Biology Reference
In-Depth Information
produce CYN in culture extracts. Bouaicha and Nasri (2004) reported for the fi rst time
C
.
raciborskii
from Algerian freshwaters. The diversity studies dealing with the
cyrB
gene adenylation domain
sequences in 17 lakes of Florida revealed that the majority (92%) of the amplicons resembled those of
Aph.
ovalisporum
with 99% identity. Curiously enough,
Aph
.
ovalisporum
is the only CYN-producing
cyanobacterium distributed throughout the United States except in Florida where the occurrence of
C
.
raciborskii
has been reported (Yilmaz and Phlips, 2011).
D) Stability of Hepatotoxins:
It is important to know the stability of MCs, nodularins and CYN
either in natural waters or when stored in the laboratory for experiments related to toxicity studies.
In natural fresh or marine waters exposure to light and interaction with dissolved organic matter may
affect the toxicity. Very few studies are devoted towards understanding the stability of these toxins.
Both MCs and nodularins are chemically stable (Harada, 1995; Harada
et al
., 1996b). MCs released by
M
.
aeruginosa
after algicide treatment retained their toxicity for nearly 10 days, afterwards a decrease
in MC content was noted (Jones and Orr, 1994). Either due to cell death during bloom senescence
or during water treatment processes, the toxins are released into the surrounding waters and these
persist for several days even after the blooms have disappeared (Lahti
et al
., 1997). A decrease of MC
content in natural waters has been attributed to (i) dilution of the toxin containing water with non-
contaminated water; (ii) adsorption on particulate material; (iii) temperature- and pH-dependent
decomposition, and (iv) photolysis and biological degradation (Harada, 1995). Nodularin is stable
for nearly 10 days in aqueous solutions irrespective of light condition, i.e. light/dark or sunlight
(Twist and Codd, 1997).
The degradation of MC-LR occurred in less than a week in reservoir water (Cousins
et al
.,
1996) when compared to sterilized reservoir water (12 days) and de-ionized water (27 days). The
authors suggested that decomposition of hepatotoxins might be taking place by the disruption of
the conjugated diene system in the Adda. The ability of
Pseudomonas aeruginosa
to degrade MC-LR
through the activity of an alkaline protease is worth mentioning (Takenaka and Watanabe, 1997).
The decrease in the concentrations of these toxins in non-sterile seawater suggested that certain
bacteria might be responsible for degrading these toxins and that nodularin is more resistant to
biodegradation than MCs.
Jones
et al
. (1994) reported the degradation of MCs by the bacterium
Sphingomonas
sp. strain
MJ-PV isolated from Australian natural waters. Bourne
et al
. (1996) observed that
Sphingomonas
could breakdown MC-LR by ring opening resulting in an open chain linear compound that is 200
times less toxic. Hydrolysis of Arg-Adda and then Ala-Leu peptide bonds involving enzymatic
degradation resulted in a linearized MC-LR (H-Adda-Glu-Mdha-Ala-Leu-MeAsp-Arg-OH) and
then a tetrapeptide (H-Adda-Glu-Mdha-Arg-OH) (Bourne
et al
., 1996). Species of
Sphingomonas
have
now been isolated from Japanese lakes as well that have the potential to degrade MCs (Park
et al
.,
2001; Saitou
et al
., 2003; Ishii
et al
., 2004). However, Imanishi
et al
. (2005) reported another bacterium,
B-9, isolated from a Japanese Lake Tsukui, that degraded MC-LR and nodularin. The degradation
of MC to non-toxic Adda by B-9 involves sequential enzymatic hydrolysis of Arg-Adda, Ala-Leu
and then Adda-Glu peptide bonds resulting in two unknown non-toxic intermediates (Imanishi
et
al
., 2005). Bourne
et al
. (2001) characterized the gene cluster for the bacterial degradation of MCs. A
new rod-shaped gram-negative, single polarly fl agellated bacterial isolate identifi ed as
Paucibacter
toxinivorans
gen. nov et sp. nov. capable of degrading MCs and nodularins was isolated from lake
sediments of Finland and physiological properties of the bacterium have now been characterized
(Rapala
et al
., 2005).
