HELGOLfl.NDER MEERESUNTERSUCHUNGEN
Helgol~inder Meeresunters. 34, 151-158 (1980)
On the s e l e c t i v e a d s o r p t i o n of c a t i o n s in the cell w a l l of
the g r e e n a l g a V a l o n i a utricularis
H. Kesseler
Biologische Anstalt Helgoland (Litoralstation};
D-2282 List/Sylt, Federal Republic of Germany
ABSTRACT: The selective adsorption of the cations Na +, K +, M g ++ a n d Ca + + by the cell wall of
the M e d i t e r r a n e a n alga Valonia utricularis (Siphonocladales, Chlorophyceae) from sea w a t e r of
40 TooS was investigated by extraction of cell-wall preparations, eluted before in 1.1 mol m e t h a n o l
{adjusted to pH 8) with 0A n formic acid in a Soxhlet apparatus. Na + a n d K + were d e t e r m i n e d b y
flame photometry, M g ++ a n d Ca + + by complexometric titration with EDTA. From calculation of
the dry weight:fresh w e i g h t ratios a n d the chloride d e t e r m i n a t i o n s in the eluates, the D o n n a n freespace fraction of the total cell-wall volume was calculated to about 35 %, a n d the analytical results
of the cation concentrations in the extracts expressed as ~Val cm -3 DFS. This calculation is b a s e d on
the assumption that the acidic groups of the noncellulosic matrix material, carrying n e g a t i v e
charges by dissociation at the reaction of sea w a t e r (ph a b o u t 8) are responsible for the adsorption of
cations by exhibition of a D o n n a n effect. The results o b t a i n e d show clearly that besides the divalent
cations M g ++ and Ca + +, w h i c h according to the physico-chemical laws of the D o n n a n distribution
must b e relatively a c c u m u l a t e d to the second power of the m o n o v a l e n t ones, potassium is also
enriched by selective adsorption, a n d t h e K + :Na + ratio increased significantly c o m p a r e d with that
in sea water. This seems to indicate that the s t r e n g t h of attraction b e t w e e n the cations a n d the
negative sites is d e p e n d e n t on the radii of the ions a n d the state of hydration a n d / o r polarisation of
the ions and b i n d i n g sites.
INTRODUCTION
I n h i g h e r p l a n t s t h e a d s o r p t i o n of i o n s b y t h e c e l l w a l l s of r o o t s is t h e p r i m a r y s t e p
a n d t h e t r i g g e r i n g e v e n t i n t h e c h a i n of r e a c t i o n s i n v o l v e d i n t h e u p t a k e a n d a c c u m u l a t i o n of salts. D e s p i t e t h e s i m p l y p h y s i c a l n a t u r e of t h i s r e a c t i o n i t is n e v e r t h e l e s s a r e a l
a c c u m u l a t i v e p r o c e s s s i n c e t h e c o n c e n t r a t i o n of t h e i o n s a t t h e a d s o r b i n g s u r f a c e s is
commonly much higher than in the surrounding soil solution.
These conditions have long been recognized, and were closely investigated by
m a n y a u t h o r s ( W i l l i a m s & C o l e m a n , 1950; B u r s t r 6 m , 1951; E p s t e i n , 1955; K r a m e r , 1956;
J a c o b s e n e t al., 1958; L u n d e g a r d h , 1958}. I n m a r i n e a l g a e , h o w e v e r , t h e r61e of t h e c e l l
wall in the mineral metabolism has scarcely been considered.
T h i s m a y b e d u e to t h e f a c t t h a t i n s o l u t i o n s of h i g h i o n i c s t r e n g t h l i k e s e a w a t e r t h e
D o n n a n effect, w h i c h is r e s p o n s i b l e for t h e a d s o r p t i o n of d i f f u s i b l e i o n s of t h e m e d i u m
b y f i x e d c h a r g e s of o p p o s i t e s i g n w i t h i n t h e c e l l w a l l s , is n o t so p r o n o u n c e d a s i n d i l u t e
m e d i a l i k e t h e s o i l s o l u t i o n a n d f r e s h w a t e r . T h e r e f o r e , its a c c u m u l a t i v e e f f e c t m a y b e
u n d e r e s t i m a t e d i n s e a w a t e r a n d its s e l e c t i v i t y n e g l e c t e d or e v e n s i m p l y o v e r l o o k e d .
