Exploring the potential of calcifying marine phytoplankton (coccolithophores) for biomass production and CO2 sequestration

The current constant increase of atmospheric carbon dioxide (CO2) pressures the scientific community to find highly-efficient strategies to achieve the net-zero carbon footprint. Bio-based techniques, such as those using microalgae, represent a sustainable alternative to traditional CO2 capturing technologies, and thus their exploitation is becoming increasingly important. Among the many species of microalgae, coccolithophores, a group of calcifying unicellular marine phytoplankton, have great potential towards carbon (C) sequestration since they are capable to fix C in both organic fraction and inorganic carbonate exoskeleton, but yet they are still not well-studied for industrial purposes. So far, only few species of coccolithophores have been investigated (mainly Emiliania huxleyi), since their main limit for industrial applications is the low cell density reached. In this context, this thesis aims to conduct exploratory tests on two target species, Helicosphaera carteri and Chrysotila pseudoroscoffensis, to increment their biomass production and CO2 sequestration, in order to establish whether they can be effectively competitive with other more commonly studied microalgae. Helicosphaera carteri was selected because it is a heavily-calcifying species, with a high ratio between the particulate inorganic C (PIC) and organic C (POC), thus promising for C capture and sequestration; C. pseudoroscoffensis PIC:POC ratio, is a species highly resistant to eutrophication that frequently proliferate in large blooms. Both nutrient and C supplies were modified in flask experiments to evaluate the optimal conditions for enhancing biomass production. The best conditions identified for each species were then replicated in photobioreactors under controlled conditions. Higher nutrient concentrations increased the cellular density in both species, with better results for C. pseudoroscoffensis. Satisfactory results were obtained also increasing the C supply for the latter species. The collected data show that despite having higher PIC:POC ratio, H. carteri returned no interesting results for exploitation. On the other hand, C. pseudoroscoffensis seems to be a better candidate for further research in the field of carbon capture and sequestration. This study highlighted the importance of conducting an initial screening to discriminate the best genus/species for industrial exploitation, as well as the best conditions for increasing cellular density of this peculiar microalgae group. The observations and data collected here represent a significant starting point for further studies on coccolithophores' industrial applications, keeping in mind that among the still poorly studied bioproducts produced by coccolithophores, the most valuable and unique product is pure calcite for zero impact cement within the context of circular economy.