“Candidatus Mystax nordicus” Aggregates with Mitochondria of Its Host, the Ciliate Paramecium nephridiatum

Extensive search for new endosymbiotic systems in ciliates occasionally reverts us to the endosymbiotic bacteria described in the pre-molecular biology era and, hence, lacking molecular characterization. A pool of these endosymbionts has been referred to as a hidden bacterial biodiversity from the past. Here, we provide a description of one of such endosymbionts, retrieved from the ciliate Paramecium nephridiatum. This curve-shaped endosymbiont (CS), which shared the host cytoplasm with recently described “Candidatus Megaira venefica”, was found in the same host and in the same geographic location as one of the formerly reported endosymbiotic bacteria and demonstrated similar morphology. Based on morphological data obtained with DIC, TEM and AFM and molecular characterization by means of sequencing 16S rRNA gene, we propose a novel genus, “Candidatus Mystax”, with a single species “Ca. Mystax nordicus”. Phylogenetic analysis placed this species in Holosporales, among Holospora-like bacteria. Contrary to all Holospora species and many other Holospora-like bacteria, such as “Candidatus Gortzia”, “Candidatus Paraholospora” or “Candidatus Hafkinia”, “Ca. Mystax nordicus” was never observed inside the host nucleus. “Ca. Mystax nordicus” lacked infectivity and killer effect. The striking peculiarity of this endosymbiont was its ability to form aggregates with the host mitochondria, which distinguishes it from Holospora and Holospora-like bacteria inhabiting paramecia.

Toxic effects of metals on two euryhaline ciliate species adapted to variable salinities

Brackish waters having salinity lower than seawater but higher than fresh water are of high importance for mankind. They serve as waterways, sources of sea food, sand or gravel, and play role in number of biogeochemical cycles including nutrients. However, human activities lead to serious pollution in these ecosystems affecting aquatic biota, migrating birds and others. The available aquatic ecotoxicity assays use either organisms from freshwaters or species from marine waters. Tools to assess the toxic effects of chemicals under the intermediate salinities are thus limited. The present study developed a 72-h growth inhibition assay in 48-well microplates using two euryhaline ciliates Paramecium dubosqui and Paramecium nephridiatum, each adapted to two different salinities: 0.5‰ and 5‰. The toxicities of metals under variable salinity conditions differed (HgCl