-disulphonate (DIDS) that blocks direct HCO3- uptake by anion exchange transport. Both inhibited photosynthesis, suggesting both diffusive uptake of CO2 via HCO3- hydrolysis to CO2 and direct HCO3- ion transport are important in this CCA. Surface pH was raised approximately 0.3 units at saturating irradiance, but less when CAext was inhibited. Surface pH was lower at pHSW 7.8 than pHSW 8.1 in the dark, but not in the light. The Ca2+ fluxes were large, complex and temporally variable, but revealed net Ca2+ uptake under all conditions. The temporal variability in Ca2+ dynamics was potentially related to localized dissolution during epithallial cell sloughing, a strategy of CCA to remove epiphytes. Simultaneous Ca2+ and pH dynamics suggest the presence of Ca2+/H+ exchange. Rapid light-induced H+ surface dynamics that continued after inhibition of photosynthesis revealed the presence of a light-mediated, but photosynthesis-independent, proton pump. Thus, the study indicates metabolic control of surface pH can occur in CCA through photosynthesis and light-inducible H+ pumps. Our results suggest that complex light-induced ion pumps play an important role in biological processes related to inorganic carbon uptake and calcification in CCA.

Highlights

Crustose coralline algae (CCA) are a cosmopolitan group of red calcifying macroalgae (Rhodophyta) that provide critical habitats for a diversity of marine organisms [1,2,3,4] and facilitate settlement for many benthic larvae [5,6,7], including new coral recruits on reefs [8,9]
Using Eq 3, we calculated that oxygen was produced within the first three to six cell layers, based on cell size from our scanning electron micrographs (SEM) images. The rate of both the pH and O2 increase or decrease upon illumination or darkening, respectively, was extremely fast, reaching steady state in < 1 min. This rapid response indicated that the pH dynamics must have been partially biologically controlled, rather than an indirect consequence of carbonate chemistry change due to CO2 uptake during photosynthesis
The results of this study indicate that this tropical CCA has the potential to metabolically control the pH, O2 and Ca2+ concentration at the surface of its thalli

Summary

Introduction

Crustose coralline algae (CCA) are a cosmopolitan group of red calcifying macroalgae (Rhodophyta) that provide critical habitats for a diversity of marine organisms [1,2,3,4] and facilitate settlement for many benthic larvae [5,6,7], including new coral recruits on reefs [8,9]. Pueschel et al [25] present data supporting metabolic control of calcification and decalcification of epithallial cells necessary for new cellular growth beyond older calcified cells in the epithallial region [27] They clearly show using transmission electron microscopy (TEM), that organic microfibrils of the cell wall produce ingrowths that they suggest increase the surface area for [H+] flux and controls decalcification. Adey et al [12] contend that CCA calcification and decalcification are highly controlled metabolic processes incorporating complex ion transport systems These processes support CaCO3 growth associated with the cell wall and formation of reproductive structures that necessitate the dissolution of several adjacent calcified vegetative cells. Different modes of calcification driven by diverse processes complicate identifying mechanisms of CCA calcification and may account for some discrepancies (i.e. strong negative effect, no effect, acclimation) on the pCO2 effects of CCA calcification in the ecological literature (e.g. [13,19,20,21,22,23,24])

Methods

Results

Conclusion