Growth of Microbial Phototrophs in Batch Culture

Phototrophic microorganisms can be grown in batch or continuous culture. A batch culture is initiated by the transfer of a small amount of cells (inoculum) into fresh growth medium; after the inoculum, nothing is added to (except light and carbon dioxide), or removed from (except oxygen), the system.

Growth Rate. Growth can be defined as the orderly increase of all the cell constituents. During this process, unicellular organisms grow to a maximum size, then divide to form two daughter cells. The daughter cells in turn repeat the process, and the number of cells doubles at regular intervals of time. The time required for a growing population to double is called the generation time, g. The rate of increase in biomass concentration is generally expressed as the specific growth rate, i, which is related to the generation time by i = ln2/g. The specific growth rate, which can be defined as a proportionality constant for the auto-catalytic growth reaction, is described by the following equation:


Figure 3. (a) Growth curve of a phototrophic microorganism in batch culture. The following six phases are shown: I, lag; II, acceleration; III, exponential; IV, deceleration; V, stationary; VI, death (after Ref. 8). (b) Changes in growth rate (u) during the six typical growth phases.


Figure 3. (a) Growth curve of a phototrophic microorganism in batch culture. The following six phases are shown: I, lag; II, acceleration; III, exponential; IV, deceleration; V, stationary; VI, death (after Ref. 8). (b) Changes in growth rate (u) during the six typical growth phases.

where i is the specific growth rate (h-1), Xis the biomass concentration (g L-1), t is time (h), and dX/dt is the variation in biomass concentration with respect to time (g L-1 h-1), that is, the culture productivity per unit volume. The volumetric productivity (dX/dt = iX of a culture in a pond or a flat reactor can be easily normalized on a per-unit-area basis by considering the culture volume (V) corresponding to the unit of illuminated surface area (A). Therefore, the areal productivity can be calculated as iXV/A, or, since V/A is the depth d of the culture, as /iXd.

Growth Curve. When a phototroph grows under suitable conditions in batch culture, its growth curve follows a sigmoidal pattern showing six principal phases (Fig. 3a). Each phase is characterized by a particular value of the specific growth rate (Fig. 3b) and reflects a particular metabolic state of the cell population. After the inoculum, growth does not necessarily start right away since most cells may be viable, but not in condition to divide, especially when the parent culture is old. The interval necessary for the transferred cells to metabolically adapt to the new situation and start to grow is called the lag phase. After the lag phase, the culture enters into the acceleration phase, during whichi increases continuously. When a constant growth rate is reached, the culture is said to be in the exponential growth phase. During exponential growth, all the cells are light saturated (if a saturating light intensity is provided at the culture surface) and growth proceeds according to equation 1. The maximumi is achieved in this phase, and in this respect phototrophs do not behave differently from chemotrophs. (Note: Organisms that obtain energy for growth from oxyreduction reactions involving either organic or inorganic electron donors.) The exponential growth phase of phototrophs in batch culture normally lasts for a relatively short period because the cells start to shade each other as their number increases; growth be comes light limited and i decreases (deceleration phase). Self-shading and the fact that the energy source is instantaneously provided at the culture surface and cannot be stored in the reactor are the two fundamental factors that differentiate photobioreactors from reactors for chemo-trophs. During the deceleration phase, although i decreases continuously, the culture productivity may remain stable at maximum value because of the continuous proportional increase in cell concentration (X). Since the increase in cell biomass becomes almost linear, sometimes the initial part of this phase is called linear phase. Conditions of linear growth are typically sought in mass cultures of phototrophs, since they lead to the highest productivity. Following the linear growth phase, the cell population continues to increase, but i 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 stationary phase is followed by the death phase, in which the population of viable cells decreases.

To achieve continuous exponential light-saturated growth, the culture process has to be changed from batch to continuous; this is accomplished by continuously removing the newly formed cells. This condition is not adopted in photobiological process targeting production of biomass or related products, since it leads to low efficiency of conversion of the irradiance impinging on the culture surface and to low productivity (see later section).

Light distribution, Growth Rate, and Volumetric Productivity in Phototrophic Cultures

If nutrients are provided in suitable amounts and temperature is maintained at the optimum value, the productivity of phototrophic cultures becomes a function of light availability and cell concentration. In dilute cultures, self-shading is minimal and all the cells receive the same amount of light, independent of their position. It is self-

evident that, at these low densities, even if the light intensity provided would permit the maximum growth rate, the culture will achieve low efficiency of light conversion and low productivity, since most of the impinging photon flux will pass through the culture unabsorbed (Fig. 4, upper curve). Mass phototrophic cultures, in order to obtain maximum productivity, must be kept dense enough to absorb all the light impinging on their surface. Under these conditions, self-shading, due to light absorption by cell pigments and scattering by the cells causes light intensity to fall off exponentially with the culture depth (Fig. 4, lower curve). In dense cultures, the light availability for each single cell is much reduced; this brings about a significant diminution of the specific growth rate, but productivity is, in any case, higher than in dilute cultures because the decrease in growth rate is more than compensated by the increase in cell concentration. For achieving maximal productivity in mass cultures it is mandatory to operate at a proper combination of culture depth and cell concentration. This concept, known as operating at the optimal population density (OPD), has been investigated by many researchers, most recently by Richmond and coworkers (9-11). In general, maximal productivity is obtained at relatively high population densities and at specific growth rates that are about one-half of the maximum. Increasing the cell concentration above the OPD is generally associated with a reduction in productivity because of the presence of a large dark zone inside the reactor, which leads to high maintenance requirements in relation to the overall

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