Grazing by Microfauna and Productivity of Heterocystous Nitrogen-Fixing Blue-Green Algae in Desert Soils

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Abstract
The presence of microfauna in cultures of heterocystous N2-fixing blue-green algae (Anabaena and Nostoc spp.) resulted in reduction of chlorophyll a content from 20.4 to 6.2 mg/m2 and of organic C from 203.3 to 136.0 mg/100 g dry soil. Total N content (fixed by the algae) was virtually unchanged (23.0 against 23.1 mg/100 g dry soil), and also NH4-N (1.6 against 1.8/100 g dry soil). These values are the amounts gained in the soil after 1 mo. of incubating the organisms in sterilized desert soil from which they were originally isolated selectively. Soils were from 2 desert sites west of Alexandria, Egypt. Incubation was under continuous 5000 lx, at 30 and 37.degree. C, and at 75, 100 and 150% W.H.C. The gain in total N and NH4-N is superior to those of the treatments: microfauna added to the algal culture 10 days after start and microfauna added to soil without algae. Since it is known that Protozoa (which were predominant) do not affect heterocysts. Protozoa may stimulate nitrogen fixation, while reducing chlorophyll a and carbon fixation by grazing on the vegetative cells.

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