partners would be great. Anaerobic ciliates, in particular, have acquired diverse endosymbionts that likely provide a variety of metabolic traits and utilities to the host ciliate. Our understanding of many of these relationships, however, remains quite poor. The availability and affordability of molecular sequencing technology over recent years has provided new opportunities to investigate the metabolic basis of these new relationships. Examples of these are giving new insights into how endosymbiosis has been a key player in enabling the oxic– anoxic boundary to be crossed, both over longer evolutionary timescales, as well as multiple transitions per day.
functions in their methane-producing metabolism. This cofactor is named according to the characteristic fluorescence that it emits when illuminated with 420 nm λ light. Thus, methanogen endosymbionts can be readily detected by simple fluorescence microscopy (Figure 1). The reason that anaerobic ciliates provide such a hospitable habitat for methanogens is that these symbionts require molecular hydrogen as an electron donor for their methane-producing energy metabolism. Hydrogen production from the ciliate hydrogenosome is, therefore, an excellent source for the symbiont. Indeed, many methanogen endosymbionts maximise their surface area that is in contact with ciliate hydrogenosomes to facilitate this acquisition of hydrogen. This exchange also benefits the ciliates as continuous hydrogen removal decreases intracellular partial pressure in the ciliate cell, which improves the efficiency and function of their hydrogenosomes. Furthermore, ciliate feeding experiments using 13 C-labelled bacteria demonstrated that Figure 1. Endosymbionts can be identified and distinguished by taxa-specific fluorescent molecular probes targeting taxa-specific regions of rRNA. Here, endosymbiont cells in the anaerobic ciliate Metopus striatus (d, f) are labelled using this method; archaea methanogens in red (b & c) and bacteria in green (a & c). The methanogen endosymbionts also naturally autofluorescence without any labelling when illuminated with 420 nm λ light (e). William Lewis
Anaerobic ciliates with methanogenic endosymbionts
Most anaerobic ciliates have endosymbionts, and most of those endosymbionts are methanogenic Archaea (methanogens). Methanogens possess a cofactor called Cofactor F 420 , which
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85 Microbiology Today October 2022 | microbiologysociety.org
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