Gons, Herman J.
NIOO-Centre for Limnology, Rijksstraatweg 6, 3631 AC Nieuwersluis, The Netherlands.
Experiments aimed at forcing change in bacterial community associated with cyanobacteria. Lake water from the shallow, highly eutrophic Lake Loosdrecht (The Netherlands) was brought into two Laboratory-Scale Enclosures (LSEs). The phytoplankton consisted almost exclusively of filamentous cyanobacteria, mainly ‘Oscillatoria c.f. limnetica’. After one week of adaptation (from winter to ‘summer’ conditions; the cyanobacterial biomass had about doubled), the LSEs received a high light dose during 4 days, while in one LSE also the mixing was interrupted. In both LSEs the cyanobacteria continued to grow well. After another week the high light during 4 days was repeated, and the mixing was interrupted in the other LSE. Again the cyanobacteria continued to grow, but at the end of this high light period lysis of the phytoplankton became apparent in one LSE, and within 24 h also occurred in the other. Within 48 h in both LSEs all species of filamentous cyanobacteria, including Aphanizomenon flos-aquae and Oscillatoria agardhii, had disappeared. Transmission electronmicroscopy of samples at the onset of lysis showed high numbers of at least two different cyanophages, occurring in suspension, attached to filaments and inside ‘ghosts’ of the cyanobacteria. In a new experiment the viral-induced collapse was reproduced. Ecological implications will be discussed.