Quantitative assessment of the major limitations on productivity ofSpirulina platensis in open raceways

Abstract

This work represents an attempt to assess the relative contribution of the factors limiting productivity ofSpirulina platensis in open raceways throughout the year. Temperature of the culture during daylight exerted the predominant effect on productivity and elevating the temperature resulted in a significant rise in productivity even in summer. Photoinhibition had a decisive role in summer in determining productivity ofSpirulina in open raceways in that growth almost ceased after mid-day. Contamination by other microorganisms, particularlyS. minor andChlorella sp. was estimated to reduce the net biomass yield by at least 15 to 20%, but measures to curtail the establishment of these species in the raceway have been devised. The effect of harvest time during the day on the yield of dry mass was examined: no conclusion could be drawn except in mid-summer, when evening harvesting resulted in a significant increase in the output rate of dry weight over morning harvesting.

It was estimated that in a subtropical climate with little cloudiness, it should be readily possible to obtain an annual output rate of dry mass of ca. 60 to 70 t ha⁻¹. Such output, however, which would reduce very substantially the cost of production to-date, is possible only if the optimal temperature forSpirulina can be maintained throughout daylight, photoinhibition essentially controlled, harvesting in summer performed in the evening, and night-loss of dry mass as well as the extent of contamination by other cyanobacteria or microalgae can be kept low. The pronounced daily fluctuations in the output rate at peak productivity in summer suggest that when environmental limitations of growth are minimal, other limitations become dominant. These should be identified to facilitate an even greater increase in the productivity ofSpirulina in outdoor cultures.