The phagotrophic growth of algae on bacteria and its potential for wastewater and waste sludge treatment

Algae exhibit diverse growth strategies, including phototrophy, osmotrophy, and phagotrophy. While phototrophic and osmotrophic growths have been extensively studied, phagotrophic growth remains relatively unexplored. This research delves into the phagotrophic growth of Ochromonas danica on bacteria, evaluating its potential for wastewater and waste sludge treatment. The study reveals that O. danica was able to grow on bacteria without light or additional nutrients, achieving a doubling time of 3.5–3.9 hours and converting 41–45 % of bacterial organic matter into algal biomass. The resultant O. danica cells were lipid-rich, containing 35–46 % lipids by dry weight. The efficiency of O. danica in treating waste sludge was highlighted, achieving a 43 % reduction in organic matter within 36 hours, outperforming conventional aerobic digestion. The study also highlights the potential of O. danica in wastewater treatment. An approach was developed to reclaim organic matter from wastewater through a two-stage process, in which bacteria were first grown on wastewater organic matter and then the grown bacteria were fed to O. danica for growth. Results show that a total of 78.2 % of the initial wastewater organic matter was removed through this approach and 27.3 % of the removed organic matter was converted into lipid-rich algal biomass. The findings underscore the potential of phagotrophic growth for waste treatment and lipid production. The simplicity of the phagotrophic process, independent of light or complex nutrient supplementation, positions it as a promising strategy for industrial applications in waste sludge and wastewater treatment.