Isolation and evaluation of a novel strain of Chlorella sorokiniana that resists grazing by the predator Poterioochromonas malhamensis

The protozoan flagellate Poterioochromonas malhamensis is one of the main contaminants in Chlorella culture; however, few effective and affordable methods have been developed to control it. Unexpectedly, one strain of an unknown green alga was observed to be capable of contaminating P. malhamensis culture and defending itself against the protozoan. Based on cell morphology and molecular information, the green microalga was identified as Chlorella sorokiniana CMBB-146. Feeding experiments showed that the addition of the predator P. malhamensis not only had no negative effect on the growth of C. sorokiniana CMBB-146, but also stimulated an increase in the biomass of C. sorokiniana CMBB-146. The grazing resistance of C. sorokiniana CMBB-146 against P. malhamensis was also verified on a pilot scale with 100-L raceway ponds. Further experiments revealed that P. malhamensis showed a strong ability in ingesting C. sorokiniana CMBB-146 cells, but the ingested cells were hard to digest. Through comparison with other species of Chlorella without grazing resistance, we found that the differences on cell size, cell morphology, and biochemical composition had no relation with the ability of C. sorokiniana CMBB-146 to resist grazing by P. malhamensis. Finally, it is speculated that variation in cell wall composition, especially the content of galactosamine, empowers C. sorokiniana CMBB-146 to resist P. malhamensis. Therefore, our study confirms that C. sorokiniana CMBB-146 is a strong candidate as a potential production organism for outdoor massive cultivation, and as a potential microalgal strain for studying how microalgae resist grazing by protozoa.