Microarchitecture and biomineralization of the envelope in Trachelomonas and Strombomonas (Euglenophyceae)

The extracellular envelopes of the euglenoid flagellates Trachelomonas and Strombomonas are composed of an organic mucilaginous component and an inorganic metal component, and vary in color from hyaline to dark brown. Envelope color, micro architecture and elemental composition are interrelated and may be selected for by manipulation of culture conditions. Analytical electron microscopy (AEM) shows that envelope microarchitecture may be granular and/or needle-like, with 5-nm-wide and 3-nm-wide deposits, respectively. Electron-microscopic energy-dispersive X-ray spectroscopy (EM-EDS) indicates that granular components are iron-enriched; needle-like components are manganese-enriched; if present in the same envelope, iron (Fe) and manganese (Mn) are spatially segregated, with Fe along the margins and Mn in the central region. These metals, present as oxides, are deposited either on or in microfibrils that anastomose to form the interwoven reticulum of the organic matrix. Cells grown in Mn-supplemented medium produce envelopes with needle-like components, whereas those grown in Fe-supplemented medium produce envelopes with granular components. Supplemental nitrogen (N) in the culture medium results in a more rapid production of heavily mineralized envelopes that contain both Mn and Fe. Mechanisms of Mn and Fe deposition may be different, since autoxidation of Fe occurs on envelope mucilage, whereas Mn does not autoxidize. Envelope development in T. lefevrei is apparently different from other Trachelomonas species thus far investigated in that a primary skin may not be required; in T. lefevrei a thin, mucilaginous layer, morphologically similar to a "skin" is produced but becomes an integral part of the subsequently mineralized envelope. Historically, the taxonomy of enveloped euglenoids has been based on gross morphological features of the envelope, e.g., size, shape, and ornamentation. These characteristics are, however, variable even within a single species. Comparative AEM of Trachelomonas and Strombomonas shows that envelopes are highly complex structures, both chemically and physically. Some species deposit only Mn, some only Fe, some both. Gross morphological features vary from individual to individual within a given species; microarchitecture does not; the characteristic granular/needle-like components persist. These data suggest that the taxonomic disposition of Strombomonas be reconsidered and that envelope microarchitecture and elemental composition may serve as useful taxonomic criteria in future studies of enveloped euglenoids.