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

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 HCO − 3. Posidonia oceanica is a marine angiosperm that uses HCO − 3 as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake of HCO − 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 NO − 3 we hypothesize that high HCO − 3 , or even CO 2 , would also promote a decrease of cytosolic NO − 3 (NO − 3 c). In this work we have used NO − 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 NO − 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 NaHCO 3 caused both a NO − 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 CO 2 also produced a diminution of the NO − 3 c, but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon (HCO − 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 cytosolic NO − 3 diminution was much lower (0.1 ± 0.08 mM), pointing to HCO − 3 as the inorganic carbon species that causes the cytosolic NO − 3 leak. The incubation of P. oceanica leaf pieces in 3 mM HCO − 3-enriched NSW triggered a shortterm external NO − 3 net concentration increase consistent with the NO − 3 c leak. As a consequence, the cytosolic NO − 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.

The differential ability of two species of seagrass to use carbon dioxide and bicarbonate and their modelled response to rising concentrations of inorganic carbon

“…For example, limitation of photosynthesis by light or temperature ( Maberly, 1985 ) or nutrients will decrease inorganic carbon limitation. In addition, Rubio et al. (2020) showed that there was an interaction between nitrate uptake and bicarbonate uptake in P. oceanica .…”

“…For example, limitation of photosynthesis by light or temperature (Maberly, 1985) or nutrients will decrease inorganic carbon limitation. In addition, Rubio et al (2020) showed that there was an interaction between nitrate uptake and bicarbonate uptake in P. oceanica. High concentrations of HCO − 3 led to loss of cytosolic nitrate via S-type anion channels, which could lead to, or increase, nitrogen limitation.…”