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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 dissolvedHC   O  3  −.  Posidonia oceanica  is a marine angiosperm that usesHC   O  3  −as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake ofHC   O  3  −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 toN   O  3  −we hypothesize that highHC   O  3  −, or even CO 2 , would also promote a decrease of cytosolicN   O  3  −(    N   O  3  −c). In this work we have usedN   O  3  −- 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 aN   O  3  −cof 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 aN   O  3  −cdecrease of 1 ± 0.04 mM and aCl  c  −decrease of 3.5 ± 0.1 mM. The saturation of NSW with 1000 ppm CO 2  also produced a diminution of theN   O  3  −c, but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon (    HC   O  3  −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 cytosolicN   O  3  −diminution was much lower (0.1 ± 0.08 mM), pointing toHC   O  3  −as the inorganic carbon species that causes the cytosolicN   O  3  −leak. The incubation of  P. oceanica  leaf pieces in 3 mMHC   O  3  −-enriched NSW triggered a short-term externalN   O  3  −net concentration increase consistent with theN   O  3  −cleak. As a consequence, the cytosolicN   O  3  −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.

Short-Term Response of Cytosolic NO3− to Inorganic Carbon Increase in Posidonia oceanica Leaf Cells