It m a y , h o w e v e r , l e a d to t h e p r e f e r e n t i a l a d s o r p t i o n n o t o n l y of p o l y v a l e n t c a t i o n s ,
a c c o r d i n g to t h e p h y s i c o c h e m i c a l l a w s for t h e D o n n a n d i s t r i b u t i o n of i o n s , b u t a l s o to
© Biologische Anstalt Helgoland
0174-3597/80/0034/0151/$ 02.00
152
H. Kesseler
that of c e r t a i n m o n o v a l e n t ones. This will d e p e n d i n detail on the very complex
i n t e r r e l a t i o n s h i p s b e t w e e n the fixed charges (,,sites") a n d the different ions according to
the s t r e n g t h of attraction by C o u l o m b a n d n o n - C o u l o m b forces i n f l u e n c i n g the states of
h y d r a t i o n a n d p o l a r i s a t i o n of sites a n d ions (Diamond & Wright, 1969).
T h e d i s r e g a r d of these c o n d i t i o n s m a y be the reason that very little information is
a v a i l a b l e a b o u t the diffusion of ions across the cell walls, their i o n - e x c h a n g e properties
a n d their spatial h o m o g e n e i t y (Hope & Walker, 1975, p. 560. The few data on this
subject are scattered in the form of occasional a n d isolated observations. Moreover, they
are c o n c e r n e d p r i m a r i l y with the c o n d i t i o n s i n freshwater algae, where, for the reasons
m e n t i o n e d above, the D o n n a n effect is more p r o n o u n c e d t h a n i n m a r i n e algae.
The only papers d e a l i n g with ion adsorption in d e a d tissue of a m a r i n e red alga are
those of E p p l e y & Blinks (1957) a n d E p p l e y (1957). T h e y reported on the adsorption of
Ca + +, Na ÷, a n d K + i n Porphyra perforata as well as o n the e x c h a n g e of a d s o r b e d Na + for
different mono-, di-, a n d trivatent cations from 0.1 n solutions. While they could not
detect a n y p o t a s s i u m i n the Porphyra tissue k i l l e d b y b o i l i n g or by i m m e r s i o n i n cold
50 % e t h a n o l the ratios of the v a l u e s reported for a d s o r b e d Ca +÷ a n d Na + correspond
rather well with our results, as will be discussed later.
Besides these s i n g l e o b s e r v a t i o n s on the ion contents i n d e a d tissue of a m a r i n e alga,
there are only three more p a p e r s d e a l i n g with the i o n - e x c h a n g e properties i n cell-wall
m a t e r i a l of the freshwater a l g a e Nitella axillaris (Diamond & Solomon, 1959) a n d Chara
australis (Dainty & Hope, 1959; D a i n t y et al., 1960). T h e former authors s t u d i e d the
e x c h a n g e kinetics of only 42K i n the three m a i n c o m p a r t m e n t s (cell wall, cytoplasm a n d
vacuole) of IV. axillaris; from this they c a l c u l a t e d the total p o t a s s i u m c o n c e n t r a t i o n in the
single compartments. D a i n t y et al. (1960), on the other hand, i n v e s t i g a t e d the e x c h a n g e
a d s o r p t i o n of l a b e l l e d Ca* a n d Na* i n the cell walls of C. australis. T h e y also d e t e r m i n e d
the c o n c e n t r a t i o n of the n o n d i f f u s i b l e a n i o n s i n the walls (0.8 Val 1-1) as well as the m e a n
dissociation constant (pK = 2.2) of the o r g a n i c acids, from w h i c h the fixed cell wall
a n i o n s are derived, a n d s h o w e d that the initial ion e x c h a n g e took place i n the D o n n a n
p h a s e of the cell walls.
