Enumeration of Microalgal Cells - Biotechnological Applications of Microalgae - page 47

saline ethanol, and freezing, can also be adopted for a wide range of phytoplankton.
Microalgal preservation is recommended so as to prevent cell disintegration and to
avoid any changes in cell population size due to zooplankton grazing (Hotzel and
Croome, 1999).
4.4 ENUMERATION METHODS
There are several methods available for the enumeration of microalgal cells.
However, due to the small size of the microalgal cells, most methods are not very
accurate. In addition, some of the methods described here require sophisticated and
expensive equipment, which is beyond the reach of some research laboratories. The
choice of counting device depends on culture density, the size and shape of the cells
or colonies being counted, and the presence and amount of extracellular threads,
sheaths, or dissolved mucilage, which can influence the filling of the counting cham-
ber (Guillard and Sieracki, 2005). Moreover, the method to be adopted for a par-
ticular sample will therefore depend on other factors such as detection range, costs,
sample throughput, health and safety considerations, inter alia .
4.4.1 s peCtrophotoMetriC a nalysis
The use of a spectrophotometer for indirectly measuring microalgal cell density
is done in conjunction with other methods, for example, gravimetric and counting
chambers for the calibration curve (see below). The wavelength for the determina-
tion of microalgal biomass is in the visible range of the electromagnetic spectrum
(400 to 700 nm). It is recommended to do a calibration curve with samples of known
microalgal cell numbers so as to extrapolate cell numbers from the standard curve.
Spectrophotometric analysis is an easy quantification method although not very accu-
rate because some other non-microalgal particles such as artifacts and dissolved and
suspended solids may contribute to light absorption. However, it is recommended to
ensure that the samples to be analyzed are free of other contaminants prior to analy-
sis because this method does not discriminate noncellular materials from microalgal
cells. One major drawback of spectrophotometric analysis is that culture conditions
greatly affect chlorophyll content, which in turn determines absorbance. Microalgal
cells grown in dark conditions have higher chlorophyll contents as compared to cells
grown under high light intensity.
The use of light absorption is suitable for the estimation of microalgal population
size rather than the determination of the actual number of individual cells.
4.4.2 G raviMetriC a nalysis
The quantitative determination of biomass by gravimetric analysis is easy, cost
effective, and the equipment required for this purpose is not very expensive.
However, this technique determines the weight of the whole biomass and does not
discriminate individual cells in the media. In general, wet or dry weights of the
biomass can be measured, although wet weight determination is not very accurate.
The microalgal cells are harvested by centrifugation and the pellet washed twice