@sfmatheson, great link!
Here’s a worthy quote for others who haven’t had a chance to read the article yet:
" It has been estimated that about 5 per cent of all algal cells in natural populations are likely to contain endosymbiotic bacteria (Surek & Melkonian 1983). Countless free-living amoebae and ciliates house bacterial endosymbionts (Fokin 2004; Schmitz-Esser et al. 2008)."
" Amoebal hosts have lately attracted considerable scientific interest as they are hypothesized to represent
(i) ‘training grounds’ for pathogens, adapting bacteria to intracellular environments (Molmeret et al. 2005), "
(ii) genetic ‘melting pots’ promoting cross-species conjugation as a result of the co-occurrence of different intracellular bacteria in amoebae (Ogata et al. 2006), and (iii) reservoirs for pathogens enabling their survival outside their specific host species (Greub & Raoult 2004; Horn 2008)."
“Unfortunately, for the majority of the endosymbiotic bacteria reported in protists, still not much more than a morphological description is available, precluding any conclusions about their physiological role as well as a clear recognition of the bacteria as endosymbionts, pathogens or prey.”