Production of fucoxanthin using a water surface-floating microalga Chromophyton sp. (Hikarimo)

Water-surface floating microalgae have a potential to be promising host organisms to produce useful compounds free from energy-consuming and costly harvesting processes. However, only a few water-surface floating microalgae have been studied for biotechnological applications. In this study, we investigated the potential of a chrysophycean alga Chromophyton sp., also known as Hikarimo in Japan, as a producer of fucoxanthin. The cells of this microalga float on the surface of freshwater in natural environments and can be harvested by attaching plastic film to the floating cells. Three strains, SH01, SH02, and SH03, were isolated from Japanese freshwater environments in Ibaraki, Nagano, and Chiba, and all three strains were identified as Chromophyton sp. by molecular phylogenetic analysis. After we prepared a less contaminated culture of these strains, culture conditions, namely medium concentrations, temperatures, and photon flux densities, were optimized to enhance the cell concentrations. As a result, the cell concentration of Chromophyton sp. reached 16.7 × 10⁶ cells/ml, which is 10.7 times higher than that before the investigation. The cultured cells did not show a water-surface floating phenotype, and thus, we should identify the trigger(s) to induce this floating phenotype. High-performance liquid chromatography analyses revealed that Chromophyton sp. was rich in fucoxanthin. The fucoxanthin content and productivity were estimated to be 48.7 mg/g of dry cell weight and 2.31 mg/L/day, respectively. This is the highest level among the microalgal species studied so far. Since Chromophyton sp. was observed to be free from a cell wall, this microalga would also be favorable for food and feed applications.