Biology Reference
In-Depth Information
The biosynthesis of digeneaside in red algae is initiated by a condensation
reaction of
L
-glycerate-P and UDP-mannose resulting in digeneaside-P. This reac-
tion is mediated by digeneaside-P synthase, and followed by a specific digeneaside
phosphatase which de-phosphorylates digeneaside-P (Kirst
1990
).
All members of the mangrove red algal genus
Caloglossa
synthesize the polyol
mannitol under salinity stress (Karsten and West
1993
). The biosynthesis of
mannitol was experimentally verified for the first time in
Caloglossa leprieurii
(Karsten et al.
1997
). This species exhibits four enzymes that control the size of the
mannitol pool. Mannitol-1-P dehydrogenase (Mt1PDH; EC 1.1.1.17) reduces fruc-
tose-6-P to mannitol-1-P. Mannitol-1-phosphatase (Mt1Pase; EC 3.1.3.22) subse-
quently dephosphorylates mannitol-1-P and releases mannitol in the anabolic
pathway. The catabolic pathway includes the conversion of mannitol to fructose
by mannitol dehydrogenase (MtDH; EC 1.1.1.67) and further to fructose-6-P by
hexokinase (HK; EC 2.7.1.1.). Both pathways involved in mannitol metabolism are
known as the so-called mannitol cycle (Karsten et al.
1997
).
A current study on the key enzyme mannitol-1-P dehydrogenase in the brown
alga
Ectocarpus siliculosus
took advantage of the recently published genome (Cock
et al.
2010
; Rousvoal et al.
2011
). Applying a biochemical and for the first time a
genomic approach, the latter authors documented a salt-induced gene expression
and upregulation for this enzyme, i.e., hypersaline conditions stimulated the forma-
tion of mannitol. Similarly, Iwamoto et al. (
2003
) documented the biochemical and
kinetic properties of purified mannitol-1-P dehydrogenase from
Caloglossa
continua
, and also reported strong enzyme regulation by salinity.
In contrast to the mannitol metabolism, the biosynthesis of sorbitol and dulcitol
as well as of trehalose in red seaweeds is unstudied. Nevertheless, a trehalose-6-
phosphate synthase gene was recently screened out from a large DNA fragment
library constructed from
Porphyra yezoensis
(Dai et al.
2004
), indicating the
genotypic presence of a trehalose biosynthesis key enzyme. This example strongly
supports the usefulness of various recent genomic projects on different seaweed
taxa, which will give a deeper look into the molecular mechanisms of biosynthesis
and regulation of organic osmolytes.
5.3.4 Antioxidants
Since seaweeds perform oxygenic photosynthesis using water as an electron donor
they steadily release molecular oxygen, which can be accumulated and easily
chemically converted to potentially damaging reactive oxygen species (ROS).
The sources and production sites of ROS are mainly related to photosynthetic
activities, such as pseudocyclic photophosphorylation and the Mehler reaction,
which stimulate the accumulation of hydrogen peroxide (Asada
1994
; see Chap.
6
by Bischof and Rautenberger). Besides these internal processes, formation of
ROS might also be induced under hypersaline conditions as reported for
Ulva
fasciata
(Sung et al.
2009
). These authors undertook a gene expression study and
