Sunday Protist -- Auranticordis: "Orange Heart"

(Also the answer to MM12, which nobody got, although I haven't particularly followed up on the hints either...)

If you're ever low on ideas for your next taxonomic adventure, Cercozoa is one endlessly fascinating clade that never fails to offer surpises. In fact, the more you explore them, the less familiar they become. Cercozoa (TC-S 1998) can offer pretty much anything -- from photosynthetic amoebae to the utterly bizarre Phaeodaria to beautiful testate Euglyphids (including a case of separate primary plastid endosymbiosis!) to a wide variety of random semi-amoeboid-semi-flagellate things that glide about on their flagella and make faces at anyone trying to classify these things by morphology. Cercozoa are a mess, but a beautiful mess.

SEM from looking down at the anterior apex of the cell. Bar = 10um; Chantangsi et al. 2008 BMC Microbiol (open access)

The baby Nautilus looking thing that was so mischievously confusing is Auranticordis quadriverbis, a rare benthic tetraflagellate of a distinctive orange colour.

A - L: lobe, arrows: orange muciferous bodies; C - arrowheads: putative endosymbionts. D - arrowhead points to ventral groove. E - phase contrast showing flagella protruding from the ventral groove. Bar = 10um. (Chantangsi et al. 2008 BMC Microbiol)

The orange colour is caused by the muciferous bodies lining the ridges along the cortex. These bodies secrete mucilage upon disturbance, and may participate in adhesion to the surface. The membranous ridges on the surface are supported by bundles of microtubules, in the absense of any cell wall. An SEM of the ridges reveals the mucilage being secreted by the pores that line them:

Sem close-up of the ridges with mucilage. Scalebars: C - 1um, D,E - 0.5um
(Chantangsi et al. 2008 BMC Microbiol)

Curiously enough, Auranticordis posesses potential endosymbionts -- thylakoid-bearing orange bodies inside the cytoplasm. While those could be food particles being digested, the lack of different extents of digestion and their intact appearance suggests these endosymbionts may be long-term (as in kleptoplasty) or permanent (endosymbiosis). The similarity of the ultrastructure to free-living cyanobacteria actually suggests this may be yet another case of primary endosymbiosis, as in Paulinella! (cyanobacteria don't have to be blue-green, by the way)

TEM of putative endosymbionts. The lines at the edges are thylakoids, surrounding a prominent invagination of the inner membrane. Bar = 2um (Chantangsi et al. 2008 BMC Microbiol)

Inside the endosymbionts one can find viruses that look similar to the ones that infect free-living cyanobacteria. The idea of a virus attacking your organelles, while not actually surprising, is still kind of cool!

Higher magnification TEM of the putative endosymbiont. B shows detail of the thylakoids C - cleavage furrow (arrowheads) supporting the idea of this body being an endosymbiont rather than a food particle, SC - sac-like vesicles. D - Viral particles at the thylakoid-free core. E - detail of the prominent inner membrane invagination. G - close up of more viral particles, (bar=0.5um) with an inset showing a complete one (bar=0.2um) (Chantangsi et al. 2008 BMC Microbiol)

Unfortunately these organisms are rare, so it's difficult to establish whether these pigmented bodies are in fact a third known case of primary plastid endosymbiosis. Wouldn't be so shocking after Paulinella, but still kind of cool. Perhaps primary endosymbiosis isn't as rare as previously thought, and that, were it true, could have a profound impact on our understanding of eukaryotic evolution. Conventionally, plastid gain is used as a very unlikely feature, and weighed heavily in analyses of parsimony. But the real picture could actually be much messier.

I hope you've enjoyed this brief encounter with an enigmatic, obscure Cercozoan. Warning: There's more where that came from, so expect more Cercozoa in the future. I really love that group...

Reference:
Chantangsi, C., Esson, H., & Leander, B. (2008). Morphology and molecular phylogeny of a marine interstitial tetraflagellate with putative endosymbionts: Auranticordis quadriverberis n. gen. et sp. (Cercozoa) BMC Microbiology, 8 (1) DOI: 10.1186/1471-2180-8-123