This p h a s e is d e f i n e d as that fraction of the free space or outer space in p l a n t cells
a n d tissues (Kramer, 1957) i n w h i c h the d i s t r i b u t i o n of the diffusible cations a n d anions
of the s u r r o u n d i n g m e d i u m is i n f l u e n c e d b y the p r e s e n c e of the indiffusible a n i o n s of the
wall material: w h i l e the m o b i l e cations a c t i n g as c o u n t e r i o n s are attracted by the fixed
n e g a t i v e charges, the diffusible a n i o n s or co-ions are repelled. Despite the u n e q u a l
d i s t r i b u t i o n of the m o b i l e ions of opposite sign i n the D o n n a n phase or D o n n a n free
space (DFS), in the state of e q u i l i b r i u m their product must be e q u a l to that i n the
e n v i r o n m e n t a l solution (Fig. 1).
In contrast to the DFS the water free space (WFS) is that part of the free space in
w h i c h the c o n c e n t r a t i o n of the solutes is e q u a l to that in the m e d i u m . For the d e t e r m i n a tion of the free space of cells or tissues one has to take into consideration, therefore, that
with different solutes, e. g. electrolytes a n d n o n e l e c t r o l y t e s or e v e n different k i n d s of
both, different v a l u e s are to b e expected. I n s t e a d of the true free space for every solute
only its a p p a r e n t free space (AFS) will be obtained, d e f i n e d as that part of free space to
w h i c h the solute s e e m s to move r e a d i l y by diffusion (for further details see Briggs &
Robertson, 1957; Briggs et al., 1961; Levitt, 1957).
While i n f o r m a t i o n on the i o n - e x c h a n g e properties of algal cell walls is far from
S e l e c t i v e c a t i o n a d s o r p t i o n i n Valonia c e l l w a l l s
153
a m p l e , n o r e l i a b l e d a t a a r e a v a i l a b l e a b o u t t h e r e a l c o n c e n t r a t i o n s a n d r e l a t i o n s of t h e
c a t i o n s of t h e n a t u r a l s u r r o u n d i n g m e d i u m i n c e l l w a l l s of m a r i n e a l g a e . T h i s s u r e l y is
d u e to t h e d i f f i c u l t i e s of o b t a i n i n g p u r e c e l l - w a l l p r e p a r a t i o n s f r o m m o s t s p e c i e s . F o r t h e
i n v e s t i g a t i o n s d e s c r i b e d b e l o w , t h e l a r g e c e l l e d c o e n o c y t i c M e d i t e r r a n e a n a l g a Valonia
utricularis w a s c h o s e n .
cell wall
I DFS
NAS
medium
SW-FS
(SW)
No+ CI-
Nd CI-
CI- Nd
c[ Nd
K÷ CI-
Nd CI-
Cl- Nd
CI-
Nd CI-
Ud CI-
CI- Nd
CI- Nd
6"x612
6+x 612
Nd
K ÷ CI-
Nd
>! K÷ Nd
K÷
Nd Nd
Nd CIion products
12+x3sum of 13articles
15
(£ osmotic potential)
K÷
Fig. 1. Diagrammatic sketch of the ion distribution b e t w e e n the cell wall of a m a r i n e alga a n d the
surrounding sea water. For reasons of simplification only the m o n o v a t e n t ions are symbolized. NAS
= non-accessible space ( ~ dry wall material); DFS = D o n n a n free space containing the b i n d i n g
sites (-) as well as their counterions of opposite charge (Na +, K +) a n d the co-ions (C1-) of the same
charge; SW-FS = Sea water free space, i.e. that part of the cell wall (large pores a n d / o r
interfibrillar spaces) w h i c h is freely accessible by diffusion to all ions of the s u r r o u n d i n g sea w a t e r
in equal distribution while single ions or anelectrolytes may also e n t e r the DFS in different
concentrations according to their charges a n d their states of hydration a n d polarisation. - . . . .
