(5.30)

where P_E is the photosynthetic rate at any irradiance E, E_λ is the spectral irradiance (in µmol m^-2 sec^-1) and K_m is the half saturation costant when P_E = P_max/2 (Figure 5.15a).

FIGURE 5.15 Photosynthesis–irradiance response curves: Ec, irradiance compensation point; Ek, saturating irradiance; and Pmax: maximum photosynthetic rate. (a) typical plot; (b) comparison of two curves with different slopes: keeping constant the number of photosynthetic units, but increasing the functional absorption cross-section, the slope increases; and (c) comparison of two curves with different maximum photosynthetic rate: increasing the number of photosynthetic units, Pmax increases.

In the light-limited region, that is, at low irradiance levels, the rate of photon absorption determines the rate of steady-state electron transport from H₂O to CO₂. In the light-limited region, the available light is insufficient to support the maximum potential rate of the lightdependent reactions, and thus limits the overall rate of photosynthesis. In this region, the rate of photosynthesis (P_E) can be described as:

(5.31)

where Eλ is the spectral irradiance and a is a measure of “photosynthetic efficiency,” or how efficiently solar energy is converted into chemical energy. α takes into account that the light absorbed by the algal cell is proportional to the functional absorption cross-section (σ_PSII) of the PSII (the effective area that a molecule presents to an incoming photon and that is proportional to the probability of absorption) and to the number of photosynthetic units (n):

(5.32)

Equation (5.31) shows that photosynthetic rate is linearly proportional to irradiance at low irradiance levels. Greater is the slope (α) more efficient is the photosynthetic process (Figure 5.15b). Keeping constant the number of photosynthetic units but increasing the functional absorption crosssection the slope will increase. The slope can be normalized to chlorophyll biomass and, if so, a superscript “B” is added to denote this normalization, thus α^B. In this case, the curve dimensions are O₂ evolved per unit chlorophyll and quanta per unit area.

At very low irradiance level, the rate of oxygen consumption will be greater than the rate of oxygen evolution; hence, respiration is greater than photosynthesis and net oxygen evolution will be negative. Therefore, the P–E curve does not pass through the origin. The irradiance value on the x-axis at which photosynthesis balances respiration is called the light compensation point (E_c). Therefore, the Equation (5.30) becomes:

(5.33)

This phenomenon (chlororespiration) is more pronounced in cyanobacteria, where the photosynthetic and respiratory pathways share common electron carriers (cytochromes). The irradiance levels needed to reach compensation point (E_c) are about 10 µmol m^-2 sec^-1 for shallow water, but are much lower in dim habitats.


As irradiance increases, more ATP and NADPH are produced, and the overall rate of photosynthesis becomes increasingly non-linear, rising towards its maximum or saturation level, P_max. This pattern will continue until some other factors becomes limiting. By definition, in the light saturated region, the rate of photon absorption exceeds the rate of steady-state electron transport from H₂O to CO₂. Saturating irradiance, E_k, is defined as the point at which the extrapolated initial slope crosses P_max. E_k represents a optimum on the photosynthesis irradiance curve.

Light-saturated photosynthetic rate is independent from the functional absorption cross-section of the photosynthetic apparatus and is only related to the number of photosynthetic unit (n) and their steady-state electron turnover rate through the PSII reaction center (1/τ_PSII):

(5.34)

This equation says that increasing the number of photosynthetic units (but not their size) P_max, that is, the saturation level, increases, and that P_max cannot be derived from measurements of light absorption (Figure 5.15c). In other words, above the saturation point (E_k), the light-dependent reaction are producing more ATP and NADPH than can be used by the light-independent reaction for CO₂ fixation, that is, increasing irradiance no longer causes any increase in photosynthetic rate. Above E_k and under normal condition, availability of CO₂ is the limiting factor, because the concentration of CO₂ in the atmosphere is very low (0.035% v/v).

Saturating irradiances show some correlation with habitat, but generally, they are low compared with full sun. Intertidal species require 400–600 µmol m^-2 sec^-1 (ca. 10% of the full sun irradiance), upper and midsublittoral species 150–250 µmol m^-2 sec^-1and deep sublittoral species less than 100 µmol m^-2 sec^-1. Diatoms under ice saturate at 5 µmol m^-2 sec^-1.

Further increase in irradiance beyond light saturation can lead to a reduction in photosynthetic rate from the maximum saturation level. This reduction, which is dependent on both the irradiance and the duration of the exposure, is often termed photoinhibition. Photoinhibition can be thought as a modification of P_max either by a reduction in the number of photosynthetic units or by an increase in the maximum turnover rate [Equation (5.34)]; thus photoinhibition leads a reduction in the photochemical efficiency of PSII, through a reduction in the population of functional (O₂ evolving) reaction centers. Increasing irradiance levels increase the probability that more than one photon, two for example, strike the same reaction centers at the same time. The added energies of two blue photons, for example, could be very harmful as the resulting energy will correspond to a UV photon and could damage the chromophores.