The Red-Water Ciliate Mesodinium rubrum and its "Incomplete Symbionts": A Review Including New Ultrastructural Observations

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Abstract
The authors provide the first ultrastructural description of the marine holotrich ciliate Mesodinium rubrum and of its unusual "incomplete symbionts," consisting of numerous functional chloroplasts associated with distinctive nonciliate mitochondria but apparently lacking associated nuclei. The pigment characteristics suggest a cryptomonad origin for the chloroplasts but they are traversed by lamellae that are either ungrouped or in groups of three's, this being contrary to ultrastructural observations on cryptomonads to date. Unusual ultrastructural features of the ciliate include the lack of alveoli in the very thin pellicle and two types of ciliary differentiation: the presence of lateral packing material in some of the cilia grouped to form cirri, and narrow distal portions from which the nine peripheral microtubule duplets are excluded. The cytostome is greatly reduced and the ciliate usually swims backwards. The problem as to the degree of permanence reflected by these features, including whether the symbiotic state is obligate or not, was not resolved. The authors discuss the evidence bearing on this question.The paper also reviews the ecology and taxonomy of the ciliate. Fifty instances of its occurrence, often in conjunction with nontoxic "red water," are summarised. The species has an extremely wide geographic distribution, including both polar and equatorial waters. Red water caused by the species occurs either within extreme coastal localities (bays, inlets) or in the vicinity of offshore upwelling. Field studies demonstrated that the ciliate–chloroplast consortium is a functional autotroph capable of playing the role of primary producer.Taxonomic difficulties are summarised. Previous designations of the species to the genus Cyclotrichium are considered inappropriate. Detailed study of widely spread populations is needed to determine the degree of taxonomic confusion involved.

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