potential barrier b e t w e e n DFS a n d SW-FS. In the state of electrochemical e q u i l i b r i u m the activity
products of the diffusible ions must be e q u a l in all compartments, w h i l e the sum of the diffusible
particles, responsible for the m a g n i t u d e of the osmotic potential, must b e greater in the DFS t h a n in
the s u r r o u n d i n g m e d i u m
MATERIALS AND
METHODS
V e s i c l e s of t h e M e d i t e r r a n e a n c h l o r o p h y c e a n Valonia utricularis, a b o u t 1.5 c m l o n g
a n d 0.5 c m i n d i a m e t e r , w e r e u s e d for t h e e x p e r i m e n t s . S o m e of t h e s e a l g a e w e r e
b r o u g h t to o u r l a b o r a t o r y f r o m t h e N a p l e s Z o o l o g i c a l S t a t i o n i n 1964. T h e a l g a e h a v e
s i n c e b e e t 1 c u l t i v a t e d i n a t e m p e r a t u r e - c o n s t a n t r o o m a t 18 °C i n l a r g e 100-1 t a n k s , m a d e
154
H. Kesseler
from p o l y v i n y l chloride, i n raw mass culture. T h e y are kept free of diatoms a n d other
e p i p h y t e s b y the littoral molluscs Littorina littorea a n d Hydrobia ulvae as well as by
different a m p h i p o d s , isopods a n d other s m a l l a n i m a l s , which u s u a l l y graze on algae in
the littoral zone. This c u l t i v a t i o n m e t h o d s i m u l a t i n g n a t u r a l c o n d i t i o n s has proved to be
successful a n d c o n v e n i e n t e v e n with other algae. I l l u m i n a t i o n is provided by fluorescent
l a m p s i n a day: n i g h t r h y t h m of 14:10 h at a n i n t e n s i t y of a b o u t 2000 Ix. Every w e e k
a b o u t one third of the sea w a t e r is r e p l a c e d b y freshly filtered water, e n r i c h e d with KNO3
a n d Na2HPO4 if required. E v a p o r a t i o n b y a e r a t i o n causes salinity c h a n g e s in the r a n g e
of a b o u t 35-40 Too. U n d e r these conditions the a l g a e flourish well: they are bright dark
g r e e n a n d turgid.
For every e x p e r i m e n t four sets of a b o u t 300 ceils w e r e used. The cell-wall preparations were o b t a i n e d b y s q u a s h i n g the vesicles a n d r u b b i n g t h e m b e t w e e n the fingers to
r e m o v e the c y t o p l a s m (Kornmann, 1934). After this t r e a t m e n t the cell walls were rinsed
several times i n sterilized sea w a t e r of 40 TooS a n d stored o v e r n i g h t i n this m e d i u m . The
next day the s a m p l e s w e r e c e n t r i f u g e d at 5000 r. p. m. (about 3000 g) to remove the
a d h e r i n g sea w a t e r a n d t h e n b l o t t e d by vigorous p r e s s i n g b e t w e e n a b s o r b e n t tissue.
After w e i g h i n g , the p r e p a r a t i o n s (about 0.5 g each) were r i n s e d five times i n 50 ml of
1.15 mol a q u e o u s m e t h a n o l solution, isotonic to sea w a t e r of 40 %0 S, for several m i n u t e s
to r e m o v e the sea w a t e r still a d h e r i n g to the surface a n d to e l u t e the diffusible ions from
the AFS of the c e l l - w a l l material. Since the r i n s i n g solution, a d j u s t e d to p H 8 with Trisbuffer, c o n t a i n e d practically not electrolytes, a n e x c h a n g e of cations adsorbed to the
fixed n e g a t i v e charges (e. g. C O O - groups) i n the D o n n a n p h a s e was prevented.
To l i b e r a t e these cations from the material, the p r e p a r a t i o n s were transferred to
150 ml of 0.1 n formic acid i n a Soxhlet apparatus, a n d extracted for about 10 h. The
extracts w e r e t h e n c o n c e n t r a t e d to 50 ml a n d u s e d for the c h e m i c a l a n a l y s e s of Ca + + a n d
M g ++ b y titration with 10.3 tool EDTA as well as for the d e t e r m i n a t i o n of Na + a n d K +
with a L e i t z - U n i c a m SP 90 atomic a b s o r p t i o n spectrophotometer.
