Screening for tolerance to natural phenols of different algal species: toward the phycoremediation of olive mill wastewater

Olive Mill Wastewater (OMWW) is a by-product of olive oil production and it is rich in nutrients (e.g. P, N and K) and phenolic compounds. These latter are aromatic compounds, and their concentrations can reach up to 11 g L−1 in OMWW. A complete remediation of OMWW is required since phenols are known to cause toxicity once released in the environment: particularly, their effect on microorganisms is species-specific and primarily depends on the chemical structure of the compound. Microalgae have already been tested to remediate OMWW, data are promising but how different phenols affect algal growth is still poorly known. In this work, ten microalgal species belonging to different phylogenetic groups and natural habitats, were grown in the presence of three phenolic compounds found in OMWW (tyrosol, coumaric acid, caffeic acid). Algal growth and removal of phenolic compounds were assessed. Tyrosol was the only compound allowing growth of each tested microalga similarly to what observed in control media. Growth of microalgae and removal of phenolic compounds were not always related, and a multi-step phenolic removal mechanism was suggested. Species such as Nannochloropsis salina and Porphyridium purpureum rapidly died after the addition of coumaric acid or caffeic acid but a high removal percentage (60–100 %) of the phenols was still observed and it was likely due to their absorbance onto the cell surface. On average, freshwater species showed a higher growth performance compared to the one of marine species; in particular, Tetradesmus obliquus and Anabaena sp. showed the best results. This work elucidates a species-specific effect for each phenolic compound on algal growth and it also highlights that growth and removal are not related phenomena.