"Surface Browsing" May Allow "Filter-Feeding" Protozoa to Exert Top-Down Control on Colony-Forming Toxic Cyanobacterial Blooms.

Blooms of the cyanobacterium threaten aquatic ecosystems. Protozoa grazing can control unicellular populations; however, blooms are composed of multicellular colonies that are thought to prevent grazing. We show that this is not so: the model ciliate has an impact on populations through grazing, even when large colonies occur, and this leads to a corresponding decrease in toxic microcystins. Notably, as the number of large colonies increased, exerted top-down control by altering its feeding behavior: once the colony size was >12-20 μm, no longer acted as a "filter feeder"; instead, it became a "surface browser," grazing around and between larger colonies, removing individual and small colonies. However, as the proportion of large colonies increased, exponentially reducing the surface area to volume ratio, the impact of decreased exponentially. This study provides new insights into how protozoa may affect populations through top-down control of blooms.