title: Biotic Control of Surface pH and Evidence of Light-Induced H+ Pumping and Ca2+-H+ Exchange in a Tropical Crustose Coralline Alga.
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.
authors.affiliation: Microsensor Group, Max Planck Institute for Marine Microbiology, Bremen, Germany.
authors.forename: Laurie C
authors.initials: LC
authors.lastname: Hofmann
authors.affiliation: Aquatic Plant Ecology Lab, Biological Sciences Department, Florida Atlantic University, Boca Raton, Florida, United States of America.
authors.forename: Marguerite
authors.initials: M
authors.lastname: Koch
authors.affiliation: Microsensor Group, Max Planck Institute for Marine Microbiology, Bremen, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: de Beer
mesh_terms: D017920:Antiporters / Q000378:metabolism*; D001639:Bicarbonates / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D002244:Carbon / Q000378:metabolism; D002256:Carbonic Anhydrases / Q000378:metabolism; D027682:Cation Transport Proteins / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008027:Light; D008954:Models, Biological; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis; D017494:Proton Pumps / Q000378:metabolism*; D012624:Seaweed / Q000502:physiology* / Q000528:radiation effects* / Q000648:ultrastructure
chemical_list: D017920:Antiporters; D001639:Bicarbonates; D027682:Cation Transport Proteins; D017494:Proton Pumps; C027614:calcium-hydrogen antiporters; D002244:Carbon; D002256:Carbonic Anhydrases; D010100:Oxygen; D002118:Calcium