Seventy axenic microbial cultures were isolated from aquatic habitats, soil and air, according to their ability to lyse live blue-green algae. These isolates comprised 62 fungi, representing the genera, Acremonium, Emericellopsis and Verticillium, four bacteria of the genera Flexibacter and Pseudomonas, and four Streptomyces spp.. In addition to these isolates, amoebae, identified as Acanthamoeba castellanii, were found predating algae in many of the initial plaques formed on Anabaena flos-aquae lawns. All isolates lysed A. flos-aquae, and in most cases, several other filamentous and unicellular blue-green algae also. The effects of varied experimental conditions on algal lysis by selected isolates were examined, as was the lytic mechanism involved. In general, the blue-green algae tested were most susceptible to microbial lysis when incubated at 25 to 30°C. Algal lysis also increased when the initial pH of the growth medium was raised. The fungi generally showed greater lytic activity than most of the other isolates towards a wider range of susceptible algae. Many of the fungal isolates retained their lytic activity after preservation under liquid nitrogen and freeze-drying. Emericellopsis and Acremonium isolates also inhibited the growth of blue-green algae and Gram positive bacteria, but did not lyse the latter. The exudates from these fungi also inhibited the Flexibacter flexilis isolate and Ac. castellanii. Acremonium and Emericellopsis spp. produced cephalosporin C and evidence is presented to suggest that the extracellular lytic and inhibitory activities of these fungi are due to the release of this ?-lactam antibiotic, Extracellular lytic activity was not detected with any of the other isolates, including the Verticillium fungi. The frequent isolation of predacious amoebae and lytic fungi from algal habitats suggests that these organisms may play an important algal-destroying role in natural ecosystems.
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