Ultrastructure and Ecology of <i>Perispira ovum</i> (Ciliophora: Litostomatea): An Aerobic, Planktonic Ciliate that Sequesters the Chloroplasts, Mitochondria, and Paramylon of <i>Euglena proxima</i> in a Micro‐oxic Habitat

Abstract

ABSTRACT. High resolution sampling of the stratified water column in a fjord‐like ecosystem revealed a green‐pigmented planktonic ciliate that was found to be a ravenous predator of Euglena proxima. The vertical distributions of both predator and prey were coincident, and maximum populations occurred across the transition from oxic to anoxic water. This ciliate was identified as Perispira ovum (family Spathidiidae; Order Haptorida). P. ovum was observed by transmission electron microscopy to retain not only the chloroplasts, but also the mitochondria and paramylon reserve of its algal prey. A mechanism of sequestration of algal organelles is demonstrated for the first time. This mechanism includes: recognition, capture, and ingestion of prey; rupture and release of algal cell contents; and enrobing of individual organelles and paramylon by the host vacuolar membrane. The structural integrity, peripheral location, and association with host endoplasmic reticulum suggests the sequestered organelles may be functional within P. ovum. The occurrence and high biomass of this aerobic ciliate in an oxygen‐limited environment also suggests that the sequestered chloroplasts are photosynthetically active and may provide additional substrates (such as oxygen) and metabolic capabilities that are crucial for its survival.