Metal-based nanoadditives for increasing biomass and biohydrogen production in microalgal cultures: A review

Abstract

Microalgae have emerged as an interesting alternative for the generation of biofuels, including bioethanol, biodiesel, biomethane, biohydrogen and syngas. They are also used for the production of biofertilizers and other products of interest, while acting as a CO2 sink and favoring water purification by absorbing nutrients. However, the economic viability of large-scale microalgae cultivation for any of these purposes is not yet a reality. One of the main factors affecting this viability refers to the microalgae growing phase. In this scenario, nanoparticles, e.g. metallic, are emerging as possible alternatives to improve the cultivation and harvesting of microalgae. The addition of certain nanoadditives in microalgal cultures can improve light conversion in the photobioreactor, as well as CO2 absorption performance. However, the way in which these nanoparticles act or the concentration at which they maximize their positive effects on the culture are aspects that remain unclear. Thus, this paper critically reviews recent advances in nanoparticulate additives when they are used as a supplement in microalgae cultures, with the specific objective of improving biohydrogen production, but also analyzing their effect on algal growth, lipid accumulation and CO2 capture. The effects of nanoparticles on the selection of metabolic pathways of microalgae and their potential toxicity are also reviewed. The reported studies reveal that the toxicity of nanoadditives depends on their concentration, typology (chemical nature, particle size, crystalline structure) and species of microalgae. Furthermore, the supplementation of certain nanoparticles clearly increases the accumulation of intracellular compounds of interest and the growth of the algae. Additional studies should focus on aspects such as optimizing the dose of nanoparticles, designing strategies to minimize toxicity risks, or working with several additives simultaneously to create synergistic effects.