CYANOBACTERIAL TOXINS - Cyanobacteria - page 488

Any substance that is poisonous in nature is termed as a toxin. Toxins produced by cyanobacteria
are known as cyanobacterial toxins or cyanotoxins. Of the 150 known genera of cyanobacteria, 40
species of cyanobacteria have been implicated in the formation of toxic blooms in fresh, marine and
estuarine waters (Carmichael, 1997; Saker et al ., 1999). The bloom algae are characterized by the
presence of reddish brown, refractile structures inside their cells known as gas vacuoles that help
them to keep afl oat in water. Due to mixing of agricultural and industrial waste-water, the nutrient
status of water bodies undergoes a change. In this context, nitrogen and phosphorus enrichment
leads to the formation of nutrient saturated water bodies defi ned as eutrophic waters. The growth of
bloom algae is thus dependent on variations in the nutrients especially of nitrogen and phosphorus,
light intensity, temperature, pH and other factors. Prepas and Charette (2003) summarized the factors
responsible for eutrophication of water bodies worldwide and suggested methods of control. The
earlier concept that a low ratio of nitrogen to phosphorus (N:P) is responsible for the predominance
of bloom algae has now been questioned. Downing et al . (2001) who analyzed data from 99 temperate
lakes suggested that it is the variations in the total N and P contents in waters more important rather
than their ratio. Due to their excessive growth, the waters emanate foul smell that is obnoxious in
nature and the waters are unfi t for use in any manner either for animal or human consumption or
for recreational purposes.
World over the drinking water requirements are generally met from surface waters, since
the groundwater supplies are unable to meet the demand of the ever increasing population. It is
estimated that at least 48% of the lakes of North America are eutrophic suggesting that they are rich
in nutrients and low in oxygen favouring the growth of cyanobacterial blooms posing problems of
water quality in the United States (Reynolds, 2004). Thus the bloom-forming cyanobacteria have
received a worldwide attention to identify the nature of toxins produced by them. The problem with
cyanobacterial toxins is that the waters are unfi t even for irrigation purposes as these toxins persist
in waters even after the disappearance or death of the bloom algae. Thus the cyanobacterial toxins
tend to accumulate (bioaccumulation) at different trophic levels leading to biomagnifi cation. World
Health Organization (WHO) recommended that the focus be made on cyanobacterial toxins by
considering cyanobacteria in drinking water as a chemical problem (Chorus and Bartram, 1999). On
the basis of the characteristics of bioaccumulation of cyanobacterial toxins, their bioavailability for
exerting the toxicity, exposure and uptake routes a theoretical predictive model has been developed
that enables to evaluate the risk to aquatic organisms and human beings (White et al ., 2005).
In general, three important features of cyanobacterial toxins are worth considering. Firstly,
the cyanobacterial toxins are colourless, odourless and low molecular weight compounds that are
formed as secondary metabolites. These do not play any role in growth and primary metabolism
of the cyanobacterium. Secondly, these compounds are highly toxic to all living organisms such
as fi shes, animals and human beings. Thirdly, the cyanobacterial toxins have proved to be highly
poisonous than the mycotoxins, and toxins of higher plant origin as well as shellfi sh toxins (Codd,
1995; Carmichael, 1997; Falconer, 1998; Sivonen and Jones, 1999; Metcalf and Codd, 2004).
Toxic bloom-forming cyanobacteria belong to the genera Anabaena , Anabaenopsis , Aphanizomenon ,
Cylindrospermopsis , Microcystis , Oscillatoria, Planktothrix , Phormidium , Nodularia and Nostoc . Table 1
summarizes the species of the respective genera with their geographic location. Amongst these,
except Nodularia that is known to inhabit marine and brackish waters, the rest of the cyanobacteria
are freshwater bloom-formers (Sivonen and Jones, 1999). Hapalosiphon hibernicus strain B2-3-1 has
been reported to be toxic but it is not clear whether it has been isolated from terrestrial or freshwater
mud-habitats (Princep et al ., 1992). Exceptionally, a lichen-associated Nostoc sp. strain IO-102I is
reported to be toxic (Oksanen et al ., 2004). Nostoc sp. strain-152, a freshwater cyanobacterium has been