Complex array of endobionts in Petalomonas sphagnophila, a large heterotrophic euglenid protist from Sphagnum-dominated peatlands

Abstract

Petalomonas sphagnophila is a poorly studied plastid-lacking euglenid flagellate living in Sphagnum-dominated peatlands. Here we present a broad-ranging microscopic, molecular and microspectrophotometric analysis of uncultured P. sphagnophila collected from four field locations in Nova Scotia, Canada. Consistent with its morphological characteristics, 18S ribosomal DNA (rDNA) phylogenies indicate that P. sphagnophila is specifically related to Petalomonas cantuscygni, the only other Petalomonas species sequenced to date. One of the peculiar characteristics of P. sphagnophila is the presence of several green-pigmented particles approximately 5 mum in diameter in its cytoplasm, which a previously published study suggested to be cyanobacterial endosymbionts. New data presented here, however, suggest that the green intracellular body may not be a cyanobacterium but rather an uncharacterized prokaryote yet to be identified by molecular sequencing. 16S rDNA library sequencing and fluorescence in situ hybridizations show that P. sphagnophila also harbors several other endobionts, including bacteria that represent five novel genus-level groups (one firmicute and four different proteobacteria). 16S rDNA phylogenies suggest that three of these endobionts are related to obligate intracellular bacteria such as Rickettsiales and Coxiella, while the others are related to the Daphnia pathogen Spirobacillus cienkowskii or belong to the Thermoactinomycetaceae. TEM, 16S rDNA library sequencing and a battery of PCR experiments show that the presence of the five P. sphagnophila endobionts varies markedly among the four geographic collections and even among individuals collected from the same location but at different time points. Our study adds significantly to the growing evidence for complex and dynamic protist-bacterial associations in nature.

Molecular Identification of Rickettsial Endosymbionts in the Non-Phagotrophic Volvocalean Green Algae

“…Group III contained Orientia tsutsugamushi and the endosymbiont ( Candidatus Cryptoprodotis polytropus) of the freshwater ciliate Pseudomicrothorax dubius [13], and was positioned most basally within the Rickettsiaceae. Just outside the Rickettsiaceae, a lineage including endosymbionts of the plastid-lacking euglenid Petalomonas sphagnophila [15] was positioned. The endosymbionts from the two volvocalean species showed 92.5–93.6% sequence similarity with the bacteria in group I, 95.5–99.5% with other group II bacteria, and 88.7–89.1% with those in group III.…”

“…These rickettsias are phylogenetically placed in separate positions within the Rickettsiaceae [13], [14]. Moreover, endosymbionts closely related to the Rickettsiaceae have been discovered within the cells of the plastid-lacking heterotrophic euglenid flagellate Petalomonas sphagnophila [15]. Little is known about the virulence or contribution of these endosymbionts to their non-arthropod hosts [16].…”

“Candidatus Finniella” (Rickettsiales, Alphaproteobacteria), Novel Endosymbionts of Viridiraptorid Amoeboflagellates (Cercozoa, Rhizaria)

“…Nucleicultrix" (clade C/N) (42). Eutreptiella and Petalomonas, two distantly related Euglenozoa (Excavata), harbor midichloriacean and paracaedibacteracean endosymbionts, respectively (41,62). A Rickettsia species was found in the cytoplasm of the naked, filose amoeba Nuclearia pattersonii (Nucleariidae, Opisthokonta) (73), and representatives of the "Ca.…”

“…(37-39), "Ca. Captivus" was found in unidentified protists that inhabit acid mine drainage (40), and the yet-undescribed Ric2 endosymbionts reside in Petalomonas sphagnophila (41).…”

A Resurgence in Field Research is Essential to Better Understand the Diversity, Ecology, and Evolution of Microbial Eukaryotes

“…(Okamoto and Inouye ) and the freshwater euglenid Petalomonas sphagnophila associating with several different kinds of endosymbiotic bacteria (Kim et al. ), have not been cultured despite numerous efforts. Furthermore, the symbiotic relationships could be altered during the culturing procedures, such as elimination of one or more partners in the case of facultative associations (e.g.…”