Biology Reference
In-Depth Information
sucrose (
Anabaena
sp. CA,
Dermocarpa violacea
,
Nodularia harveyana
, two strains) and 23% accumulated
trehalose. Of the 49 freshwater strains tested 76% accumulated sucrose in response to salt stress, 18%
synthesized trehalose and 6% showed GG. There was absolutely no relationship between habitat
or genus and the type of osmolyte synthesized. In another study, Stal and Reed (1987) tested 47
strains of cyanobacteria belonging to taxonomically different groups from marine microbial mats for
osmolyte accumulation. There was no correlation between the chemical nature of the organic solute,
the salt optimum or salt tolerance of the cyanobacterium examined. Sucrose was the main osmolyte
accumulated in species of
Nostoc
/
Anabaena
group while trehalose accumulation was noted in marine
heterocystous
Calothrix
sp. GG accumulation was noted characteristically in strains belonging to
LPP-group. However, some members of this group accumulated sucrose or trehalose as well. There
was variation between marine
Oscillatoria
spp. (trehalose accumulation) and freshwater
Oscillatoria
spp. with broad salinity tolerance (that accumulated sucrose). Samples from marine microbial mats
revealed the presence of glycerol, GG, sucrose and trehalose. Of the total 130 cyanobacterial strains
tested, half of them (66 strains) accumulated sucrose whereas 20, 24 and 18 strains accumulated
trehalose, GG and glycine betaine, respectively (Erdmann, 1992; Reed
et al
., 1986; Stal and Reed, 1987).
The accumulation of the low molecular weight carbohydrates has been correlated with the habitat
distribution of cyanobacteria in the marine environment. The synthesis of sucrose and trehalose was
more rapid than that of GG. Strains that synthesized the former two compatible solutes inhabited
brackish waters where as those that showed the synthesis of the latter commonly occurred in
habitats of less variable salinity. Thus sucrose synthesis has been noted in case of
Anabaena variabilis
ATCC 29413,
Nostoc entophytum
CCAP1453/14 and
S. elongatus
PCC 6301 while tehalose synthesis
was noted in case of
Microchate grisea
CCAP1451/1 and GG as well as sucrose was synthesized by
Synechococcus
sp. strain CCMP/SYN WH 5701 (Warr
et al
., 1987). The synthesis of sucrose by salt-
sensitive (freshwater) strains of cyanobacteria such as
Synechococcus
has been reported (Mackay
et
al
., 1984: Reed
et al
., 1986; Joset
et al
., 1996; Hagemann and Erdmann, 1997). The detection of sucrose
synthase gene in some unicellular cyanobacteria (
Microcystis aeruginosa
PCC 7806,
G
.
violaceus
PCC
7421 and
T
.
elongatus
BP-1) and increase in its transcript levels in these organisms after salt treatment
is suggestive of sucrose synthesis as an adaptive mechanism (Kolman
et al
., 2012). The synthesis of
GG has been fi rst detected in
Synechococcus
sp. strain N100 (Borowitzka
et al
., 1980). Subsequently,
GG synthesis has been detected in the cyanobacterial strains that exhibited intermediary tolerance
such as
Synechocystis
sp. strain PCC 6803 (Hagemann
et al
., 1987; Erdmann
et al
., 1992; Joset
et al
.,
1996; Hagemann and Erdmann, 1997). Salt-tolerant strains of cyanobacteria such as
Synechococcus
sp.
strain PCC 7418 (later redesignated as
Aphanothece halophytica
) synthesize glycine betaine (Mackay
et al
., 1984; Reed
et al
., 1986).
Blumwald
et al
. (1983) have suggested that changes in cell volume caused by different osmotic
environments could be monitored by electron spin resonance (ESR) methods. According to them
these enable us to identify changes in internal cell volume by measuring ESR spectra with 2,2,6,6-
tetramethyl-4-oxopiperidinoxy free radical (TEMPONE) as a spin probe. The accumulation of
sucrose by
Synechococcus
sp. strain 6311 and
Nostoc muscorum
both freshwater strains is indicative
of common osmoregulatory response where internal cellular sucrose concentrations reached up to
580 mM in presence of salt (0.6 M NaCl) for 40 h.
a) GG synthesis
:
Hagemann and Erdmann (1994) have suggested that the precursor for the synthesis
of GG is ADP-Glucose (ADP-Glc) and not UDP-glucose as proposed earlier by Reed and Stewart
(1985). ADP-Glc and glycerol-3-phosphate (G3P) react in presence of ADP-glucose: glycerol-3-
phosphate 2-glycosyl transferase (GGP synthase) to produce ADP and GGP. The latter is further
