Abstract
The concentration of CO(2) 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 CO(2), 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 NaHCO(3) 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 CO(2) 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 CO(2)) 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 CO(2)-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.