Bioenergetic strategy of microalgae for the biodegradation of phenolic compounds: exogenously supplied energy and carbon sources adjust the level of biodegradation.

The biodegradation of phenolic compounds by microalgae seems to be not a simple feature of a particular organism, but mostly a bioenergetic process depending on the growth conditions, especially on the exogenously supplied energy (carbon and light) sources. By using chlorophyll fluorescence induction measurements to estimate the molecular structure and function of the photosynthetic apparatus and therefore the tolerance/sensitivity of microalgae incubated with phenols, it can be assumed that, at least in low concentrations, phenol have no toxic effects on the cultures and can be used as alternative carbon source in them. Halophenols (chlorophenols, bromophenols and iodophenols) are quite toxic for the microalgal cultures. In halophenols the first step of the biodegradation is the split of the halogen substituent (dehalogenation). This is strongly determined by the bond dissociation energy of the corresponding substituent and therefore the energetic requirement for the biodegradation of halophenols increases following the sequence: iodophenol<bromophenol<chlorophenol. Additionally, the meta-position of the halogen on the phenol ring needs more energy than the ortho- and the para-one. These are possible explanations of the fact that the biodegradation of halophenols needs additional energy sources that can be exogenously supplied as organic carbon (glucose) or inorganic carbon (CO(2)).