3.4.3 Principles of large-scale culture management

The objective in culture management is to obtain the greatest possible daily yield of algae so that the culture systems are operated cost effectively. This yield must be sustained for long periods of time to maintain the hatchery output of juveniles. Ineffective management of algal culture greatly influences the potential for production and ultimately the selling price of the bivalve seed.

Figure 25:
The relationship between the productivity of a culture system (yield) and light energy input.
See text for explanation.

The operation of semi-continuous, internally illuminated cultures will be considered in this section. The general principles are applicable to any culture facility and at any scale of production. The basic yield relationship with light energy input is shown in Figure 25. Yield is calculated as the number of litres of algae harvested per day at a standard cell density per ?l.
The use of the term standard cell density requires explanation. In order to compare yields of different species in a culture system, a common factor based on dry weight biomass of harvested algae is applied. Different alga species vary widely in linear dimensions and in weight per cell, as already seen in Table 1. Knowing the weight per cell, an equivalent number of cells can be calculated for each species to provide a given biomass. For some of the important species this approximates to:
250 cells of Chaetoceros calcitrans = 100 cells of lsochrysis galbana = 60 cells of Skeletonema costatum = 10 cells of Tetraselmis suecica, on a dry weight basis.
Thus, for Skeletonema and Tetraselmis the standard cell densities used in yield calculation are 6 000 and 1 000 cells per ?l respectively (alternatively, 6 million and 1 million cells per ml).
Another term requiring explanation is the concept of post-harvest cell density (PHCD).
PHCD = cell density per unit volume (cells per ?l) immediately after daily harvesting and replacement of the volume removed with fresh culture medium.
It is the cell density (following harvesting and replenishment of the culture volume with new medium) relative to light intensity that will largely dictate growth of the culture in the following 24-h period. Reference to Figure 25 shows that yield is at a maximum at the optimum PHCD when light energy input is not limiting. At PHCD values below the optimum, cell division rate (K), described by the equation:

K = l.443 x logn Nt (Nt = cells per ?l at harvest) t (days) logn N0 (N0 = PHCD)

is at its maximum but the PHCD is too low for maximum productivity. Above the optimal PHCD, light becomes increasingly limiting due to the self-shading effect of cells at higher culture densities. Photosynthesis decreases, therefore, cell division rate and daily yields decrease. Yield is maximal at a particular light intensity and can be increased or decreased by altering the light energy input.
The effect of increasing light intensity in 200 l cultures of Tetraselmis by increasing the number of 80W fluorescent lamps from 4 to 8 is shown in Figure 26. Four lamps provide an illumination intensity of 7.6 mW per cm2 (7.6 milliwatts per centimetre squared which provides an illumination intensity of 28 000 lux) and 8 lamps, 14.0 mW per cm2 (52 000 lux). Maximum yields, increased from 67 l per day at 1 000 cells per ?l at 28 000 lux to 96 l per day at the same cell density at the highest illumination intensity. Improvements in yield result from an

Figure 26: The effect of light intensity on yield of Tetraselmis in 200 l internally illuminated culture vessels.

accelerated cell division rate and, because of the greater
light energy input, cultures can be operated at a higher PHCD. Yields from 8 and 6 lamp units are similar. This is because the cultures approach light saturation at the highest illumination level, therefore, yield relative to cost of the extra energy input decreases with 8 lamp units.
The influence of PHCD on cell division rate (K) in Tetraselmis in 200 l cultures is shown in Figure 27A. Increasing PHCD values result in exponentially decreasing K values, as light becomes progressively more limiting. Data in Figure 27B and C show that values of K decrease, therefore, yield decreases with increasing pH and salinity. This highlights the need to control these parameters by (a), increasing carbon dioxide input in the case of increasing pH and (b), by diluting the culture medium in the case of elevated salinity. Devices to control pH by varying the rate of input of carbon dioxide are available from suppliers of aquaculture equipment.

Figure 27: Effects of A – post-harvest cell density (PHCD) and B – pH on cell division rate, and the influence of salinity on the productivity of cultures of Tetraselmis suecica – C.
Culture techniques that improve maximum yield can also have the effect of altering the size of cells at harvest (Figure 28). With increasing PHCD and onset of light limitation, cells decrease in size measured as either dry or organic weight. However, within normal operating limits of PHCD the overall effect on maximum yield, on a biomass basis, is small.
The nutrient content of the culture medium also has an important effect on the maximum yield achievable in large-scale culture systems. This is shown in Figure 29, which provides data on the culture of the diatom, Skeletonema costatum. Diatoms require silica, which is provided in the form of SiO3-Si, to allow development of the silicaceous frustrules that enclose the cytoplasm. If silica is limiting, cell growth and division rates decrease and yields diminish. This is clearly shown in the comparison of 6, 80 W fluorescent lamp units at 30 mg per l Si (Figure 29A) and at 5 mg per l Si (Figure 29C). Cultures at 30 mg per l Si provided a maximum daily yield of 160 l (from a culture volume of 200 l at 6 000 cells per ?l), whereas at 5 mg per l the maximum

Figure 28: Relationships between post-harvest cell density (PHCD) and the size of cells in terms of weight and the productivity of semi-continuous culture of Tetraselmis suecica.

Figure 29: Relationships between post-harvest cell density and yield at standard cell density of Skeletonema costatum cultures operated semi continuously at two light intensities and silicate concentrations.
yield was only 74 l – less than from a 4 lamp unit at the highest Si level (Figure 29B). The maximum yield (Figure 29) is considerably greater than was obtainable from efficiently operated cultures of Tetraselmis and reflects the much higher cell division rates hence, productivity achievable with diatoms.