Although it was long concerned that the contamination in mass microalgal cultivation by various organisms can drastically reduce the overall productivity of algal cultivation, there are few numbers of systematic studies on how to identify and manage contaminating organisms in algal cultures.

A Chinese research group led by Prof. HU Qiang and Dr. GONG Yingchun from Institute of Hydrobiology (IHB) of Chinese Academy of Sciences reported, for the first time, a mixotrophic flagellate grazer from mass culture of Chlorella, and conducted systematic studies on how to control this predatory protozoan.

In this work, the team firstly reported that a mixotrophic flagellate Poterioochromonas malhamensis is the most harmful grazer in mass cultivation of Chlorella based on a long-time tracking survey, which often caused the cultures crash within several days throughout the year.

They did further work to explore the impact of environmental factors on the growth of the flagellate, including temperature, light intensity, pH value, and CO₂ concentration. The results showed that a low medium pH maintained by high concentration of CO₂ is effective to reduce the intracellular pH of flagellate, thus destroying the flagellate. Till now, the CO₂ method has widely applied to the control of this flagellate in the field cultivation of Chlorella.

The team further studied the interaction between Chlorella and the predatory flagellate. They found that the flagellate almost graze on all microalgae with smaller size than itself, and the cell wall thickness and growth rate of the microalgae are the two main factors that impact the feeding behavior of the flagellate.

Moreover, the team isolated one Chlorella strain with grazing-resistance against flagellate. Their preliminary studies showed that the resistance was formed by the cell wall lack some certain structure consisting of galactosamine.

The above findings are very important to understand the impact of the predatory protozoan on the growth of microalgae and develop solutions to control the predatory zooplankton in microalgae culture, and also provide evidential insight into the understanding of the interactions between protozoan and phytoplankton, as well as basic and constructive data to help in screening for grazing-resistant microalgae.

This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences, and SDIC Biotech Investment Co., Ltd., the State Development and Investment Corporation, China.

 
Process of ingestion and digestion of mixotrophic flagellate Poterioochromonas malhamensis on Chlorella.(Image by IHB)