Methanosaeta sp., the major archaeal endosymbiont of Metopus es

Epifluorescence microscopy and whole cell in situ hybridization analysis revealed the presence of Methanosaeta sp. as endosymbionts in Metopus es. Direct microscopic observation under epifluorescent microscope showed the presence of long slender rods with an average length of 3.4 μm. The number of methanogenic rods varied from 792 ± 12 in a single M. es cell with a biovolume of 3.4 × 105 μm3. At the exponential growth stage, a single symbiotic methanogen in M. es produced about 1 fmol methane/h leading to a methane production rate of 0.85 pmol/ciliate/h. The presence of endosymbiotic methanogens in the domain archaea and Methanosaeta sp. was confirmed by FISH with ARCH 915 and MX 825 oligonucleotide probes specific to domain archaea and Methanosaeta respectively. The homogenized cells of M. es also showed bright fluorescing rods with MX 825 hybridization. The culture obtained on inoculation of the released endosymbiotic organisms on Methanosaeta-specific medium lent support to the growth of long slender rods having the same range of mean length (3.6 μm) as that of the endosymbiotic methanogens observed. Both intra- and extracellular production of acetate was detected in M. es culture.