Some rumen ciliates have endosymbiotic methanogens

Area of Science:

• Microbiology
• Rumen microbiology
• Symbiotic relationships

Background:

• Rumen ciliates host various microorganisms, including bacteria.
• The precise nature and abundance of intracellular bacteria in ciliates are not fully understood.
• Methanogenic archaea are key players in rumen function and methane production.

Purpose of the Study:

• To investigate the identity and prevalence of intracellular bacteria within rumen ciliates.
• To determine if these intracellular bacteria are methanogens and their role in the ciliate symbiosis.
• To compare the abundance of intracellular versus extracellular methanogens in rumen ciliates.

Main Methods:

• Transmission electron microscopy (TEM) stereology for estimating bacterial volume.
• Coenzyme F420 autofluorescence microscopy to detect potential methanogens.
• Archaea-specific oligonucleotide probe hybridization for bacterial identification.

Main Results:

• Intracellular bacteria were consistently found within the cytoplasm of Dasytricha ruminantium and Entodinium spp. ciliates.
• These bacteria exhibited coenzyme F420 autofluorescence and were identified as Archaea.
• The volume of autofluorescing bacteria correlated with TEM estimates, suggesting they are endosymbiotic methanogens.

Conclusions:

• Rumen ciliates harbor endosymbiotic methanogens in their cytoplasm, not in digestive vacuoles.
• These intracellular methanogens likely utilize hydrogen from the host ciliate for methane production.
• Intracellular methanogens are significantly more abundant than extracellular ones, highlighting their importance in the ciliate microbiome.