The most common culture system is the batch culture, due to its simplicity and low cost. This is a
closed system, volume-limited, in which there is no input or output of materials, that is, resources
are finite. The algal population cell density increases constantly until the exhaustion of some limiting
factor, whereas other nutrient components of the culture medium decrease over time. Any products
produced by the cells during growth also increase in concentration in the culture medium.
Once the resources have been utilized by the cells, the cultures die unless supplied with new
medium. In practice this is done by subculturing, that is, transferring a small volume of existing
culture to a large volume of fresh culture medium at regular intervals. In this method algal cells
are allowed to grow and reproduce in a closed container. A typical batch culture set-up can be a
250 ml Erlenmeyer culture flask with a cotton/gauze bung; in some cases, the bung can be fitted
with a Pasteur pipette and air is bubbled into the culture to maintain high levels of oxygen and
carbon dioxide and provide mixing.
Batch culture systems are highly dynamic. The population shows a typical pattern of growth
according to a sigmoid curve (Figure 6.1a), consisting of a succession of six phases, characterized
by variations in the growth rate (Figure 6.1b); the six phases are summarized in Table 6.19.
|FIGURE 6.1 Growth curve of an algal population under batch culture conditions (a) and corresponding variations of the growth rate (b).
The growth curve, relative to the Phases 3, 4 and 5, without the lag, acceleration and crash
phases, can be described with a rectangular hyperbolic function similar to the Michaelis-Menten
formulation that describes the nutrient uptake kinetics, and the dynamic relationship between
photosynthetic rate and irradiance.
After the inoculum, growth does not necessarily start right away, because most cells may be
viable, but not in condition to divide. The interval necessary for the transferred cells to adapt to
the new situation and start growing is the first phase of the growth curve, the lag phase. This lag
or induction phase is relatively long when an algal culture is transferred from a plate to liquid
culture. Cultures inoculated with exponentially growing algae have short lag phases, which can
seriously reduce the time required for upscaling. The lag in growth is attributed to the physiological
adaptation of the cell metabolism to growth, such as the increase of the levels of enzymes and
metabolites involved in cell division and carbon fixation. During this phase the growth rate is zero
After a short phase of growth acceleration, characterized by a continuously increasing growth
rate, up to its maximum value, which is achieved in the following exponential phase, the cell
density increases as a function of time t according to the exponential function:
where N2 and N1 are the number of cells at two successive times and µ is the growth rate. During
this phase, the growth rate reached is kept constant. The growth rate is mainly dependent on
algal species and cultivation parameters, such as light intensity, temperature, and nutrient
The exponential growth phase normally lasts for a very short period, because cells start to shade
each other as their concentration increases. Hence, the culture enters the phase of retardation, and
cell growth rate decreases because mainly light, but also nutrients, pH, carbon dioxide, and other
physical and chemical factors begin to limit growth. Following this phase, the cell population continues
to increase, but the growth rate decreases until it reaches zero, at which point the culture
enters the stationary phase, during which the cell concentration remains constant at its maximum
value. The final stage of the culture is the death or “crash” phase, characterized by a negative
growth rate; during this phase water quality deteriorates, mainly due to catabolite accumulation,
and nutrients are depleted to a level incapable to sustain growth. Cell density decreases rapidly
and the culture eventually collapses.
In practice, culture crashes can be caused by a variety of reasons, including the depletion of a
nutrient, oxygen deficiency, overheating, pH disturbance, or contamination. The key to the success
of algal production is maintaining all cultures in the exponential phase of growth. Also, the nutritional
value of the produced algae is inferior once the culture is beyond Phase 4 due to reduced
digestibility, deficient composition, and possible production of toxic metabolites.
In batch cultures, cell properties such as size, internal nutrient composition, and metabolic function
vary considerably during the above growth phases. This can often make interpretation of the
results difficult. During the exponential growth phase, cell properties tend to be constant. However,
this phase usually only lasts for a short period of time, and if one wishes to estimate growth rates of
the exponential phase of batch cultures, daily sampling appeared to be insufficient to allow a
reasonably accurate estimate. Moreover, the accuracy of growth rate determination is highest in
artificial, defined media as compared to cells grown in natural surface water media.
A significant advantage of batch culture systems is their operational simplicity. The culture
vessels most often consist of an Erlenmeyer flask with a sample to flask volume ratio of about
0.2 in order to prevent carbon dioxide limitation. This volume ratio is only critical if the flasks
are shaken by hand once a day during the culturing run. If the flasks are cultured on a rotating
shaker table a sample to flask volume ratio of 0.5 is permitted.
Batch culture systems are widely applied because of their simplicity and flexibility, allowing to
change species and to remedy defects in the system rapidly. Although often considered as the most
reliable method, batch culture is not necessarily the most efficient method. Batch cultures are harvested
just prior to the initiation of the stationary phase and must thus always be maintained for a
substantial period of time past the maximum specific growth rate. Also, the quality of the harvested
cells may be less predictable than that in continuous systems and, for example, vary with the timing
of the harvest (time of the day, exact growth phase).
Another disadvantage is the need to prevent contamination during the initial inoculation and
early growth period. Because the density of the desired phytoplankton is low and the concentration
of nutrients is high, any contaminant with a faster growth rate is capable of outgrowing the culture.
Batch cultures also require a lot of labor to harvest, clean, sterilize, refill, and inoculate the