Viability of the cyanobacterium Planktothrix rubescens in the cold and dark, related to over-winter survival and summer recruitment in Lake Zürich

Abstract

Over the winter many phytoplankton in deep lakes encounter prolonged periods of dark and cold. Survival times of the planktonic cyanobacterium Planktothrix rubescens were assessed by isolating single filaments of six strains in separate tubes that were stored at 4–5° C in the dark for 2–16 weeks and then incubating them at 20° C in the light. Viable filaments grew to produce cultures, usually within 3–5 weeks. Filaments survived for longer in isolation than they did in samples of cultures containing many filaments. After 8 weeks in the cold and dark, five of the Planktothrix strains had viable filaments, with 14–67% of filaments sampled surviving. In the most long-lived of these, Planktothrix strain 9736, 50% of the isolated filaments survived for 10 weeks and the maximum survival time exceeded 11 weeks. Combining this information with information on filament floating velocities, it is calculated that filaments would float up 25 m before half of them lost viability. Such calculations can be used in estimating the proportion of the lake population that can be recruited to the spring and summer populations in the metalimnion.