SYNCHRONOUS DIVISION OF AN ENDOSYMBIOTIC METHANOGENIC BACTERIUM IN THE ANAEROBIC CILIATE PLAGIOPYLA-FRONTATA KAHL

The life cycle of a methanogenic bacterium, symbiotic within the marine, free-living anaerobic ciliate Plagiopyla frontata, was studied using light microscopy and transmission electron microscopy (TEM). The bacteria are disc-shaped. During the growth phase of the host, bacteria and hydrogenosomes (organelles which ferment pyruvate into acetate and hydrogen) are arranged in conglomerates resembling stacks of coins in which bacteria and hydrogenosomes alternate; hydrogenosomes always cap the ends of the stacks. During the growth phase, numbers of hydrogenosomes and bacteria remain constant (about 5,000 and 3,500 per cell, respectively). Hydrogenosomes increase in volume shortly after cell division. Methanogens increase in volume slowly during the growth phase of the ciliate and rapidly when the ciliate begins to divide. The hydrogenosomes divide mainly during the initial phases of cell division while the methanogens divide synchronously during the last phase of ciliate division. The timing of reproduction of the symbionts is controlled by the host-cell cycle. The ciliate is known to receive an energetic advantage from its symbionts. The suppression of continuous bacterial reproduction may trigger the secretion of excess bacterial production as soluble organic compounds, for use by the ciliate.