Flow cytometry (FCM) is an automated cell counting technique that captures the fluorescence and scatter properties of the microalgal cells. The major advantage of automated cell counting techniques over optical microscopy is that they mini­mize the errors associated with human counting (Marie et al., 2005). The use of this highly sophisticated technique is hindered by the cost of the equipment as well as the requirement of highly skilled and trained personnel to operate the instrument. In addition, this technique is also plagued by limited sensitivity at lower microalgal cell concentrations. The solid-phase cytometer method for conducting total direct counts of bacteria is less biased and has performed significantly better than any of the microscopic methods (Lisle et al., 2004). Basic image analysis methods do not generally discriminate between phytoplankton and other material such as detritus and sediment in samples, thereby presenting a problem in the application to routine field samples. This technique may be more useful for the analysis of cultures and mono-specific high-density blooms (Karlson et al., 2010).

1.5 CONCLUSION

Microalgal enumeration can be tedious and cumbersome due to the small size of microalgal cells. Furthermore, this is exacerbated by the prohibitive cost of the available sophisticated equipment. To date, however, microalgal enumeration has been accomplished by gravimetric analysis, counting chambers, and flow cytometry.

Sedgewick-Rafter

Cultures and high cell numbers

1000 cells L-1 Limit of Detection (LOD) A rapid estimate of cell concentrations

Accurate results only when sample contains high algal cell densities Method-easy to learn and use; highly trained taxonomist needed for verification of species identification Compound Microscope Cover slips Pipettes Sedgewick-Rafter slides Compound microscope: £2500/US$3250 Sedgewick-Rafter slides: Perspex: £50/ US$65 Glass: £166/US$213 £1/US$1.3 20 min

This depends on the sample density

This depends on the sample density

Only one sample at a time Dependent on preservative used

Palmer-Maloney

Cultures and high cell numbers as in bloom situations 10,000 cells L-1 (LOD)

A rapid estimate of high cell concentrations

Accurate results only when sample contains very high algal cell densities Method-easy to learn and use; highly trained taxonomist needed for verification of species identification Compound Microscope Cover slips Pipettes Palmer-Maloney slides Compound microscope: £2500/US$3250 Palmer-Maloney slides: Ceramic-£60/ US$80 Stainless steel-£170/US$230 £1/US$1.3 5 min

10-30 min/sample depending on cell density

14-30 min/sample depending on the sample density Only one sample at a time Dependent on preservative used

Haemocytometer

Cultures and extremely high cell concentrations of small organisms

10,000,000 cells L-1 (LOD)

A rapid estimate of extremely high cell concentrations

Accurate results only when sample contains extremely high algal cell densities

Method-easy to learn and use; highly trained taxonomist needed for verification of species identification

Compound Microscope Pipettes Haemocytometer slide with cover glass.

Compound microscope: £2500/US$3250 Haemocytometer slide: £200/US$230

£1/US$1.3

5 min

<20 min/sample depending on the sample density

<30 min dependent on target species

Only one sample at a time

Dependent on preservative used

Source: Adapted from LeGresley and McDermott, 2010.

The invention and development of advanced techniques such as fluid imaging

alleviates the drawbacks of flow cytometry.

ACKNOWLEDGMENTS

The authors hereby acknowledge the National Research Foundation (South Africa)

for financial contribution.

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