Endosymbiosis-related changes in ultrastructure and chemical composition of Chlorella variabilis (Archaeplastida, Chlorophyta) cell wall in Paramecium bursaria (Ciliophora, Oligohymenophorea)

Rina Higuchi,Chihong Song,Ryo Hoshina,Toshinobu Suzaki

doi:10.1016/j.ejop.2018.09.003

Rina Higuchi, Chihong Song + Show 2 more

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https://doi.org/10.1016/j.ejop.2018.09.003

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Abstract

Chlorella variabilis, a symbiotic alga, is usually present in the cytoplasm of Paramecium bursaria, although it can be cultured in host-free conditions. Morphological and chemical properties of its cell wall were compared between its free-living and symbiotic states. Transmission electron microscopy (quick-freezing and freeze-substitution methods) revealed that the cell wall thickness of symbiotic C. variabilis was reduced to about half that of the free-living one. Chemical properties of the cell wall were examined by treatment with three fluorescent reagents (calcofluor white M2R, FITC-WGA, and FITC-LFA) having specific binding affinities to different polysaccharides. When the algae were re-introduced into Paramecium host cells, calcofluor fluorescence intensity reduced by about 50%. Calcofluor stains β-d-glucopyranose polysaccharides such as cellulose, N-acetylglucosamine, sialic acid, and glycosaminoglycans. Because treatment with cellulase showed no effect on calcofluor fluorescence intensity, we consider that cellulose is not majorly responsible for the stainability of calcofluor. Staining intensities of FITC-WGA and FITC-LFA were similar in the free-living and symbiotic conditions, suggesting that N-acetylglucosamine and sialic acid are also not responsible for the reduction in the stainability of calcofluor associated with intracellular symbiosis. The amount of glycosaminoglycans on the cell wall may decrease in C. variabilis present in the cytoplasm of P. bursaria.

Highlights

Hundreds of Chlorella cells may be present in the cytoplasm of a single Paramecium bursaria
Free-living and symbiotic Chlorella living in P. bursaria were observed using a transmission electron microscope
Chlorella within the host cell is enclosed by a membrane called the peri-algal vacuole (PV) membrane (Karakashian et al 1968; Meier et al 1984)

Summary

Introduction

Hundreds of Chlorella cells may be present in the cytoplasm of a single Paramecium bursaria. The host P. bursaria supplies nitrogen sources (Albers and Wiessner 1985) and CO2 (Reisser 1980) to C. variabilis, and while living inside the host cells, C. variabilis seems to be able to avoid infection by Chlorella virus The host P. bursaria shows an enhanced tolerance to high temperature and UV due to the presence of the symbiotic Chlorella (Iwatsuki et al 1998; Summerer et al.2009). P. bursaria and C. variabilis are considered to be in a mutually beneficial symbiotic relationship.

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