Photosystem II is an enzimatic chlorophyll-protein complex which, in higher plants and photosynthetic organisms, catalyses primary charge separation and is responsible for splitting water to form molecular oxygen, thus supporting electron transfer and the photosynthetic process. The Photosystem II as a biosensor has several advantages. The most important is the simple nature of the biological transduction which is directly usable without resorting to makers or competitors. The decrease of the oxygen-evolution, the electron transfer, and the fluorescence can be easily monitored using Clark’s electrodes, potenziometric and optical systems, respectively. Given the sensitivity of the PSII complex to several pollutants and stress conditions, it has the potential for a wide variety of applications.

The aim of the present work was to study the potential of Photosystem II for use as a biosensor to detect ionizing radiation. Atomic physics reveals that the passage of radiation through matter is characterised by a transfer of energy to the target material. Exposure of biological material to ionising radiation leads to a loss of function due to the destruction of critical structures. The larger the structure, the more likely it will be hit. Because the energy deposition is so large, function is completely destroyed by a single hit. The only activity remaining after radiation exposure is a result of units which have escaped ionisation and are fully active. Therefore, the level of PSII activity decline in oxygen-evolution, fluorescence or electron transfer can be directly correlated to the dose of radiation.

We studied the effects of g -ray exposure in a ⁶⁰Co-cell of 8 Mrad on Photosystem II activities. We observed that at –20°C, the activity of isolated PSII particles is inhibited to 90% after 10h g -ray exposure; in 1h the functionality is decreased by about 6-10%, corresponding to an absorbed energy of 7000 Gy (Joule/Kg). An application of the biosensor could be the measurement of the space radiation. Galactic cosmic radiation consists of about 83% of protons with energies between 10-1000 MeV, 16% helium nucleus and 1% electrons. Despite the high energies involved, the number of these particles is very reduced to a flux of 0.01 erg. cm^-2.sec^-1. Under such a condition, the biosensor tested in our laboratory would not be sensitive enough since it would adsorb an energy of 7000 Gy only after 800 days of exposition. However, solar particle radiation, contains energetic particles emitted more copiously during magnetic disturbances. The energy released during a sun flare is about 10³² erg and the flux can be a thousand times higher than galactic cosmic radiation. Under these conditions we calculated that our actual biosensor system could absorb energy enough to inhibit its functionality in 1-2 days.

We studied the effects of radiation on various organisms coupling the biomediator to a PEA fluorescence transduction system. It was found that various organisms respond to radiation in a different way and the sensitivity to radiation was correlated to the lipid composition. We designed a semiautomatic system to renew the inhibited biomediator.

In comparison to the present non-traditional system for measuring ionising radiation with the current methods (counters and visual detectors), this device provides many advantages: i) miniature size, ii) modulated sensitivity depending on the biological preparation; iii) measurements on line; iv) low expense; v) operative for long-term experiments. Moreover, this system directly measures the radiation effect on living photosynthetic organisms which are expected to be used as an oxygen-producing system on shuttles.