Besides these d e t e r m i n a t i o n s the first 50 ml of the eluates were also a n a l y s e d for Cl-,
Na +, a n d K +. The C1- v a l u e s o b t a i n e d b y titration with 2 10.2 n A g N O 3 were related to
the C1- c o n t e n t of the sea water; from these results the chloride a p p a r e n t free space (C1AFS) was c a l c u l a t e d as per cent fresh w e i g h t (% FW). The m e a n v a l u e from 12 determin a t i o n s was 50 % (50.13 _ 2.6 %).
However, as c a n b e s e e n from F i g u r e 1, the actual a n i o n c o n c e n t r a t i o n in a D o n n a n
p h a s e c o n t a i n i n g n e g a t i v e b i n d i n g sites must always be c o n s i d e r a b l y lower t h a n in the
s u r r o u n d i n g m e d i u m . C o n s i d e r i n g that the n o n a c c e s s i b l e space of the material, calcul a t e d from the dry w e i g h t to fresh w e i g h t ratios, was a b o u t 30 % a n d the C1-AFS 50 %, it
has to b e c o n c l u d e d that the DFS must b e greater t h a n 20 % a n d the WFS soaked with
sea water, s m a l l e r t h a n 50 %. Therefore, it c a n n o t be far from reality to suppose that both
spaces of Valonia cell wails are i n the r a n g e of a b o u t 35 %. This is i n good a g r e e m e n t
with a c o m m u n i c a t i o n of Preston (1974) who m e n t i o n s that the matrix m a t e r i a l in the cell
walls of Valonia, w h i c h is s u p p o s e d to be the seat of the n e g a t i v e b i n d i n g sites, a m o u n t s
to a b o u t one third of the cell-wall volume.
The a n a l y t i c a l results listed i n T a b l e 1 are expressed i n ~Val cm -3 DFS. A l t h o u g h
such a d e f i n i t i o n of " c o n c e n t r a t i o n " m a y be s o m e w h a t arbitrary, the author is conscious
of the fact, that the a p p l i c a t i o n of this term to a get p h a s e of h e t e r o g e n e o u s composition
is always problematic.
Selective cation adsorption in Valonia cell walls
155
RESULTS
T h e first c o l u m n of T a b l e 1 (I S W 40 %0) s h o w s t h e c o n c e n t r a t i o n s of t h e s i n g l e
c a t i o n s i n s e a w a t e r of 40 %o S c a l c u l a t e d a f t e r D i e t r i c h e t al. (1975), In t h e s e c o n d c o l u m n
t h e r e s u l t s of t h e a n a l y s e s of t h e f o r m i c a c i d e x t r a c t s c a l c u l a t e d a s / ~ V a l c m -3 D F S a r e
l i s t e d (I DFS); t h e s e f i g u r e s a r e t h e m e a n v a l u e s of s i x d e t e r m i n a t i o n s e a c h . T h e t h i r d
a n d f o u r t h c o l u m n s g i v e t h e i o n i c r e l a t i o n s of t h e s i n g l e c a t i o n s c o m p a r e d w i t h t h e t o t a l
c a t i o n c o n c e n t r a t i o n i n s e a w a t e r (I SW/2~ I SW) a n d i n t h e D F S (I DFS/~: I DFS). T h e l a s t
c o l u m n f i n a l l y l i s t s t h e r a t i o s of t h e v a l u e s of c o l u m n f o u r to t h o s e of c o l u m n t h r e e
(t D F S : 2~ I D F S / I S W : ~YI SW).
T h e s e d a t a s h o w c l e a r l y t h a t i n t h e D F S of t h e c e l l - w a l l m a t e r i a l a s i g n i f i c a n t
c h a n g e of t h e i o n i c r e l a t i o n s , a s c o m p a r e d w i t h t h o s e i n s e a w a t e r , t o o k p l a c e . T h e y a l s o
d e m o n s t r a t e t h a t a s t r o n g s e l e c t i v e a d s o r p t i o n h a s o c c u r r e d , e s p e c i a l l y of C a + + a n d - to
a l e s s e r e x t e n t - M g + +, b u t a l s o of t h e m o n o v a l e n t i o n K +, w h i c h is t h e m a i n i n t r a c e l l u l a r i o n of m o s t p l a n t c e l l s . O f s p e c i a l i n t e r e s t is, h o w e v e r , t h a t t h e r e l a t i v e c o n c e n t r a t i o n
of s o d i u m , t h e m o s t i m p o r t a n t c a t i o n of s e a w a t e r w h i c h is c o m m o n l y p r e s e n t i n
r e l a t i v e l y s m a l l a m o u n t s i n t h e c e l l s a p of m o s t p l a n t s , is s i g n i f i c a n t l y r e d u c e d i n t h e
D F S of t h e c e l l - w a l l m a t e r i a l of V a l o n i a .
