Seaweed Acclimation to Salinity and Desiccation Stress - Seaweed Biology - page 93

In contrast to the phase I, phase II is a rather slow process in seaweeds lasting
from several hours up to 2-3 days (Kirst 1990 , Karsten et al. 1996b ). As a result, the
internal osmotic potential becomes adjusted by changing the concentrations of ions
and organic osmolytes to restore the original turgor pressure. These processes are
under direct metabolic control.

5.3.1 Inorganic Ions

To adjust inorganic ion concentrations to homeostatic conditions, ions are actively
extruded under hypersaline conditions but must be imported under hyposaline
stress. The main ions involved in osmotic acclimation are K þ ,Na þ ,Cl and to a
lesser extent sulfate, nitrate, or phosphate. The ionic composition of algal cells and
in particular of their vacuoles varies widely depending on cellular number and
volume of vacuoles (Kirst 1990 ). In most seaweeds, the Cl concentration usually
follows the fluctuation in salinity. With respect to the Na þ content, particularly
siphonous green algae such as members of the genera Caulerpa , Halimeda or
Bryopsis with their huge vacuolar system accumulate this cation. In contrast,
other seaweeds with small vacuoles and high cytoplasmic fraction prefer to accu-
mulate K þ under salinity stress (Kirst 1990 ). In contrast to the toxic properties of
Na þ (and Cl ), K þ is fully compatible to metabolic activities, although an expla-
nation for this fact is still missing (Maathuis and Amtmann 1999 ). Both cations
exhibit similar physico-chemical properties, as the smaller Na þ together with its
rather large hydration shell mimics the size of K þ . Consequently, uptake systems
for K þ have difficulties discriminating between both ions, and high external Na þ
amounts may result in K þ deficiency. Inside the cell, Na þ can compete for K þ -
binding sites of proteins, which contribute to their stabilization, resulting in the
inhibition of K þ -dependent metabolic processes (Hagemann 2011 ). Therefore, all
organisms tend to ensure a defined usually high K þ /Na þ ratio in the cytoplasm
(Maathuis and Amtmann 1999 ).
During osmotic adjustment, the changes in and control of ion composition are
achieved by regulating the activity of specific transport systems, which are particu-
larly well studied in cyanobacteria (Hagemann 2011 ) and probably act also in
seaweeds. Ion concentrations in algae are mainly regulated by ion-selective carriers
driven by the membrane potential (Gimmler 2000 ). In addition, facilitated diffusion
via ion-selective channels may play a role during rapid changes and recovery of
ionic composition (Kirst 1990 ).
The generally low cytoplasmic Na þ concentrations observed in most seaweeds
clearly indicate that active sodium ion export mechanisms exist in these plants. The
complete genome sequence of the cyanobacterial strain Synechocystis 6803 showed
six different genes, annotated as Na þ /H þ antiporters (Kaneko et al. 1996 ), of which
at least three were functioning (Inaba et al. 2001 ). The occurrence of rather large
gene families for Na þ /H þ antiporters clearly point to a group of proteins that fulfill
many important functions in osmotic acclimation of cyanobacteria and probably of