Short-Term Response of Cytosolic N O 3 - to Inorganic Carbon Increase in Posidonia oceanica Leaf Cells.

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 HC O 3 - . Posidonia oceanica is a marine angiosperm that uses HC O 3 - as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake of HC 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 to N O 3 - we hypothesize that high HC O 3 - , or even CO2, would also promote a decrease of cytosolic N O 3 - ( N O 3 - c ). In this work we have used N 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 a N O 3 - 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 N O 3 - c decrease of 1 ± 0.04 mM and a Cl c - decrease of 3.5 ± 0.1 mM. The saturation of NSW with 1000 ppm CO2 also produced a diminution of the N O 3 - c , but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon ( HC O 3 - 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 N O 3 - diminution was much lower (0.1 ± 0.08 mM), pointing to HC O 3 - as the inorganic carbon species that causes the cytosolic N O 3 - leak. The incubation of P. oceanica leaf pieces in 3 mM HC O 3 - -enriched NSW triggered a short-term external N O 3 - net concentration increase consistent with the N O 3 - c leak. As a consequence, the cytosolic N 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.

Na⁺-Dependent High-Affinity Nitrate, Phosphate and Amino Acids Transport in Leaf Cells of the Seagrass Posidonia oceanica (L.) Delile.

Posidonia oceanica (L.) Delile is a seagrass, the only group of vascular plants to colonize the marine environment. Seawater is an extreme yet stable environment characterized by high salinity, alkaline pH and low availability of essential nutrients, such as nitrate and phosphate. Classical depletion experiments, membrane potential and cytosolic sodium measurements were used to characterize the high-affinity NO3-, Pi and amino acids uptake mechanisms in this species. Net uptake rates of both NO3- and Pi were reduced by more than 70% in the absence of Na⁺. Micromolar concentrations of NO3- depolarized mesophyll leaf cells plasma membrane. Depolarizations showed saturation kinetics (Km = 8.7 ± 1 µM NO3-), which were not observed in the absence of Na⁺. NO3- induced depolarizations at increasing Na⁺ also showed saturation kinetics (Km = 7.2 ± 2 mM Na⁺). Cytosolic Na⁺ measured in P. oceanica leaf cells (17 ± 2 mM Na⁺) increased by 0.4 ± 0.2 mM Na⁺ upon the addition of 100 µM NO3-. Na⁺-dependence was also observed for high-affinity l-ala and l-cys uptake and high-affinity Pi transport. All together, these results strongly suggest that NO3-, amino acids and Pi uptake in P. oceanica leaf cells are mediated by high-affinity Na⁺-dependent transport systems. This mechanism seems to be a key step in the process of adaptation of seagrasses to the marine environment.