T h i s m a y b e s e e n e v e n m o r e c l e a r l y f r o m t h e r a t i o s of K +, C a + + a n d M g ++ to t h e
N a + c o n t e n t of s e a w a t e r (I : N a SW) a n d of t h e c e l l - w a l l e x t r a c t s (I : N a DFS) l i s t e d i n
T a b l e 2. T h e r e l a t i v e a c c u m u l a t i o n of t h e s e c a t i o n s b e c o m e s e v e n m o r e e v i d e n t b y t h e
ratio I : N a D F S / I : N a SW.
Table 1. Valonia utricularis. Cation concentrations and relations in sea water of 40 % S and in the
Donnan free space (DFS) of cell-wall preparations. I = concentration of the respective ion in sea
water (SW) and in DFS; X I . . , = total concentration of the listed ions in sea water or DFS (for
further explanation see text)
Cations
Na +
K+
Mg ++
Ca ++
I SW 40%o
(~Val cm -3)
547
12
125
24
I DFS
I SW
I DFS
I D F S : .YI DFS
(~Val cm -3)
Z I SW
Z I DFS
I SW : ~: ISW
0.773
0.017
0,177
0,034
0.390
0.063
0.350
0,195
0,505
3.705
1.975
5.375
651
105
582
324
_+ 19
_+ 2
_ 35
+_ 29
Table 2. Valonia utricularis. Relations and relative accumulations of the main cations of sea w a t e r
and of the Donnan free space (DFS) as compared with sodium. I = concentration of the respective
ion in sea water (SW) and in DFS ~: I . , . = total concentration of the listed ions in sea water or DFS
for further explanation see text)
Cations
K+
Mg + +
Ca + +
I
Na + (SW)
0.022
0.228
0.044
I : Na + (DFS)
I : Na + (DFS)
I : Na + (SW)
0.161
0.894
0.498
7.32
3.92
11.32
156
H. K e s s e l e r
DISCUSSION
T h e results c o m p i l e d in T a b l e s 1 a n d 2 s h o w c l e a r l y that e s p e c i a l l y the d i v a l e n t ions
Ca + + a n d M g + + are s e l e c t i v e l y a d s o r b e d in the c e l l - w a l l m a t e r i a l of Valonia utricularis.
E p p l e y & Blinks (1957) r e p o r t e d s i m i l a r findings. In k i l l e d tissue of the r e d a l g a Porphyra
perforata t h e y f o u n d a C a ++ c o n t e n t of 8-10 m i l l i m o l e s or 16-20 reVal p e r k g fresh
w e i g h t . T h e sucrose free s p a c e w a s d e t e r m i n e d to 43.6 % of the l i v i n g tissue. Since
u n c h a r g e d n o n e l e c t r o t y t e m o l e c u l e s are n e i t h e r a t t r a c t e d nor r e p e l l e d by t h e c h a r g e s of
t h e DFS, t h e n o n e l e c t r o l y t e FS s h o u l d also c o m p r i s e the DFS. In Porphyra peHorata the
latter c a n b e a t t r i b u t e d to t h e p o r p h y r a n c o n t e n t of t h e w a l l m a t e r i a l w h i c h a c c o r d i n g to
C r o n s h a w et al. (1958) a m o u n t s to a b o u t 50 % a n d contains l a r g e a m o u n t s of s u l p h a t e d
p o l y s a c c h a r i d e s (O'Colla, 1962; M a c k i e & Preston, 1974) c a r r y i n g n e g a t i v e c h a r g e s at
t h e p H of s e a water. R e l a t i n g t h e C a ++ v a l u e to t h e p o r p h y r a n content of the w a l l
m a t e r i a l w h i c h w i l l c o r r e s p o n d r o u g h l y to a DFS v a l u e of 20 % fresh w e i g h t , an actual
c o n c e n t r a t i o n of a b o u t 80 to 100/~Val cm -3 DFS is o b t a i n e d . Sodium, on the other hand,
w a s p r e s e n t only in a c o n c e n t r a t i o n of a b o u t 200/~Val c m -3 DFS. C a l c u l a t i n g t h e ratio of
C a + + : N a + in the DFS of Porphyra a v a l u e of 1 0 0 : 2 0 0 = 0.5 is o b t a i n e d w h i c h
c o r r e s p o n d s rather e x a c t l y to that c a l c u l a t e d for Valonia to 0.498. C o m p a r e d with the
ratio of Ca + + : N a + in s e a w a t e r (---- 0.043) a m o r e t h a n tenfold r e l a t i v e a c c u m u l a t i o n has
o b v i o u s l y o c c u r r e d in both m a r i n e a l g a e .
