Biology Reference
In-Depth Information
seaweeds as well. Various studies showed that proton-pumping activity can be
directly switched on in salt-treated cyanobacteria due to increased respiratory
activity or because of stimulated activities of specific ATPases (Wiangnon et al.
2007
, Hagemann
2011
). Other Na
þ
-export systems such as primary active sodium
pumps or transporters might exist in seaweeds, but experimental proof is still
lacking.
The much higher concentration of intracellular K
þ
than in the external medium
clearly points to an active uptake of this important cation from the seaweed exterior.
In the genome sequences of cyanobacteria, various K
þ
transporters and putative K
þ
channels have been assigned as potential candidates for performing the uptake of
this cation (Hagemann
2011
), and more recently it was reported that all
cyanobacteria possess the structural genes for a functional ATP-dependent K
þ
transport system consisting of a K
þ
permease, an ATPase that provides the energy,
and a structural stabilisator (Ballal et al.
2007
). Most interestingly, this complex K
þ
transport system requires but does not transport Na
þ
(Matsuda et al.
2004
). Such a
direct activation by the presence of Na
þ
would nicely explain how the K
þ
transport
activity is rapidly enhanced under salt stress.
In contrast to Na
þ
and K
þ
transport mechanisms much less is known about Cl
extrusion/uptake systems. Although Cl
was involved in the osmotic acclimation of
various eulittoral green algae from Antarctica, the concentration remained rela-
tively low even under hypersaline treatments (Karsten et al.
1991b
). This is in
accordance with the related temperate green algae such as
Ulva prolifera
(as
Enteromorpha prolifera
) (Young et al.
1987
). Except
Acrosiphonia arcta
, all
other green algal species studied from Antarctica have typically small cells and a
large cytoplasmic:vacuolar ratio (Karsten et al.
1991b
). They resemble cytoplasm-
rich microalgae, which also tend to maintain low Cl
values in their cytoplasm
(Dickson and Kirst
1986
). Cl
like Na
þ
has adverse effects on many enzymes
(Gimmler et al.
1984
). In addition, ribosomes of plants are not functional in the
presence of high Cl
contents (Ritchie
1988
).
In the green alga
Acetabularia
spp. a negative membrane potential was
measured and interpreted as primarily caused by an electrogenic Cl
pump
(Wendler et al.
1983
). Nevertheless, in eukaryotic algae and cyanobacteria Cl
transport is still badly understood. In contrast, in some bacteria and other
eukaryotes such as invertebrates and humans, various genes encoding Cl
channels
or Cl
/H
þ
exchangers have been described (Jentsch
2008
). The underlying proteins
assemble to dimers, with each monomer containing an ion translocation pathway
(Jentsch
2008
). From the available data on these organisms it is reasonable to
assume similar Cl
transport systems for seaweeds.
Although K
þ
,Na
þ
and Cl
represent the major inorganic ions involved in
osmotic acclimation, some seaweeds such as
Laminaria digitata
use NO
3
to
satisfy osmotic and nitrogen requirements (Davison and Reed
1985a
). However,
there are strong seasonal changes in the cytoplasmic composition of major inor-
ganic and organic osmolytes in this kelp.
Laminaria digitata
accumulates high
nitrate concentrations in spring. During summer, this anion is completely
metabolized, and the gap in the osmotic potential is filled through the biosynthesis
