Microalgae are promising sustainable feedstock for the production of biofuels, food, feed, chemicals, nutraceuticals and pharmaceuticals. Green marine microalga Picochlorum displays a high growth rate, broad salinity tolerance and high-temperature tolerance, making this genus suitable for outdoor cultivation. One of the major bottlenecks in the commercialization of microalgal biomass production is energy-intensive harvesting methods. Bio-flocculation is an efficient, low-cost method for harvesting microalgal biomass. Conventional chemical flocculants, commonly used to improve the harvesting efficiency, contaminate growth medium and make biomass unsuitable for food and feed production. Bio-flocculation uses microorganisms as bio-flocculants, especially fungi, which could also improve the quality of isolated biomass.
To investigate the growth productivity, Picochlorum sp. was cultivated phototrophically in a 35 L bubble column bioreactor. Cultivation was conducted in two phases using f/2 medium without pH and temperature control. During the first cultivation phase, a high growth rate was maintained by feeding the culture with nitrogen and phosphate sources. When cells entered the stationary phase on the 16th day of cultivation, nitrogen and phosphate sources were depleted. In the second phase of cultivation, the culture was fed with an inorganic carbon source (1 g L-1), sodium bicarbonate. The culture was also fed on the 21st and 23rd days with 1 and 0.5 g L-1 of sodium bicarbonate, respectively. Maximal cell number and dry cell weight concentrations were 3.46×108 cells per millilitre and 1.68 g L-1, respectively. Biomass productivity was 0.0646 g L-1 d -1. Cell biomass was analyzed for macromolecular composition (carbohydrates, lipids and proteins), fatty acid composition and chlorophyll content. Proteins were the most abundant macromolecule in cell biomass, followed by carbohydrates as the main source of reserve energy with a maximal 37.7 % in dry cell weight. Picochlorum sp. had a low capacity for lipid accumulation; at the end of cultivation, the lipid content was only 8 % in dry cell weight. The most dominant fatty acid was linoleic (C18:2), followed by heptadecanoic (C17:0) and palmitic (C16:0). These fatty acids comprised more than 70 % of all fatty acids. During the second cultivation phase, cells also accumulated a significant amount of linolenic (C18:3) acid.
Microalgal biomass was harvested by bio-flocculation under optimized conditions using pellets of fungus Mortierella isabellina. This fungus is known for its ability to accumulate a significant quantity of lipids abundant in unsaturated fatty acids. The recovery efficiency of microalgal biomass was more than 80 % (g g-1) after 150 min. The developed bio-flocculation system is a suitable method for chemical-free harvesting of microalgae that could be used as feedstock for feed or food production.