Algal culture studies related to a closed ecological life support system

Long-term cultures of Scenedesmus obliquus were maintained in an annular air-lift column operated as a turbidostat. We observed a linear relationship between the dry weight of the cultured cells, their cell number, and their chlorophyll content over a broad range of cell density at constant illumination. Thus, the cells did not appear to be adapting to differences in growth rate or light intensity during these experiments. Productivity vs dry wt rose linearly until the cell density reached a level at which light became limiting; at this point approximately 89% of the photosynthetically active radiation (PAR) was being absorbed. The maximum dilution rate of the system corresponded to a doubling time of 13.8 hr, about half the maximum growth rate generally observed at this temperature. Productivity at the maximum was approximately 80% of the maximum theoretical productivity. The rather low incident intensities (approximately 10% of full sunlight) were a main contributing factor to the high light utilization efficiencies obtained in this system, since the cells were never driven into light saturation. In many respects, algae would be ideal plant components for a biologically-based closed life support system, since they are eminently suited to the closely coupled functions of food production and atmosphere regeneration. In this communication, we report some findings on the (steady-state) continuous culture of Scenedesmus obliquus, a physiologically well-characterized green alga with good growth characteristics.