M

(b)

i i i i

l i l i

400 600 800 1,000

Wavelength (nm)

400 600 800 1,000

Wavelength (nm)

Figure 1. In vivo absorption spectra of (a) a microalga (Scene-desmus acutus), (b) a cyanobacterium (Synechocystis sp.), and (c) a purple bacterium (Rhodopseudomonas viridis).

a result of water photolysis. Green and purple photosyn-thetic bacteria lack photosystem II and cannot use water as an electron source; hence, they do not produce oxygen photosynthetically. For this reason they are called anoxy-genic phototrophs. These definitions apply irrespectively of whether the organism obtains its cellular carbon principally from inorganic (autotrophically) or organic (hetero-trophically) sources. Microalgae and cyanobacteria are mostly aerobic (they use oxygen as electron acceptor for respiratory metabolism) and autotrophic; purple bacteria are mainly anaerobic and heterotrophic; green bacteria are prevalently anaerobic and autotrophic. For a clear, comprehensive treatment of both plant and bacterial photosynthesis, see Hall and Rao (7).

Although this article deals exclusively with photobi-oreactors for the cultivation of oxygenic phototrophs (mi-croalgae and cyanobacteria), some of the aspects covered and most of the conclusions can be extended to other pho-totrophic groups. For brevity, the term microalgae is used here in its botanical meaning, which includes cyanobac-teria.

Response of Phototrophs to Light Quality and Quantity

The rate of photosynthesis achieved by a phototrophic cell depends on the rate of photon absorption and on the effi ciency with which the absorbed photons are used. The rate of absorption and the efficiency of conversion of absorbed photons are in turn determined by the characteristics of the photosynthetic apparatus and by the spectral quality and intensity* of the incident light.

Photosynthesis as a Function of Light Quality. The wavelengths absorbed by a phototroph depend on its primary and accessory pigment content as shown by its absorption spectrum (Fig. 1). Only absorbed wavelengths are used, but not all the wavelengths absorbed are equally efficient for photosynthesis. Therefore, the spectral suitability of a light source for a phototroph is better represented by the action spectrum of photosynthesis (i.e., the curve obtained by plotting the specific rate of photosynthesis [P]\ against wavelength across the photosynthetic range for the cultured organism). Although action spectra give more information than absorption spectra, the latter are more commonly reported because they are easier to determine and interpret. The curve obtained when growth, rather than photosynthesis, is plotted against wavelength is called a growth action spectrum.

Photosynthesis as a Function of Light Intensity. The rate of photosynthesis is not simply proportional to the rate of photon absorption, since photons may be captured by the pigments much faster than the photosynthetic apparatus can make use of them. If photosynthesis is measured at different light intensities, the specific photosynthesis rate (P) can be plotted as a function of light intensity (I) in the well-known light response curve of photosynthesis, or the P-Icurve (Fig. 2). In the dark, where there is, of course, no photosynthetic activity, O2 is consumed and CO2 is released due to cellular respiration. As the light intensity gradually increases, photosynthetic O2 evolution (and CO2 consumption) takes place and a light intensity value, called the light compensation point or compensation irra-diance (I), is eventually reached, at which value photosyn-thetic O2 evolution balances respiratory O2 consumption. Beyond this value, production exceeds consumption and net photosynthesis is achieved. From here on, P increases linearly with Iup to a certain point, beyond which the rate of increase of P per increment in light intensity declines and eventually levels off. The slope of the linear portion of the curve, called alpha (a), represents the rate of photosynthesis per unit incident light or relative quantum yield (also called maximum photosynthetic efficiency or maximum quantum yield), and is a measure of the efficiency

*The spectral quality of light depends on the wavelength of each single photon. The energy content of a photon varies with its wavelength according to e = hc/k, where e is energy content, h is Planck's constant, c is the speed of light, and k is the light wavelength in nm. Light intensity is given by the number of photons impinging on unit area in unit time (expressed as imol photons m"2 s"1). More properly the incident photon flux on a unit area in unit time is defined as photon flux density (PFD). jPhotosynthesis can be measured in terms of either CO2 uptake or O2 release. Rates of photosynthesis can be expressed per unit biomass, or any quantity proportional to it, to obtain the specific photosynthetic rate (P). Typical units for P are imol evolved O2 (or fixed CO2) mg Chla"1 h"1.

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