Cyanobacteria
are the microscopic bacteria that are photosynthetic. They are formally known
as “blue-green algae” and they are most ancient life form in the earth. Cyanobacterial
blooms have been implicated in a wide range of social, economic and
environmental impacts and are of particular concern for animal and human
health. When the blooms are formed, the risk of toxin contamination of surface
water increases especially for some species of cyanobacteria algae with the
ability to produce toxins and other noxious chemicals. Amongst the
cyanobacteria, the Microcystis sp. is
the most frequently encountered in freshwater and it produces hepatotoxins
called microcystins (MCs). WHO has established a drinking water standard for
cyanobacteria at 1 μg/L for MC-LR. EPA has compiled information on freshwater
cyanobacterial blooms including, causes, detection, treatment, health and
ecological effects, current research activities in the US, and policies and the
regulations for cyanotaxins at the state and international levels.
The
lack of definitive correlation between a cyanobacterial bloom formation and MCs
production necessitates the need for the development of rapid and more reliable
methods for routine monitoring of Microcytis as well as MCs and their
utilization in environmental sampling. Cyanobacterial pigments have been
analyzed by spectrophotometric, fluorometric analysis, and remote sensing to
provide more reliable information about the extent of a bloom and to assess the
status of water bodies. ELISA, PPIA (protein phosphate inhibition assay), HPLC,
LC-MS are the analytical techniques for analyzing cyanobacterial pigments. Molecular
techniques can also be combined with biochemical assays to provide a powerful
monitoring approach for environmental samples. To develop a model that
simulates the magnitude and timing of blooms it is also necessary to determine
those key factors which govern the dynamics of a cyanobacterial bloom
formation.
Different
approaches can be use for the monitoring of cyanobacterial blooms like
biological and physiological methodologies. For the biological methods, cell
counting and estimation of pigments (Chlorophyll a and phycocyanin) can be
done. Biological controls can be done by identifying Microcystis sp. by flow cytometry. The pigment dependent spectral
signatures of cyanobacterial blooms cannot distinguish a bloom-less shallow
water area from bloom-containing deep water area. Remote sensing can also be
used for quantitative mapping of phycocyanin. Cyanobacterial bloom using remote
sensing devices suffers from high cost, dependency on meteorological conditions
with long monitoring intervals, which limits their use for routine monitoring.
For
molecular techniques, PCR based method is useful for quantitative analysis of
cyanobacteria. Various genes like 16S rRNA, internal transcribed spacer (ITS),
and phycocyanin intergenic spacer (PC-IGS) have been employed for
characterizing toxic and nontoxic Microcystis
colonies in natural populations. SYBR Green and TaqMan based quantitative
real-time PCR using specific primers/probes have been used for rapid monitoring
of total cyanobacteria and nontoxic/toxigenic Microcystis sp. in freshwater bodies using specific primers/probes.
However, real-time PCR has limitations, as the amplification efficiency
decreases with increase in the length of the amplification product and
optimization becomes difficult and time-consuming.
Microarray
can be used to detect sequence variations and monitor gene expression levels on
a genomic scale. It can be used for the rapid identification of cyanobacterial
groups undetectable or present in low quantities, making it suitable for
monitoring toxic as well as nontoxic strains in the large number of
environmental samples. For biochemical and physiochemical methods, nutrients
and other parameters (water temperature, pH, salinity, etc) can regulate the
growth of cyanobacteria. So these parameters are considered valuable in
assessing the potential for future bloom development. Variations in total
phosphorous along with salinity and water temperature were taken into account
for predicting variations in Chl a
and cyanobacteria. Various physiochemical analyses are available for the rapid
screening of a large number of samples, the regular monitoring of sites where
the toxic patterns are well established, and the monitoring of new toxic
cyanobacterial metabolites.
Enzyme-Linked
Immunosorbent Assay (ELISA) has been developed using either monoclonal or
polyclonal antibodies against cyanotoxins like MCs. ELISAs are highly sensitive
and specific and require minimum sample processing for rapid monitoring and
detection of MC concentrations that are within the levels set by WHO. Protein
phosphate inhibition assay (PIPA) is cost effective and can be used for the
routine culture screening of environmental samples but are not specific and may
respond to other protein phosphate inhibitors. HPLC and LC-MS can be used for
the accurate determination of natural blooms. Matrix-Assisted Laser Desorption/Ionization
Time of Flight (MALDI-TOF) is a rapid and sensitive technique, with high
resolution, allowed exact mass measurements and detection of compounds, based
on molecular formula. The GC-MS approach has been used to monitor MCs in water
bodies containing blooms. NMR has been useful tool for structural determination
of MCs. Capillary Electrophoresis (CE) is based on separation of charged
molecules in a buffer solution under the influence of a strong electric field. This
method demonstrated poor sensitivity compared to HPLC and not recommended for
routine monitoring of environmental samples. Monitoring approaches must involve
the identification of unknown toxins in environmental samples which can be
performed by MS/MS. MALDI-TOF is useful for identification of MCs with very
small sample volumes and provides the molecular mass of all the peptides and MC
variants present. Further research must focus on the development of
approaches/techniques and guidelines which can be applied in both temperate and
tropical climates because differences may arise depending upon temperature and
thermal stratification.
A
strategy for monitoring cyanobacterial blooms in water bodies will depend on
various local aspects such as the intended use and types of water bodies. Important
objectives include the identification of problematic areas, identification and
quantification of cyanobacterial populations and toxins, causes and regulation
of blooms, recognition of associated health risks and providing input for the
development of guidelines for drinking water and use of recreational sites. The
presence of other metabolites in addition to MCs in drinking water samples can
be detected by chromatographic techniques and the dissolved toxins and other
metabolites can then be processed with water treatment process such as,
physical process lije activated carbon, chemical process like chlorination, and
biological process like sand filtering or GAC supporting a healthy biofilm.
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