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and accumulation of the polyol mannitol. This seasonal increase in mannitol
concentration compensates for the intracellular decrease in nitrate rather than for
changes in external salinity (Davison and Reed
1985a
).
In some green seaweeds such as
Codium decorticatum
(Bisson and Gutknecht
1975
),
Acrosiphonia arcta
and
Ulva rigida
(Karsten et al.
1991b
)SO
4
2
plays an
important role as an osmolyte. Some brown seaweeds of the genus
Desmarestia
even contain this anion in the form of free sulfuric acid inside the vacuole resulting
in a pH of 1-2 (Eppley and Bovell
1958
; Anderson and Velimirov
1982
).
The energy requirements for ion transport in seaweed species such as
Ulva
lactuca
are equivalent to 10-30% of the energy provided by respiration (Ritchie
1988
). It seems that all inorganic ion transporters in seaweeds are generally quite
fast systems with much lower requirements for metabolic energy compared with the
cost for biosynthesis or degradation of organic osmolytes (Kirst
1990
). The forma-
tion of an intracellular osmotic potential of 1 osmol kg
1
requires 1.2 mol ATP
when created by KCl uptake only, but 66 mol ATP when based on sorbitol or
mannitol biosynthesis.
5.3.2 Organic Osmolytes
The main physiological strategy of all seaweeds studied so far under saline
conditions is to keep particularly the Na
รพ
and Cl
concentrations in the cytoplasm
as low as possible. Since protein and organelle function (e.g., ribosomes,
mitochondria), enzyme activity, membrane integrity and structural macromolecules
in seaweeds are adversely affected by these ion concentrations (Kirst
1990
), it is
now generally accepted that the biosynthesis and accumulation of organic
osmolytes in the cytoplasm, although energetically costly, permits the generation
of low water potentials without incurring metabolic damage (Karsten et al.
1996a
;
Yancey
2005
; Eggert and Karsten
2010
).
In many, but not all, cases the organic osmolytes are identical with the main
photosynthetic product, and thus there are preferences in different seaweed groups.
Polyols such as mannitol are characteristic for most Phaeophyceae, while the
Chlorophyta contain typically sucrose, some taxa in addition to proline, glycine-
betaine, or dimethylsulphoniumpropionate (DMSP) (Kirst
1990
). Although manni-
tol is present as the sole osmotically significant low-molecular weight organic
solute in most brown seaweeds, some species are capable of synthesizing a second
polyol (Davison and Reed
1985b
). The top-shore Phaeophyceae
Pelvetia
canaliculata
contains the heptitol volemitol as an additional intracellular organic
solute (Kremer
1976
), while the lower shore brown alga
Himanthalia elongata
from the Northern hemisphere as well as various Southern hemisphere Fucales
(e.g.,
Bifurcariopsis capensis
,
Hormosira banksii
,
Xiphophora chondrophylla
)
accumulate the hexitol altritol as isomeric solute in addition to mannitol (Chudek
et al.
1984
; Wright et al.
1987
).
In the Rhodophyta a multitude of low-molecular-weight carbohydrates have
been identified in recent years (Eggert and Karsten
2010
). The main photosynthetic
