Abstract
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 . Posidonia oceanica is a marine angiosperm that uses as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake of 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 we hypothesize that high , or even CO2, would also promote a decrease of cytosolic (). In this work we have used - 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 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 decrease of 1 ± 0.04 mM and a decrease of 3.5 ± 0.1 mM. The saturation of NSW with 1000 ppm CO2 also produced a diminution of the , but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon ( 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 diminution was much lower (0.1 ± 0.08 mM), pointing to as the inorganic carbon species that causes the cytosolic leak. The incubation of P. oceanica leaf pieces in 3 mM -enriched NSW triggered a short-term external net concentration increase consistent with the leak. As a consequence, the cytosolic 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.
Highlights
Nitrate ion (NO−3 ) is the main source of inorganic nitrogen for plants in aerobic conditions
The saturation of natural seawater (NSW) with 1000 ppm CO2 produced a diminution of the NO−3c, but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon (HCO−3 or CO2) in NSW would have an effect on the cytosolic anion homeostasis mechanisms in P. oceanica leaf cells
We have previously reported that in P. oceanica mesophyll leaf cells, incubated in light conditions, the addition of NSW enriched with 3 mM HCO−3 evoked an initial and transient plasma membrane depolarization that turned into a transient hyperpolarization to stabilize at a depolarized value (Rubio et al, 2017)
Summary
Nitrate ion (NO−3 ) is the main source of inorganic nitrogen for plants in aerobic conditions. To survive in the conditions of low NO−3 availability, seagrasses have evolved high affinity uptake systems to capture NO−3 through their leaves These systems have been characterized for Zostera marina in which the uptake of NO−3 and inorganic phosphate (Pi) are driven by the inwardly directed electrochemical gradient for Na+ (Garcıá -Sánchez et al, 2000; Rubio et al, 2005). Similar high-affinity and Na+-dependent uptake mechanisms operate in P. oceanica for both nutrients and some amino acids (Rubio et al, 2018) In both cases, the low semi-saturation constants observed (2.3 and 8.7 μM NO−3 for Z. marina and P. oceanica, respectively: Garcıá -Sánchez et al, 2000; Rubio et al, 2018) indicate that those systems are very efficient for NO−3 uptake at the very low concentrations of NO−3 in seagrass meadows (Touchette and Burkholder, 2000; Romero et al, 2006). As with other vascular plants, seagrasses must maintain intracellular NO−3 homeostasis to preserve the N metabolic flux
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