BIOACTIVITY POTENTIAL OF A MEDITERRANEAN BRACKISH DIATOM CULTIVATED IN PHOTOBIOREACTORS

Benthic microalgae developed strategies through evolution to cope with a variety of abiotic conditions as well as grazing and competition for resources so to tolerate and also thrive in a wide range of aquatic ecosystems. We isolated a strain of the benthic, colonial diatom Staurosirella pinnata (Ehrenberg) D.M. Williams & Round from sediments of a Mediterranean dystrophic lagoon (Cabras, Sardinia, Italy). The stock culture was maintained in Diatom Medium (DM) and used as inoculum for mass cultivation in an indoor 30 L photobioreactor, at 25 °C, irradiance of 80 μmol photons m-2 s-1 and 12:12h L/D cycle. The biomass was harvested at the stationary phase, by settling and centrifuging, and freeze-dried. A crude extract was obtained using a methanol aqueous solution (20% v/v) and separated into two different fractions, hydrophilic and lipophilic. Bioactivity of the hydrophilic fraction was then assessed by means of cytofluorimetric analysis on HaCaT, human immortalized keratinocytes, and CHL-1, human melanoma cell lines. The antiproliferative activity was tested in terms of cell death induction and cell cycle variations in a 24 h dose-response assay. Results showed a strong, dose-dependent cytotoxic effect on the CHL-1 cell line, up to 69.75% for the highest extract concentration tested (10 mg/ml). Conversely, extract administration to the HaCaT cells did not induce significant cell death levels. Further, we quantified the percentage of cells in each phase of the cell cycle. HaCaT showed a progressive and significant dose-dependent increase of cells in the S phase, at the lower doses. Whilst, administration to CHL-1 cells induced a completely different behaviour: i) a progressive dose-dependent decrease in the percentage of cells in the G1 phase (from 48.87% of the control cells down to 25.20% at 3.2 mg/ml); ii) an increase of cells in the S phase (from 29.72% up to 42.10% at 3.2 mg/ml); iii) an increase of cells in G2/M phase (from 20.27% up to 30.02% at 3.2 mg/ml). The compared analysis of cell death and cell cycle data suggests that the hydrophilic fraction of the crude extract of the diatom S. pinnata contains bioactive molecules that affect different targets or cellular pathways in malignant and normal cells. A more exhaustive and detailed analysis of the soluble compounds present in this fraction, as the future identification, isolation and characterisation of new active metabolites will open novel perspectives for biomedical exploitation.