ULTRASTRUCTURE AND ECOLOGY OF PERISPIRA-OVUM (CILIOPHORA, LITOSTOMATEA) - AN AEROBIC, PLANKTONIC CILIATE THAT SEQUESTERS THE CHLOROPLASTS, MITOCHONDRIA, AND PARAMYLON OF EUGLENA-PROXIMA IN A MICRO-OXIC HABITAT

High resolution sampling of the stratified water column in a fjord-lik e ecosystem revealed a green-pigmented planktonic ciliate that was fou nd to be a ravenous predator of Euglena proxima. The vertical distribu tions of both predator and prey were coincident, and maximum populatio ns occurred across the transition from oxic to anoxic water. This cili ate was identified as Perispira ovum (family Spathidiidae; Order Hapto rida). P. ovum was observed by transmission electron microscopy to ret ain not only the chromoplasts, but also the mitochondria and paramylon reserve of its algal prey. A mechanism of sequestration of algal orga nelles is demonstrated for the first time. This mechanism includes: re cognition, capture, and ingestion of prey; rupture and release of alga l cell contents; and enrobing of individual organelles and paramylon b y the host vacuolar membrane. The structural integrity, peripheral loc ation, and association with host endoplasmic reticulum suggests the se questered organelles may be functional within P. ovum. The occurrence and high biomass of this aerobic ciliate in an oxygen-limited environm ent also suggests that the sequestered chloroplasts are photosynthetic ally active and may provide additional substrates (such as oxygen) and metabolic capabilities that are crucial for its survival.