The necessary conditions for achieving vigorous algal growth are: (a) a supply of carbon dioxide and mineral nutrients in adequate concentrations, 1 The survey of literature pertaining to this review was concluded in June, 1956. 309 Annu. Rev. Plant. Physiol. 1957.8:309-334. Downloaded from www.annualreviews.org by University of Sussex on 10/22/12. For personal use only. Quick links to online content Further ANNUAL REVIEWS Annu. Rev. Plant. Physiol. 1957.8:309-334. Downloaded from www.annualreviews.org by University of Sussex on 10/22/12. For personal use only. MASS CULTURE OF ALGAE 311bolism have not been sufficiently investigated in connection with algal cul ture in sewage. Determination of the suitable relative concentration of each nutrient substance is a matter of importance in obtaining active growth of algae. While some species grow only within a narrow range of salt concen trations (52, 53), green algae such as ChIarella and Scenedesmus can grow with the same vigor over a wide range (54), a fact which makes these algae suit able for large scale mass culture. Nevertheless, to obtain steady growth in long range cultures measures must be taken to keep the concentrations of nutrients as well as that of hydrogen ions at suitable levels. The simplest way to achieve this may be to resuspend the algal cells in fresh medium at certain intervals (25). More rational is the replenishment method adopted by Krauss (33,56, 57), who added the elements to the solution at the same rate at which they were removed by the algae (Scenedesmus). The rate of removal of each element from the solution had been made clear by analyses of both nutrient media and algal cells. Different in principle are the steady-state culture techniques devised by Cook (28) and by Myers and co-workers (31, 36). These devices maintain a constant population in a container by means of a photoelectric ceH circuit which activates a solenoid valve allowing the en try of new medium. When the cell concentration increases above a prede termined point, fresh medium is automatically added, and the amount of overflow during a given period can then be used as a measure of growth. In their studies of algal culture in sewage, Gotaas et al. used two continuous culture techniques. One is a simple bubbling culture using a vertical glass cylinder (30, 58, 59). A definite volume of culture, depending upon the "de tention period" to be studied, was removed each day and was replaced by an equal volume of fresh sewage water. The other type of culture, using an apparatus called a "symbiocon" (60, 61), is a closed system in which algae and bacteria are maintained in symbiosis without supply of atmospheric air. Nutrient sewage is continuously added and an equivalent volume of the culture withdrawn at a constant rate. The difficulty of furnishing an adequate supply of microelements to the algae has been largely eliminated by the use of chelating agents which make possible concentrations of the elements higher than can otherwise be tol erated by the algae. Since Myers et al...