Environmental Advances (Jul 2022)
First evidence of extracellular enzymatic degradation of benzo(a)pyrene by the phytoplankton species Selenastrum capricornutum and the influence of temperature
Abstract
Microbial degradation of pollutants such as polycyclic aromatic hydrocarbons (PAHs) is carried out by bacteria, fungi and microalgae. The degradation pathways have been fully elucidated for bacteria and fungi, but only partially for microalgae. The early PAH emergent metabolites formed by microalgae are known to be of the cis dihydrodiol type, suggesting the presence of a dioxygenase. Even so, the enzyme responsible for the degradation of PAH in microalgae has still not been identified. In this work, the presence of this enzyme was evidenced in extracellular and intracellular crude extracts (EE and IE, respectively) obtained from cultures of Selenastrum capricornutum activated by exposure to benzo(a)pyrene (BaP) for 72 h at 34 and 25 °C. BaP degradation was evaluated in both extracts by 3 h of incubation at room temperature, and subsequent quantitative analysis of the 4,5-dihydrodiol benzo(a)pyrene metabolite (4,5-dBaP) was performed. The results showed that the amount of metabolite produced was higher in the enzymatic extracts from cultures activated at 34 °C than in those activated at 25 °C: 0.195 μg vs. 0.091 μg in EE and 0.085 μg vs. 0.008 μg in IE. This suggests that enzymatic degradation was highly dependent on temperature and that the enzyme is produced in cells and subsequently secreted to the exterior to carry out degradation there. Extracts obtained from non-activated cells did not show degradation. Thus, an extracellular enzyme was evidenced for the first time in this organism, as was enzymatic induction by exposure to contaminants. The activation of the degradation metabolism of the microalgae was preserved and enhanced when re-exposure to the contaminant was performed. These findings are of utmost importance, as they provide solid support to understand the degradative metabolism of this microalgae and the interaction of its extracellular enzyme with a polluted environment as a protective mechanism.
