Manuel Méndez García, Martha Patricia García de Llasera, Francisco Rojo Callejas
Benzo(a)pyrene (BaP) and benzo(a)anthracene (BaA) are environmental pollutants with a well-known toxicity; however, documented information is not enough to fully understand their biodegradation pathways in Selenastrum capricornutum, a microalga that degrades polycyclic aromatic hydrocarbons (PAHs) very efficiently. Thus, this research aimed to identify the BaP and BaA metabolite formation in the degradation process performed by S. capricornutum, focusing on different microalgal components (biomass, liquid medium, and microalgal extracts). Hence, extracts containing metabolites isolated from microalgal components by the use of analytical techniques were selectively and characteristically analyzed by high-performance liquid chromatography (HPLC) coupled to mass spectrometry (MS) with different detection modes: (i) HPLC-ESI-QqQ-MS/MS analysis in multiple reaction monitoring (MRM) mode that confirms the formation of PAH-dihydrodiols in the initial PAHs oxidation process, but it also shows other metabolites that coincided with the MRM transitions, which was verified by (ii) HPLC-ESI-QqQ-MS analysis using a full mass scan, where new metabolites containing four and three aromatic rings (for BaP) and three and two aromatic rings (for BaA) were also identified that can be related to monooxygenases, dehydrogenases, dioxygenases, aromatic ring-opening oxygenases, and decarboxylases enzymes. Likewise, catechol identification during the biodegradation of both PAHs suggests a common metabolic pathway. Also, a key result was the identification of PAH metabolites in extracellular extracts, indicating that PAH-degrading enzymes are secreted by microalgal cells into the external environment. Overall, this work proposes a biodegradation pathway for BaP and BaA by S. capricornutum, opening the possibility of using microalgal extracts to develop ecological PAHs bioremediation strategies.