CORRELATIONS BETWEEN CHARACTERISTICS OF SOME FREE …

[Pages:10]J. Cell Set. 74, 257-266 (1985)

257

Printed in Great Britain ? Company of Biologists Limited 1985

CORRELATIONS BETWEEN CHARACTERISTICS OF SOME FREE-LIVING CHLORELLA SP. AND THEIR ABILITY TO FORM STABLE SYMBIOSES WITH HYDRA VIRIDIS

M. RAHAT AND V. REICH

Department of Zoology, The Hebrew University ofJerusalem, Jerusalem 91904, Israel

SUMMARY

Aposymbiotic polyps of Hydra viridis were infected with 17 strains of in vitro cultured Chlorella sp. Larvae of Artemia fed with the chlorellae were used as an infecting vector.

Of the 17 strains, seven formed stable symbioses and one formed a transient infection that disappeared within several weeks. Chlorellae of the nine other strains were cleared out of the infected hydra within 2-3 days.

There was a distinct correlation between the ability of the chlorellae to form stable symbioses and their ability to adapt and grow in media enriched with 0-5 % proteose peptone. Only strains that grew in the latter medium formed symbioses with the hydra.

The symbioses formed with the different strains of chlorellae differed from one another. Hydra infected with some strains greened completely while those infected with other strains greened only partially. The degree of infection varied also within each population, and there were differences in the distribution of the various chlorellae along the stalk and inside the digestive cells of the hydra. Growth rates of the infected hydra were all less that those of aposymbiotic hydra or of hydra hosting native zoochlorellae.

We conclude that adaptability to a nutrient-rich environment inside the perialgal vacuole of the digestive cell and a sufficient growth rate therein are crucial to the ability of chlorellae to form stable symbioses with H. viridis. In time, co-adaptation of hydra and chlorellae would restore the normal growth rate of the former and bring about regularity to the form and extent of infection by the latter.

INTRODUCTION

Several strains of free-living Chlorella live also as endocytobionts in intracellular vacuoles of Hydra viridis or Paramecium bursaria (Muscatine, Cook, Pardy & Pool, 1975; Karakashian, 1975). Aposymbiotic specimens of these organisms form stable symbioses if re-infected with suitable chlorellae.

Of the endosymbiotic chlorellae, some are restricted to their intracellular habitat, but others have been cultured in vitro. Chlorella sp. isolated from P. bursaria has been propagated in modified Bristol's medium (Karakashian, 1963) and in a salt medium enriched with vitamins B| and B)2 (Reisser, 1975). Some attempts to culture Chlorella isolated from hydra have failed (Park, Greenblatt, Mattern & Merril, 1967; Muscatine et al. 1975), but one strain isolated from the European green hydra has been cultured in vitro (Jolley & Smith, 1978). This Chlorella was found to have "no

Key words: Chlorella, symbiosis, Hydra.

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M. Rahat and V. Reich

fastidious requirements" and "it grew rapidly on liquid media but very slowly on agar". In a later study these authors (Jolley & Smith, 1980) stated that chlorella strains with a "capacity for mesotrophic growth were in general more acceptable (to hydra) than those lacking these characteristics".

Chlorellae that live inside the vacuoles of their host cells must survive the specific conditions of this extreme habitat (Richmond & Smith, 1979; Smith, 1980), and satisfy there all their nutritional requirements. The required nutrients can obviously come only from or through the host cell.

A comparative study of nutritional requirements of symbiotic and non-symbiotic chlorellae should thus disclose some of the factors that determine whether respective chlorellae can or cannot live as endosymbionts.

In a recent study (Rahat & Reich, 1984), we described an intracellular infection of aposymbiotic H. viridis by a free-living Chlorella that resulted in a stable symbiosis. The chlorellae reproduced rapidly in the intracellular vacuoles of their host, as well as in nutrient-enriched media in vitro. Consequently, we tried to infect aposymbiotic H. viridis with other in vitro-cxAXurtd chlorellae, some of symbiotic and some of non-symbiotic origin. Of these chlorellae, some did not infect our hydra, but others formed stable symbioses with various degrees of infection.

We describe below correlations between some characteristics of 17m w'irw-cultured Chlorella strains, and their ability to form stable symbioses with H. viridis. All experiments reported here refer to populations of hydra infected with respective chlorellae for at least 6 months.

MATERIALS AND METHODS

Stock cultures

The various strains of Chlorella used in our experiments are listed in Table 1. We thank Professor D.C. Smith, F.R.S., Oxford, and the Culture Centre of Algae and Protozoa, Cambridge, for presenting us with these algae.

A Swiss strain of symbiotic//, viridis (Ssh) and Swiss aposymbiotic hydra (Sah) were used in all experiments. Some experiments were also done with symbiotic and aposymbiotic hydra of an Israeli strain (Rahat & Reich, 1980).

Growth conditions

Stock cultures of Chlorella were grown in BBMGPLY (see Table 1), inoculated at a ratio of 1:5, v/v (i.e.' 1 ml of inoculum into 5 ml of fresh medium), in standard 16mm X 150mm test tubes placed on a Tissue Culture Rollerdrum, at 5 rev./min. Chlorellae examined for their ability to grow in diverse media were inoculated with a bacterial loop into 5 ml of fresh medium ( ................
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