The problem of living in the sea: the uptake of inorganic carbon and nutrients in Posidonia oceanica (L.) Delile
Rubio-Valverde, Lourdes,García-Pérez, Delia,García-Sánchez, María Jesús,Fernández-García, José Antonio
- Published
- 2015-06-02
- Language
- en
Abstract
The genus Posidonia exhibits a peculiar geographical distribution. It is composed by nine species, eight of which are distributed along the Australian coasts and only one, Posidonia oceanica (L.) Delile, is a Mediterranean endemism. Like other angiosperms, P. oceanica has adapted secondarily to the marine environment, and has developed anew mechanisms to face a liquid and alkaline medium (pH 8.2) that contains a high salt concentration (0.5 M NaCl). The liquid environment limits the diffusive flow of CO2 and nutrients and, furthermore, CO2 dissolves in water and forms HCO3-, the more abundant chemical species of inorganic carbon at pH 8.2. Like other green plants P. oceanica uses CO2 for photosynthesis. In addition, this species shows a transport system in the plasma membrane for the direct uptake of HCO3-, that uses H+ as the driving ion. The addition of HCO3- provokes a transient hyperpolarization of the plasma membrane followed by a depolarization; at the same time, the cytosolic pH (pHc) becomes transiently acidic and next it gets alkaline, and remains alkaline throughout the HCO3- pulse. The alkalinization of the pHc is due to the cytosolic accumulation of HCO3- and OH- and it is sensitive to the addition of ethoxyzolamide, an inhibitor of the internal carbonic anhydrase. The increase of negative charges in the cytosol triggers the release of Cl- to recover the values of the resting membrane potential. The plasmalemma of P. oceanica exhibits a reduced Na+ permeability and shows a H+/Na+ antiporter activity that keeps low and relatively constant the cytosolic Na+ concentration (17 mM Na+). The inside negative membrane potential (-178 mV) and the low [Na+]c generate a tremendous Na+-motive force that this plant uses for the high affinity transport of NO3- (Km= 21 µM), and of the amino acids alanine (Km= 37 µM) and cysteine (Km= 10 µM). The uptake of these compounds shows a strict dependence on the presence of Na+ in the medium. Moreover, the addition of micromolar concentrations of NO3-, alanine or cysteine gives rise to millimolar increments of [Na+]c. Experiments with external LIX pH mini-electrodes show that the uptake of glucose is not Na+ but H+ dependent. Thus, the model for the ion transport energization in this species seems to be mixed, with a H+-ATPase as the primary pump and a series of carriers that use H+ (HCO3-, Na+, glucose) or Na+ (NO3-, amino acids) as the driving ion. Project Funding: CTM 2011-30356 (MEC)
Full text
! The! problems! of! living! in!the! sea:! the! uptake! of! inorganic! carbon! and! nutrients!in!Posidonia(oceanica!(L.)!Delile.! ! Lourdes!Rubio,!Delia!GarcíaAPérez,!María!Jesús!GarcíaASánchez!and!José!Antonio! Fernández! Dpto.& Biología& Vegetal.& Facultad& de& Ciencias.& Universidad& de& Málaga.& Campus& de& Teatinos&s/n.&29071&Málaga&(Spain)& [email protected] 0 0 The0 genus0 Posidonia0exhibits0 a0 peculiar0 geographical0 distribution.0 It0 is0 composed0 by0 nine0 species,0 eight0 of0 which0 are0 distributed0 along0 the0 Australian0 coasts0 and0 only0 one,0 Posidonia& oceanica0(L.)0 Delile,0 is0 a0 Mediterranean0 endemism.0 Like0 other0 angiosperms,0 P.& oceanica0has0 adapted0secondarily0to0the0marine0environment,0and0has0developed0anew0mechanisms0to0face0 a0liquid0and0alkaline0medium0(pH08.2)0that0contains0a0high0salt0concentration0(0.50M0NaCl).0The0 liquid0 environment0 limits0 the0 diffusive0 flow0 of0 CO20and0 nutrients0 and,0 furthermore,0 CO20 dissolves0in0water0and0forms0HCO3 Q,0the0more0abundant0chemical0species0of0inorganic0carbon0 at0pH08.2.0 Like0other0green0plants0P.&oceanica0uses0CO20for0photosynthesis.0In0addition,0this0species0shows0 a0transport0system0in0the0plasma0membrane0for0the0direct0uptake0of0HCO3 Q,0that0uses0H+0as0the0 driving0 ion.0 The0 addition0 of0 HCO3 Q0provokes0 a0 transient0 hyperpolarization0 of0 the0 plasma0 membrane0 followed0 by0 a0 depolarization;0 at0 the0 same0 time,0 the0 cytosolic0 pH0(pHc)0 becomes0 transiently0acidic0and0next0it0gets0alkaline,0and0remains0alkaline0throughout0the0HCO3 Q0pulse.0 The0alkalinization0of0the0pHc0is0due0to0the0cytosolic0accumulation0of0HCO3 Q0and0OHQ0and0it0is0 sensitive0to0the0addition0of0ethoxyzolamide,0an0 inhibitor0 of0the0internal0carbonic0anhydrase.0 The0increase0of0negative0charges0in0the0cytosol0triggers0the0release0of0ClQ0to0recover0the0values0 of0the0resting0membrane0potential.0The0plasmalemma0of0P.&oceanica0exhibits0a0reduced0Na+0 permeability0and0shows0a0H+/Na+0antiporter0activity0that0keeps0low0and0relatively0constant0the0 cytosolic0Na+0concentration0(170mM0Na+).0The0inside0negative0membrane0potential0(Q1780mV)0 and0the0low0[Na+]c0generate0a0tremendous0Na+Qmotive0force0that0this0plant0uses0for0the0high0 affinity0 transport0of0 NO3 Q0(Km=0 210 µM),0 and0of0the0 amino0acids0 alanine0 (Km=0 370 µM)0and0 cysteine0(Km=0 100 µM).0 The0 uptake0 of0 these0 compounds0 shows0 a0 strict0 dependence0 on0the0 presence0of0Na+0in0the0medium.0Moreover,0the0addition0of0micromolar0concentrations0of0NO3 Q,0 alanine0or0cysteine0gives0rise0to0millimolar0increments0of0[Na+]c.0Experiments0with0external0LIX0 pH0miniQelectrodes0show0that0the0uptake0of0glucose0is0not0Na+0but0H+0dependent.0Thus,0the0 model0for0the0ion0transport0energization0in0this0species0seems0to0be0mixed,0with0a0H+QATPase0as0 the0primary0pump0and0a0series0of0carriers0that0use0H+0(HCO3 Q,0Na+,0glucose)0or0Na+0(NO3 Q,0amino0 acids)0as0the0driving0ion.00 0 Project0Funding:0CTM02011Q303560(MEC)0 0
