AUTHOR=Rubio Lourdes , García-Pérez Delia , Davies Julia M. , Fernández José A. TITLE=Short-Term Response of Cytosolic NO3− to Inorganic Carbon Increase in Posidonia oceanica Leaf Cells JOURNAL=Frontiers in Plant Science VOLUME=11 YEAR=2020 URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.00955 DOI=10.3389/fpls.2020.00955 ISSN=1664-462X ABSTRACT=

The concentration of CO₂ in the atmosphere has increased over the past 200 years and is expected to continue rising in the next 50 years at a rate of 3 ppm·year⁻¹. This increase has led to a decrease in seawater pH that has changed inorganic carbon chemical speciation, increasing the dissolved HCO3−. Posidonia oceanica is a marine angiosperm that uses HCO3− as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake of HCO3− is the diminution of cytosolic Cl⁻ (Cl⁻c) in mesophyll leaf cells due to the efflux through anion channels and, probably, to intracellular compartmentalization. Since anion channels are also permeable to NO3− we hypothesize that high HCO3−, or even CO₂, would also promote a decrease of cytosolic NO3− (NO3−c). In this work we have used NO3−- and Cl⁻-selective microelectrodes for the continuous monitoring of the cytosolic concentration of both anions in P. oceanica leaf cells. Under light conditions, mesophyll leaf cells showed a NO3−c of 5.7 ± 0.2 mM, which rose up to 7.2 ± 0.6 mM after 30 min in the dark. The enrichment of natural seawater (NSW) with 3 mM NaHCO₃ caused both a NO3−c decrease of 1 ± 0.04 mM and a Clc− decrease of 3.5 ± 0.1 mM. The saturation of NSW with 1000 ppm CO₂ also produced a diminution of the NO3−c, but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon (HCO3− or CO₂) in NSW would have an effect on the cytosolic anion homeostasis mechanisms in P. oceanica leaf cells. In the presence of 0.1 mM ethoxyzolamide, the plasma membrane-permeable carbonic anhydrase inhibitor, the CO₂-induced cytosolic NO3− diminution was much lower (0.1 ± 0.08 mM), pointing to HCO3− as the inorganic carbon species that causes the cytosolic NO3− leak. The incubation of P. oceanica leaf pieces in 3 mM HCO3−-enriched NSW triggered a short-term external NO3− net concentration increase consistent with the NO3−c leak. As a consequence, the cytosolic NO3− diminution induced in high inorganic carbon could result in both the decrease of metabolic N flux and the concomitant biomass N impoverishment in P. oceanica and, probably, in other aquatic plants.