For the cell w a l l of Chara australis D a i n t y et al. (1959) m e n t i o n a C a ++ v a l u e of
0.4/~Val cm -2 at a w a l l t h i c k n e s s of 10 ~ n , c o r r e s p o n d i n g to 400 #Val cm -3 w a l l material.
T h e s e authors also f o u n d a strong i n c r e a s e of t h e ratio Ca + + : Na + in the cell w a l l s w h e n
the m e d i u m c o n c e n t r a t i o n of s o d i u m w a s only slightly r e d u c e d in favour of Ca + +. This
i n c r e a s e of the ratio Ca ++ : N a + o b s e r v e d in slices of PorphyrapeHorata tissue as w e l l as
in c e l l - w a l l m a t e r i a l of Chara australis, a n d c o n f i r m e d by our results on c e l l - w a l l
p r e p a r a t i o n s of Valonia utricularis is, h o w e v e r , h a r d l y surprising. A c c o r d i n g to the
p h y s i c o - c h e m i c a l laws g o v e r n i n g the D o n n a n distribution of ions, an e x c h a n g e b e t w e e n
m o n o - a n d d i v a l e n t ions c a n only t a k e p l a c e if two m o n o v a l e n t ions (I +) from one
c o m p a r t m e n t (e. g. DFS) a n d one d i v a l e n t ion (Io+ +) of the n e i g h b o u r i n g c o m p a r t m e n t
(e. g. WFS) cross the b o r d e r i n g a r e a or p o t e n t i a l b a r r i e r in opposite directions s i m u l t a n e ously.
t
I++
4
,
*
i+ +
I+
I
T h e p r o b a b i l i t y of such an e x c h a n g e is o b v i o u s l y directly r e l a t e d to the activity
p r o d u c t s of t h e ions e n g a g e d . H e n c e e q u i l i b r i u m will b e r e a c h e d if t h e activity products
of the e x c h a n g i n g ions from b o t h c o m p a r t m e n t s are e q u a l , i.e. if
(io+) • (i+) - (i++) =
( i + ) . (I+) - (i++)
The corresponding relations are then
(I++ )
(I+) 2
(Ig÷)
(Io+)2
In o t h e r words, in a D o n n a n s y s t e m c o n t a i n i n g m o n o - a n d d i v a l e n t cations as
c o u n t e r i o n s to t h e f i x e d a n i o n s the a c c u m u l a t i o n rate of the d i v a l e n t cations by ion
e x c h a n g e is e q u a l to the s e c o n d p o w e r of the a c c u m u l a t i o n rate of the m o n o v a l e n t ones,
S e l e c t i v e cation adsorption in Valonia cell w a l l s
157
This t h e o r e t i c a l concept, h o w e v e r , t a k e s only t h e v a l e n c y of the ions e n g a g e d into
account. It is of poor practical v a l u e for t h e e s t i m a t i o n of t h e real d i s t r i b u t i o n of different
kinds of ions of the s a m e charge, as p r e s e n t in sea water. B e c a u s e of different states of
hydration a n d / o r p o l a r i s a t i o n of ions a n d b i n d i n g sites c o m p o u n d s m a y be f o r m e d w h i c h
are only partly dissociated, as is the case w i t h C a - a t g i n a t e in b r o w n a l g a e (Kreger, 1962).
