Control of Toxic Marine Dinoflagellate Blooms by Serial Parasitic Killers

The marine dinoflagellates commonly responsible for toxic red tides are parasitized by other dinoflagellate species. Using culture- independent environmental ribosomal RNA sequences and fluorescence markers, we identified host- specific infections among several species. Each parasitoid produces 60 to 400 offspring, leading to extraordinarily rapid control of the host's population. During 3 consecutive years of observation in a natural estuary, all dinoflagellates observed were chronically infected, and a given host species was infected by a single genetically distinct parasite year after year. Our observations in natural ecosystems suggest that although bloom- forming dinoflagellates may escape control by grazing organisms, they eventually succumb to parasite attack.