Unraveling the effect of olive-mill waste biostimulant particles during photosynthetic biogas upgrading coupled with pigment production.

The addition of biostimulant particles (BPs) derived from olive-mill waste (OMW) has emerged as a promising technique to improve microalgae performance during photosynthetic biogas upgrading (PBU) and has motivated the use of pigment-producing microalgae to improve the cost-competitiveness of the process. However, a clear understanding of the interaction between the particles and microalgae is necessary to design proper materials that maximize both PBU overall performance and pigment production. Thereby, this study aims to unravel the biostimulant effects of OMW-BPs on Arthrospira platensis cultures during batch PBU tests. Three BPs with different Fe contents (0, 6 and 15 % dw) were synthetized and added to A. platensis cultures at a concentration of 100 mg L-1. The results showed that the three BPs boosted A. platensis growth rate and phycocyanin (C-PC) content regardless of the Fe content. However, the higher content of humic substances in non-iron containing BPs (hydrochar) resulted in a superior growth performance and in a C-PC content up to 325 mg g-1dw. A. platensis cultures supplemented with hydrochar BPs under light-stress conditions (800 μmol m-2s-1) exhibited a similar C-PC content and a specific growth rate 15 % higher than the non-supplemented cultures. Transcriptomic and SEM-EDS analysis demonstrated that hydrochar BPs interacted extracellularly (by acting as metal chelators and improving nutrient and ions uptake) and intracellularly (by penetrating the cells and improving electron flux and PSII efficiency). Hence, this study demonstrates the potential of hydrochar BPs to improve the photosynthetic efficiency of A. platensis and ultimately the overall PBU process.