Purine‐Excretory Nature of Refractile Bodies in the Marine Ciliate Parauronema acutum*

Abstract

SYNOPSIS. A method was developed for the isolation and purification of crystalline, highly refractile bodies found in the cytoplasm of a symbiote‐free strain of the marine hymenostome ciliate, Parauronema acutum, strain 110–3. Chemical analysis of the purified refractile bodies revealed an abundance of the purines, hypoxanthine and guanine. It was evident from studies involving the use of 14C‐labeled precursors that both hypoxanthine and guanine are derived from higher purine derivatives. We postulate that these bodies are excretory in function and that guanine and hypoxanthine are major endproducts of purine metabolism of P. acutum.

Proposal for an Abridged Nitrogen Turnover Cycle in Certain Marine Planktonic Systems Involving Hypoxanthine-Guanine Excretion by Ciliates and their Reutilization by Phytoplankton

“…Since then, an important investigation on the excretory produds of marine ciliates has unwittingly answered this question. Using axenic cultures of a small hymenostome ciliate, Soldo et al (1978) isolated cytoplasmic crystalline bodies composed principally of hypoxanthine and guanine in the ratio 1 : 4 , and these bodies were inferred from other evidence to be excretory in function. Since similar cytoplasmic inclusions have been observed in other marine ciliates, these investigators believe that hypoxanthine-guanine excretion may be a common metabolic feature of ciliates.…”

“…Strictly speaking, the ciliate-excreted purinoid bodies should be considered metabolically equivalent to the zooplankter fecal pellets (see recent review by Turner and Ferrante, 1979), but their microscopic size (ca 5 X 2 pm, according to Soldo et al, 1978) and solubility (see below) probably qualifies them in a category intermediate between excreted dissolved organic matter and the fecal pellets; this size qualification entails the ecological implication that they are probably too small for degradation from the bacterial surface adhesion commonly assumed for the fecal pellets. Soldo et al (1978) report the ciliate purinoidexcretory bodies to be insoluble in water, but it is not clear whether they are talking of immediate solubility or whether the purines in the bodies are in a peculiar physical or chemical state.…”

Interactions of bactivorous grazers and heterotrophic bacteria with dissolved organic matter

“…Heterotrophic microflagellates (Pro-tozoa-Mastigophora) i n culture and at a coastal station i n the Southern California Bight have been shown to enhance the production of dissolved free amino acids (Andersson et al i n press). In axenic laboratory cultures, Soldo et al (1978) (Stanier et al 1976, Geesey 1982 (Fig. ?a) (Fig.…”

“…Hence the role of products released by bactivores in bacterial metabolism cannot be adequately assessed by gross quantitative measurements. Direct measurement of grazer carbon exudation, although highly desirable, is only possible with axenically grown or physically separated grazers as reported in Soldo et al (1978). Such treatment, however, does not permit investigation of phagotrophic organisms under relevant ecological and physiological conditions.…”

Purine and pyrimidine metabolism by estuarine bacteria

“…These metabolic differences may be related to the availability of the different purines in the habitat of the microorganism. For example, guanine and hypoxanthine are produced as excretion products by a marine ciliate (Soldo et al 1978) and guanine is a major component of fish skin and scales (Nicol 1960, Staley & Ewing 1992. The prevalence of guanine and hypoxanthine excretion by ciliates and other zooplankton in marine environments is not known, but the high lability of guanine and hypoxanthine to marine bacteria may well reflect its rate of production by higher trophic levels and its availability in the water column.…”