Seaweed Acclimation to Salinity and Desiccation Stress - Seaweed Biology - page 97

West 1993 ; Karsten et al. 1994 , 1999 , 2003b ; Eggert and Karsten 2010 ), which are
otherwise uncommon for red algae.
With the exception of digeneaside that plays no more than a minor role in
osmotic acclimation of Rhodophyta (Karsten et al. 2005 ; Eggert and Karsten
2010 ), all other low-molecular-weight carbohydrates act as organic osmolytes.
Since these organic compounds can be accumulated and tolerated at high intracel-
lular concentrations, and permit the generation of low water potentials without
incurring metabolic damage (Yancey 2005 ), the term “compatible solute” was
introduced by Brown and Simpson ( 1972 ). In general, the intracellular
concentrations of these organic osmolytes are actively adjusted by photosynthesis
driven de novo biosynthesis or by remobilization of storage products and are
directly proportional to external salinity (Kirst 1990 ).
Although differing in their chemical structure, compatible solutes in seaweeds
have some features in common: they are typically highly soluble, in most cases have
no net charges at physiological pH, and are non-inhibitory at high concentrations
(Kirst 1990 ; Karsten et al. 1996a ). The interactions of these organic compounds with
intracellular macromolecules are not completely understood and several mechanisms
have been suggested. Bisson and Kirst ( 1995 ) discussed different models to explain
the protection of enzyme systems: (1) binding of the solute to the protein, (2)
colligative action of the solute, (3) buffering of potentially damaging changes in
solution properties, (4) inhibition of conformational changes resulting in formation of
inter- or intramolecular disulfide bridges, and (5) preferential exclusion of the solute
from the protein surface. These models can be basically summarized into two types:
(1) those that hypothesize direct solute-protein interactions and (2) those that postu-
late that protein stability is mediated by solute-induced changes in water structure
(Roberts 2005 ;Yancey 2005 ). However, there is little experimental evidence in
seaweeds for any of these models.
Desiccation tolerant seaweeds such as supralittoral green algal Prasiola or
brown algal Pelvetia species typically exhibit morphological adaptations such as
thick cell walls that protect against rapid water loss and the capability to synthesize
and accumulate polyols as “water-keeping” substances (Jacob et al. 1991 , 1992 ).
5.3.3 Biosynthesis of Organic Osmolytes
Kremer and Kirst ( 1981 ) showed that exogenously applied inorganic 14 C is rapidly
taken up by various red algae and assimilated into floridoside, which is thus acting
as a major photosynthetic product. The biosynthesis of floridoside is initiated by a
condensation reaction of l-glycerol-3-P and UDP-galactose resulting in floridoside-P.
This reaction is mediated by a respective heteroside-P synthase (Kremer and Kirst
1981 ). Floridoside-P is subsequently de-phosphorylated by a specific phosphatase.
In this anabolic pathway l-glycerol-3-P serves as precursor, and the condensation
reaction takes place at the C-2 position of glycerol. In contrast, the biosynthesis of
D - and L -isofloridoside in the Bangiales is unknown.