Critical assessment of the filamentous green microalga Oedocladium carolinianum for astaxanthin and oil production

The filamentous microalga Oedocladium carolinianum that was isolated from soil was found to be capable of producing large amounts of astaxanthin under stress culture conditions. When the culture conditions were optimized, the maximum specific growth rate of 0.37 d−1 and the maximum biomass concentration of 10.12 g L−1 were obtained after 18 days of cultivation in 1-liter glass columns (inner diameter: 5 cm) under laboratory conditions. When subjected to culture conditions of nitrogen starvation and 2 g L−1 NaCl-induced salinity stress, however, the cells produced up to 3.91% w/w astaxanthin and a high astaxanthin productivity of 24.2 mg L−1 d−1 was obtained. Analysis by HPLC-MS revealed that the majority of astaxanthin was in fatty acid esterified forms with a typical molecular ratio of free, monoester and diester astaxanthin of 1:18:81. The biosynthesis of astaxanthin coincided with that of fatty acids, and the total fatty acid content reached 40% w/w or more. The technical feasibility of mass culture of O. carolinianum was tested in a 7000-L inclined thin-layer photobioreactor and a 10,000-L tubular photobioreactor in a greenhouse. This demonstrated that O. carolinianum grew rapidly in a nitrogen replete BG-11 culture medium and the maximum biomass concentrations obtained in the inclined thin-layer photobioreactor and tubular photobioreactor were 3.74 g L−1 and 3.07 g L−1 respectively, resulting in maximum biomass productivities of 276 mg L−1 d−1 and 198 mg L−1 d−1, respectively. Although small populations of a few zooplankton species occurred in the two types of photobioreactors, none grazed on O. carolinianum and they grazed on invading unicellular microalgae instead. It was therefore concluded that O. carolinianum is a promising microalga for sustainable co-production of astaxanthin and fatty acids.