MIXOTROPHIC GROWTH OF THE ALGA CHLORELLA PYRENOIDOSA IN A CLOSED CULTURE SYSTEM

Abstract

The alga Chlorella pyrenoidosa was cultured in a closed stirred tank fermenter, where gas exchange between the culture and the external environment was not permitted. The culture media contained 0.3M, 0.1M or 0.01M glucose. The average light intensity measured at the inner surface of the reactor vessel was 220 and 40 ?mol/m2/s.

It was observed that the specific growth rate (µ) of the cultures increased rapidly in the first 6 hours, reaching a maximum value of 0.16 h-1 and 0 . 1 33h-1 for cultures exposed to 220 and 40 µmo l/ m2 / s light intensity respectively. The µ then decreased with time and the degree of decrease was determined by the glucose concentration. However, in 40 µmol / m2/s light, the kinetic changes in µ of the cultures containing 0.3 and 0.1 M glucose coincided with each other, indicating that growth was limited by light. The dissolved oxygen partial pressure (P 0 2) in the culture increased initially and then decreased before an equilibrated stage was reached. During this prolonged period of equilibrium, the P02 remained unchanged. The changes in the dissolved carbon dioxide partial pressure (Pco2) were cyclical inverted mirror images of P02. After a short lag, the specific uptake rate of glucose (qg) increased rapidly in the first 10 hours and then decreased subsequently in all the culture conditions studied. The uptake of glucose, however, was not inhibited by light. Instead, the higher light intensity enhanced qg, at high glucose concentration. It appeared that the regulatory mechanism for the uptake of glucose in Chlorella was a strain dependent system. The photosynthetic activity of the alga was not inhibited by glucose. Photosynthesis in this case served to recycle the carbon released from respiration and provide oxygen for the respiration process. It was possible to maintain an actively growing, high density algal culture in a closed mixotrophic system. The study suggested that the alga Chlorella could be cultured in the mixotrophic mode without an external supply of carbon dioxide and oxygen.