Trichocyst

Abstract: Alveolates are a recently recognized group of unicellular eukaryotes that unites disparate protists including apicomplexan parasites (which cause malaria and toxoplasmosis), dinoflagellate algae (which cause red tides and are symbionts in many corals), and ciliates (which are microscopic predators and common rumen symbionts). Gene sequence trees provide robust support for the alveolate alliance, but beyond the common presence of membranous sacs (alveoli) subtending the plasma membrane, the group has no unifying morphological feature. We describe a family of proteins, alveolins, associated with these membranous sacs in apicomplexa, dinoflagellates, and ciliates. Alveolins contain numerous simple peptide repeats and are encoded by multigene families. We generated antibodies against a peptide motif common to all alveolins and identified a range of apparently abundant proteins in apicomplexa, dinoflagellates, and ciliates. Immunolocalization reveals that alveolins are associated exclusively with the cortical regions of apicomplexa, dinoflagellates, and ciliates where the alveolar sacs occur. Alveolins are the first molecular nexus between the unifying structures that defines this eukaryotic group. They provide an excellent opportunity to explore the exceptional compartment that was apparently the key to a remarkable diversification of unique protists that occupy a wide array of lifestyle niches.

Abstract: SYNOPSIS. Intact algal chloroplasts were found by electron microscopy in the peripheral cytoplasm of 2 ecologically important species of planktonic marine ciliates. The exact origin of the chloroplast is uncertain and the periods of their retention by the protozoa is unknown. The 2 ciliate species may function partially or fully as primary producers. In other ciliates algal eye-spots are retained and may actually be utilized. Ecologic and evolutionary implications of the observations are discussed.

Abstract: We found two tide-pool ciliates, Strombidium oculatum and Strombidium stylifer, that ingest ulvaceous green macroalgae and retain their chloroplasts. Sequences of the form I ribulose bisphosphate carboxylase/oxygenase (rubisco) large subunit gene (rbcL1) from chloroplasts found in the ciliates cluster with the Ulvophyceae sequences on GenBank and with those of ulvaceous macroalgae from pools in which the ciliates were collected. In addition, we have cultivated S. stylifer in vitro on filtered seawater supplemented with pieces of Enteromorpha thalli that had been treated with light and temperature shock to maximize production of reproductive unicells (swarmers). An average growth rate of 1.08 ± 0.07 SE [standard error] d -1 was measured when S. stylifer was grown this way. Because both ciliates and the algal swarmers contain eyespots while vegetative cells in the Ulvophyceae do not, we speculate that these reproductive cells are the source of both the chloroplasts and the pigments used in the ciliates' eyespots. This ciliate will not grow in the dark and is required to ingest fresh chloroplasts every few days, making it an obligate mixotroph. Ingestion of macroalgae by ciliates constitutes an upside-down trophic link, contrary to the usual pattern for planktonic food webs, in which production passes from very small organisms to successively larger ones. Our finding suggests that macroalgal production, heretofore believed to contribute predominantly to detrital or macroherbivore food chains, may be an important food source in the nearshore plankton and that ciliates may play an important role in this trophic pathway.

Abstract: We examine the origin of harpoon-like secretory organelles (nematocysts) in dinoflagellate protists. These ballistic organelles have been hypothesized to be homologous to similarly complex structures in animals (cnidarians); but we show, using structural, functional, and phylogenomic data, that nematocysts evolved independently in both lineages. We also recorded the first high-resolution videos of nematocyst discharge in dinoflagellates. Unexpectedly, our data suggest that different types of dinoflagellate nematocysts use two fundamentally different types of ballistic mechanisms: one type relies on a single pressurized capsule for propulsion, whereas the other type launches 11 to 15 projectiles from an arrangement similar to a Gatling gun. Despite their radical structural differences, these nematocysts share a single origin within dinoflagellates and both potentially use a contraction-based mechanism to generate ballistic force. The diversity of traits in dinoflagellate nematocysts demonstrates a stepwise route by which simple secretory structures diversified to yield elaborate subcellular weaponry.