Comparison of two different modes of UV-B irradiation on synthesis of some cellular substances in the cyanobacterium Synechocystis sp. PCC6803

Abstract

Two different modes of UV-B irradiation of the cyanobacterium Synechocystis sp. PCC 6803 are compared: turbidostatic control and additional physiostatic control. Under turbidostatic control, the cells were exposed to different constant UV-B irradiances, whereas under physiostatic control, an electronic control loop modulated UV-B irradiance in such a way that photosynthetic efficiency ϕ _PSII was kept constant at a fixed set point. The UV-B-induced stimulation of the synthesis of pigments, α-tocopherol, and the antioxidative potential of methanolic soluble components of Synechocystis showed significant differences depending on the mode of irradiation, even though the overall doses were equal. For example, compared to the initial values, the concentrations of myxoxanthophyll and zeaxanthin increased to 226–244% and 453% upon constant UV-B irradiation in turbidostatic processes, whereas maxima of 600% and 740% were reached in turbidostatic process with additional physiostatic control. The α-tocopherol concentration increased under constant UV-B irradiances, up to a maximum of 150%. Under physiological control, however, maximum increases of 390% over the initial values were measured. Furthermore, a reaction scheme is given explaining the higher yield under physiostatic control.

Appendix

Using the transfer functions and states (pool sizes) as given in Fig. 8, the following relationships (in the frequency domain, Hansen 1978) can be derived:

The measured quantum yield ϕ is the quantum yield ϕ _A of the non-UV-B-radiated system diminished by the action of the ROS pool R via the transfer function f

$$ \phi = {\phi_A} - fR $$

(5)

In the physiostat, the measured ϕ is compared with the set point ϕ _S

$$ U = a\left( {\phi - {\phi_S}} \right) $$

(6)

By virtue of the introduction of U _T into Eq. 6, the following calculations hold for pure turbidostatic control (amplification of the physiostat a = 0, U _T  ≠ 0) and for physiostatic control (a ≠ 0; U _T  = 0)

$$ U = a\left( {\phi - {\phi_S}} \right) + {U_T}\, $$

(7)

Insertion of Eq. 5 into Eq. 7 leads to

$$ U = a\left( {{\phi_0} - fR} \right) + {U_T}\,\,\,\,\,\,\,{\hbox{with}}\,\,\,\,\,{\phi_0} = {\phi_A} - {\phi_S} $$

(8)

ϕ ₀ comprises the terms which are not dependent on UV-B irradiation and is the sum of ϕ of the undamaged photosystem (i.e., ϕ _A ) and the negative set point of the physiostat ϕ _S   = ϕ _PSII,SP.

Synthesis of pigments and antioxidants is stimulated by R. R ₀ accounts for the possibility that the organism tolerates a basic level R ₀.

$$ P = s\left( {R - {R_0}} \right) $$

(9)

R is generated by U, and P scavenges R. This leads to

$$ R = ua\left( {{\phi_0} - fR} \right) - ps\left( {R - {R_0}} \right) + u{U_T} $$

(10)

$$ R = \frac{{ps{R_0} + u{U_T} + ua{\phi_0}}}{{1 + ps + uaf}} $$

(11)

$$ P = \frac{{ssp{R_0} + su{U_T} + sua{\phi_0}}}{{1 + ps + uaf}} - s{R_0} $$

(12)

The transfer functions a, u, and f can be treated as constant factors within the temporal resolution of the investigations here: a is the electronic amplification (Marxen et al. 2005); u and f are fast because there is an immediate response of p upon irradiation (Marxen et al. 2005; Fig. 1).