Cyanobacteria‐cyanophage Interactions in Continuous Culture

When the filamentous Plectonema boryanum 594 or the unicellular Aphanothece stagnina were grown in chemostats with phages LPP‐DUN1 and Aph‐1, respectively, an initial series of reciprocal oscillations in cyanobacterial and phage numbers occurred; their magnitudes decreased successively until an equilibrium population of cyanobacteria and virus was established in each system. Evolution of the protagonists occurred as the interactions proceeded. Most of the A. stagnina population appeared resistant to Aph‐1 at the end of the experiment. Two types of resistant P. boryanum , designated PR1 and PR2, were isolated. PR1 was resistant to wild‐type LPP‐DUN1 but susceptible to a mutant phage; PR2 was unaffected by the phage. The sequence of appearance and interaction of mutant phage and P. boryanum accounted for the initial fluctuations in population numbers, and the coexistence of host and pathogen in the climax population. Plectonema boryanum PR2 grew more slowly than wild‐type, or PR1 and competed poorly with them in the absence of phage. The ecological implications of the results are discussed.