Most of the information that is used to grow Spirulina in outdoor culture is derived from observations made on natural blooms of these algae in natural lakes. Of special significance is their adaptation to highly alkaline (up to 400 meq/l) and very high pH (up to 11) of tropical and subtropical regions of the world. This harsh chemical environment essentially prohibits the growth of other algae. It is therefore not uncommon to find almost unialgal populations of Arthrospira in these lakes. The crater lakes around the Great African Rift Valley are good examples of these. These lakes support high-standing crops of Spirulina (up to 2.0 g Chl a l-1) and support huge populations of flamingoes (Figure 1.1). It is estimated that adult and juvenile flamingoes consume on the average about 66 g of Spirulina per day on a dry weight basis. This means that the whole flamingo population, about a million individuals at the time, extracted 50-94% of the daily primary production or 0.4-0.6% of the algal biomass.21
The productivity of these natural ecosystems has not been studied to any great extent. However, there have been some studies looking into photosynthetic productivity22 (Belay, unpublished). On the basis of these studies some of these soda lakes are among the most productive natural systems. The photosynthetic productivity reported for these lakes is the highest ever recorded for any natural systems and are comparable to those observed in some waste treatment ponds or mass culture
facilities. The values of 43-57 g m-2 d-1 recorded by Talling for Lake Arenguadie in Ethiopia are among the highest ever recorded for natural systems. Comparable values have also been recorded recently for this same lake (Belay, unpublished). The high productivity of these soda lakes is a result of (a) high algal contents in the euphotic zone, (b) high photosynthetic capacity favored by high temperature, and (c) a surplus of dissolved inorganic phosphate and an especially huge reserve of CO2. Maximal photosynthetic rates (mg C l-1 h-1) and photosynthetic efficiency (mg C [mg Chl a]-1 h-1) obtained at light saturation in a continuous culture replete with nutrients were comparable to those obtained in these highly productive lakes.23 Aerial biomass concentrations (mg Chl a m-2) for Lakes Arenguadie and Kilole have been found to be close to the maximum possible on theoretical grounds (180-300 mg Chl am-2).22
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