*2.1. Bio-Photolysis*

Bio-photolysis in a biological system generally means the dissociation of the water molecule in the presence of light (photons). In such a lytic process, photosynthetic microorganisms such as microalgae and cyanobacteria are involved, whereby photosystems (PSI, PSII) absorb light energy and the excited electrons pass through a sequence of energy carriers, eventually receiving two protons when a water molecule splits [28], as depicted in Figure 1.

**Figure 1.** Schematic representation of direct photolysis for hydrogen production.

In the direct process, the system follows the following reactions:

$$\begin{array}{rcl} 2\mathrm{H}\_{2}\mathrm{O} + \mathrm{hv} & \rightarrow \mathrm{O}\_{2} + 4\mathrm{H}^{+} + \mathrm{Fd}^{(4\mathrm{e}^{-})} \rightarrow \mathrm{Fd}\_{(\mathrm{red})}. \\\\ 4\mathrm{e}^{-} + 4\mathrm{H}^{+} + \mathrm{Fd}\_{(\mathrm{red})} & \stackrel{\text{Hydrogenase}}{\rightarrow} \mathrm{Fd}\_{\mathrm{Ox}} + 2\mathrm{H}\_{2}^{\mathrm{Fd}\,(\mathrm{red})} \end{array}$$

.

Although photoproduction of hydrogen in the biosystem is environmentally friendly and can be undertaken by partial activation of photosystem II in *Chlamydomonas* spp., it is not economically feasible because it is inefficient for development at the industrial level. When discussing the drawbacks of this mechanism, many physiological factors are taken into consideration, such as hydrogenase O2 sensitivity, competition with other metabolic pathways, downregulation of electron transport by non-dissipation of a proton gradient, and performance under non-saturating illumination [30].