Such c h e m i c a l i n t e r r e l a t i o n s h i p s also s e e m to g o v e r n t h e distribution of t h e m o n o v a l e n t ions N a + a n d K + in the c e l l - w a l l m a t e r i a l of Valonia w h i c h is not in a c c o r d a n c e
with a simple D o n n a n m o d e l , d i s c r i m i n a t i n g only b e t w e e n cations of different c h a r g e .
The actual relations, therefore, of the cations a d s o r b e d in the cell w a l l s of a l g a e can be
e s t i m a t e d only by direct c h e m i c a l d e t e r m i n a t i o n . T h e r e a l d i s t r i b u t i o n of t h e cations
r e v e a l e d t h e r e b y m a y b e i m p o r t a n t for the further steps of m i n e r a l m e t a b o l i s m .
Thus the p r e f e r e n t i a l m e t a b o 1 i c a c c u m u 1 a t i o n of p o t a s s i u m by the cells of
most m a r i n e a n d e s p e c i a l l y f r e s h w a t e r a l g a e m a y b e e n h a n c e d by the p r e f e r e n t i a l
nonmetabolic
adsorption
of this ion by the cell walls. By e x c h a n g e w i t h
h y d r o g e n ions, l i b e r a t e d d u r i n g t h e m e t a b o l i s m of t h e cells, the u p t a k e of p o t a s s i u m into
the c y t o p l a s m m a y b e facilitated by f l a t t e n i n g the steep c o n c e n t r a t i o n g r a d i e n t b e t w e e n
the cell sap and the s u r r o u n d i n g m e d i u m . This m a y r e d u c e the e x p e n d i t u r e of e n e r g y
w h i c h is n e e d e d for osmotic work, e s p e c i a l l y in the case of ion a c c u m u l a t i o n from
d i l u t e d m e d i a such as p o n d w a t e r l t h e n u m b e r of t h e b i n d i n g sites a v a i l a b l e for the
adsorption of ions s e e m s to be i n d e p e n d e n t of the outer m i n e r a l c o n c e n t r a t i o n a n d only
r e l a t e d to the pH of the e n v i r o n m e n t r e s p o n s i b l e for the d e g r e e of dissociation of the
acidic groups of the w a l l material. This, h o w e v e r , w o u l d m e a n that e v e n u n d e r such
conditions the a m o u n t of ions a d s o r b e d by t h e b i n d i n g sites w o u l d l i k e w i s e d e p e n d on
the r e 1 a t i o n of t h e different ions in t h e s u r r o u n d i n g m e d i u m a n d not on t h e i r
absolute c o n c e n t r a t i o n .
This v i e w is s u p p o r t e d by a result of D i a m o n d & S o l o m o n (1959) on Nitella ax111aris.
In the c e l l - w a l l m a t e r i a l of this a l g a e x p o s e d to artificial p o n d w a t e r c o n t a i n i n g o n l y
104 n K +, t h e y f o u n d a c o n c e n t r a t i o n e v e n of t h e m o n o v a l e n t p o t a s s i u m ion of 7200
p i c o m o l e s p e r s q u a r e c e n t i m e t r e cell wall. Since the w a l l t h i c k n e s s w a s 2 10 -4 cm, the
c o n c e n t r a t i o n of the a d s o r b e d K + w a s 3.6 10-2n K +, c o r r e s p o n d i n g to an a d s o r p t i v e
a c c u m u l a t i o n from the e n v i r o n m e n t of 360 times! It s e e m s r e a s o n a b l e to s u p p o s e that
such strong n o n m e t a b o l i c p r e a c c u m u l a t i o n of a m e t a b o l i c a l l y i m p o r t a n t i o n m u s t also
affect the process of its m e t a b o l i c u p t a k e into the cytoplasm. F u r t h e r i n v e s t i g a t i o n s w i l l
prove w h e t h e r this c o n c e p t is valid.
Acknowledgement. I am indebted to Miss E. Herre for technical assistance and careful performance
of the chemical analyses.
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