Page 254 - Algae

Algal Culturing 237

extended periods. Open cultures such as uncovered ponds and tanks (indoors or outdoors) are more
readily contaminated than closed culture vessels such as tubes, flasks, carboys, bags, etc. Axenic
cultivation can be also chosen, by using algal cultures free of any foreign organisms such as bac-
teria, but this cultivation is expensive and difficult, because it requires a strict sterilization of all
glassware, culture media, and vessels to avoid contamination. These constraints make it impractical
(and very expensive) for commercial operations. On the other hand, non-axenic cultivation, though
cheaper and less laborious, are more prone to crash, less predictable, and often of inconsistent
quality.
Different types of algal cultures are used worldwide, the most routinely adopted include batch,
continuous, and semicontinuous ponds and photobioreactors.

BATCH CULTURES
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 limit-
ing factor, whereas other nutrient components of the culture medium decrease over time. Any pro-
ducts 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.
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:
N 2 ¼ N 1 e m (6:1)

where N 2 and N 1 are the number of cells at two successive times and m 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
availability.