Short-Term Response of Cytosolic [Formula: see text] to Inorganic Carbon Increase in Posidonia oceanica Leaf Cells

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 [Formula: see text]. Posidonia oceanica is a marine angiosperm that uses [Formula: see text] as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake of [Formula: see text] 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 [Formula: see text] we hypothesize that high [Formula: see text] , or even CO(2), would also promote a decrease of cytosolic [Formula: see text] ([Formula: see text]). In this work we have used [Formula: see text]- 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 [Formula: see text] 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 [Formula: see text] decrease of 1 ± 0.04 mM and a [Formula: see text] decrease of 3.5 ± 0.1 mM. The saturation of NSW with 1000 ppm CO(2) also produced a diminution of the [Formula: see text] , but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon ([Formula: see text] 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 [Formula: see text] diminution was much lower (0.1 ± 0.08 mM), pointing to [Formula: see text] as the inorganic carbon species that causes the cytosolic [Formula: see text] leak. The incubation of P. oceanica leaf pieces in 3 mM [Formula: see text]-enriched NSW triggered a short-term external [Formula: see text] net concentration increase consistent with the [Formula: see text] leak. As a consequence, the cytosolic [Formula: see text] 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.