Patterns of cell death in freshwater colonial cyanobacteria during the late summer bloom

The occurrence of senescence (Evans blue staining) and programmed cell death (Hoechst staining/TUNEL reaction) was studied in the colonial cyanobacteria Anabaena flos-aquae and Microcystis flos-aquae during the late summer bloom in a eutrophic lake. Algae were analysed over a seven-day period (three sampling days) within mixed phytoplankton samples obtained from the upper epilimnion (trawl-net sample) and lower epilimnion (sediment-trap sample). Death of Anabaena in the trawl-net population was attributed to widespread infection by a chytrid fungus, resulting in breakdown and positive Evans blue staining of most filamentous colonies. Associated akinetes were a mixture of senescent cells (30% of total, Evans blue stained) and viable cells (unstained) plus a few dead cells (no contents). Sedimentation resulted in loss of filamentous colonies, leaving groups of akinetes with about 30% nonviable cells (senescent and dead). Over the sampling period, 20-50% of Microcystis colonies in the trawl-net samples showed some cells with clear Evans blue staining, indicating widespread senescence in the algal population. The occurrence of programmed cell death in some colonies of Microcystis was indicated by a positive TUNEL reaction and by condensed regions of nucleoid DNA in Hoechst-stained material. The results suggest that senescence was a general response to adverse environmental conditions (no fungal infection), with at least 30% (minimum count) of cells in affected colonies switching to programmed cell death. The induction of cell death within individual cells occurred randomly throughout the colony (nearest-neighbour comparison to random distribution) and was unrelated to the cell cycle - since both stained and unstained cells included the full range of cell size, from dividing cells to large nondividing cells. Very few colonies of Microcystis were observed in the sediment-trap samples, suggesting that senescence and cell death in the planktonic population of this alga were preceding the major phase of sedimentation.