Ultrastructural observations on prokaryotic associates of benthic foraminifera: Food, mutualistic symbionts, or parasites?

Because prokaryotes (Eubacteria, Archaea) are ubiquitous in the marine realm, it may not be surprising that they are important to the diet of at least some foraminifera. Over recent decades, Transmission Electron Microscopy (TEM) has revealed that, at the ultrastructural level, additional intimate relationships exist between prokaryotes and foraminifera. For example, the cytoplasm of a variety of benthic foraminiferal species contains intact prokaryotes. Other benthic foraminiferal species support prokaryotic populations on their exterior. Some of these prokaryote-foraminifera associations are sufficiently consistent to be considered symbioses. Symbiotic relationships include beneficial associations (mutualism; commensalism) to detrimental associations (parasitism). Here, we provide a synopsis of known foraminiferal-prokaryotic symbioses and TEM micrographs illustrating many specific associations. We further comment on and illustrate additional interactions such as bacterial scavenging on foraminifera and foraminiferal feeding on prokaryotes. Documenting and understanding all of these microbial interactions will contribute to a more comprehensive knowledge of benthic marine ecology and biology.