Enhancing Gephyrocapsa huxleyi growth with recycled calcium from oyster shells

Abstract Coccolitophores, such as Gephyrocapsa huxleyi , can have multiple interesting biotechnological applications thanks to their ability to produce complex structures made of CaCO 3 , called coccolith, and a biomass rich in valuable molecules. Unfortunately, they usually reach low cellular density in standard conditions, hindering actual industrial applications. To enhance growth, CaCO 3 may be used instead of standard CaCl 2 . Here, shell-derived CaCO 3 was also tested to valorise oyster shells, which are not recyclable and can cause environmental harm if not disposed of correctly. G. huxleyi cultures were grown in f/2 medium prepared using standard artificial seawater, artificial seawater with pure CaCO 3 in place of CaCl 2 , and artificial seawater with the pulverised shells of Magallana gigas (Pacific oyster) instead of CaCl 2 . Cultures were monitored for 14 days, and final dry biomass concentration, photosynthetic pigment and total protein concentrations were evaluated. Moreover, coccolith morphology was checked through scanning electron microscopy. Replacing CaCl 2 with pure CaCO 3 increased cell density roughly sixfold relative to the control; using pulverised oyster shells yielded the greatest effect, with cell densities about 26 times higher than CaCl 2 . Cells grown in the medium added with pulverised oyster shells were smaller, exhibited higher pigment content per dry weight and a protein content comparable with the control. The bulk composition of the oyster shell revealed the presence of trace elements, like Mn, Fe and Cu, that could have enhanced the algal growth. Results indicate pulverised oyster shells can be a sustainable and effective calcium source to valorise waste and enhance G. huxleyi biomass production.