CYANOBACTERIAL TOXINS - Cyanobacteria - page 509

the earlier studies as the content of MC was expressed in proportion to the increase in biomass or
protein content of cultures. Red and green lights supported higher ratio of toxin to protein content
than in white light (Utkilen and Gjolme, 1992). Five strains of O . agardhii produced demethyl-3-
MC-RR at high nitrogen concentrations (84 mg -1 L), low light intensity (12 to 95 microeinsteins
m -2 s -1 ), phosphorus concentration (0.4 mg -1 L) and temperature (25ºC) (Sivonen, 1990).
Photosynthetically active radiation (PAR) in between 10 and 403 µmol of photons m -2 s -1 with
a light-dark (12h:12h) cycles in continuous cultures of M . aeruginosa strain PCC 7806 infl uenced the
content of MC in between 34.5 and 81.4 fg cell -1 . This was signifi cantly positively correlated with
growth rate under PAR-limited growth but not under PAR-saturated growth. Intracellular MC
content increased to 10 to 20% higher at a PAR of 10 and 40 µmol of photons m -2 s -1 than at the end
of the dark period while at higher light intensities the MC content was inhibited (Wiedner et al .,
2003). Kaebernick et al . (2000) have chosen mcyB (for peptide synthetase) and mcyD (for PKS) genes
for demonstrating the effect of light intensity/quality and oxidative stress on the transcriptional
regulation of these genes for MC biosynthesis in M . aeruginosa PCC 7806. Under high light intensities
and red light, the transcript levels of mcyB and mcyD increased while blue light and ceratin stress
factors (such as methylviologen and sodium chloride) reduced the level of transcripts. It may be
added that MC synthetase production is initiated at certain threshold light intensities such as between
dark and low light and medium and high light. The infl uence of high and low light intensities on
transcriptional regulation of mcy gene cluster has already been described earlier (Kaebernick et al .,
2002). Total MC content and selective production of a particular variant of MC in P . agardhii depended
on light intensity in the range of 60-100 µmol m -2 s -1 . The transcripts of the mcyA gene and total MC
production rate started increasing up to 60 µmol m -2 s -1 light intensity but the levels of these decreased
with irradiance greater than 100 µmol m -2 s -1 . Increase of light intensity decreased the content of
MC-deRR but with a concomitant three-fold increase in the content of MC-deLR signifying that
P . agardhii becomes more toxic at high light intensities (Tonk et al ., 2005).
The rest of the studies are devoted to understand MC production in light, temperature and/or
nitrogen/phosphorus enrichment or the supply of iron. Anabaena sp. cultured under light limitation
at different temperatures has been assessed for production of MCs and neurotoxins (Rapala and
Sivonen, 1998). Growth and MC content of different strains of M . aeruginosa have been compared
under different light intensities and various nutrients (Hesse and Kohl, 2001). According to some
workers, variation in light intensity had no signifi cant effect on MC content of both Planktothrix
agardhii and M . aeruginosa grown in turbidostats (Boettcher et al ., 2001). A study on the infl uence of
trace metals such as Zn and Fe revealed that Zn enhanced growth and MC production while low
Fe concentrations decreased growth but supported higher MC formation (Lukac and Aegerter,
1993). Iron uptake is shown to be light-dependent and the decrease in iron concentration resulted
in a decline in the ratio of toxin to both dry weight as well as protein content. The effects of light
intensity on toxin production reported earlier (Watanabe and Oishi, 1985; Van der Westhuizen and
Eloff, 1985; Sivonen, 1990; Utkilen and Gjolme, 1992) has been suggested to be due to the effect of
light intensity on iron uptake (Utkilen and Gjolme, 1995).
Utkilen and Gjolme (1995) observed that nitrate- and phosphate-limited conditions decreased
the ratio of toxin content to dry weight in M . aeruginosa CYA 228/1. This decrease was not due to a
decrease in MC production but was due to enhanced carbohydrate content under low nitrate and
phosphate concentrations. However, the MC content remained constant when expressed in relation
to protein content of the cells that appeared to be a better parameter for expression of MC content
than dry weight (Utkilen and Gjolme, 1995). Phosphorus-limited chemostat cultures of M . aeruginosa
UTEX 2388 showed a specifi c growth rate constant (µ) of 0.1 to 0.6 day -1 and was a function of cellular