Single-walled carbon nanotubes for enhanced photosynthesis and high-value compound production in microalgae and cyanobacteria

Carbon nanomaterials such as single-walled carbon nanotubes (SWCNTs) can improve algal photosynthetic efficiency and increase the production of valuable compounds without decreasing biomass productivity. This work investigates the effects of SWCNTs dispersed by different proteins or functionalized with carboxylic groups (SWCNT-COOH) on several microalgae and a cyanobacterium to boost photosynthetic performance and produce valuable compounds. The best SWCNTs' dispersion in water was achieved with lysozyme (LSZ@SWCNT), histone (HST@SWCNT) and SWCNT-COOH. In most cases, no toxicity was observed when the phototrophs were exposed for 72 h to these SWCNTs. The cyanobacterium Arthrospira platensis and the marine diatom Thalassiosira sp. were selected for evaluating the effects of exposure to LSZ@SWCNT for up to 14 days. Increased production of valuable algal compounds (i.e., phycocyanin + 38 % in A. platensis after 14 days, docosahexaenoic acid + 92 % and eicosapentaenoic acid + 63 % in Thalassiosira sp. after 7 days) was observed without any impairment of the photosynthetic efficiency (+19 % for Thalassiosira sp.). The effects observed on both the surface of the cells and intracellular structures (i.e., cell wall modifications, an increase of mucus, and vacuolization) suggested the interaction with LSZ@SWCNT as responsible for the changes in biochemical composition and photosynthetic performance. Therefore, the proposed nanobiotechnological approach coupling cyanobacteria and microalgae with SWCNTs may tune the photosynthetic pathways towards the production of high-value compounds exploitable in cosmetics and nutraceuticals, ultimately improving the light-to-chemicals conversion processes, without negatively impacting the growth.