The polymorphic diatom Phaeodactylum Tricornutum: Ultrastructure of its Morphotypes

The ultrastructure of the oval, fusiform and triradiate morphotypes of Phaeodactylum tricornutum Bohlin is described. The organization and structure of the cytoplasmic organelles is similar in all three morphotypes, except that the vacuoles occupy the extra volume created by the arms of the fusiform and triradiate cells. The frustule in fusiform and triradiate cells is organic; in the oval type it may be organic or one of the valves may have a silica frustule surrounded by an organic wall. In all cells, the organic cell wall has up to 10 silica bands (13 nm wide) embedded in its surface in the girdle region, lacks girdle bands, and has an outer corrugated cell wall layer, except in the girdle region. Cell division, organic wall formation and silica deposition are described in detail. Four types of oval cells are also described. The relation to other diatoms is discussed

Ultrastructure of a chain-forming diatom Phaeodactylum Tricornutum

The ultrastructure of a chain‐forming clone of the polymorphic diatom Phaeodactylum tricornutum Bohlin has been studied by scanning and transmission electron microscopy. Both fusiform and tri‐radiate cells are capable of forming chains. The cells, lacking any silica shell, are attached to each other at the central region of the theca, leaving the arms free. Neither homogenization nor sonication completely disrupts the chains. The attachment is due to fusion of the cell wall in the central region of the cell during cell wall deposition. This fusion results from failure of the cytoplasmic cleavage furrow to separate the plasma membranes of the two daughter cells sufficiently so that a single wall is deposited instead of two separate walls. Possible explanations for this are discussed

Poly-gamma-glutamate in bacteria.

International audiencePoly-gamma-glutamate (PGA), a natural polymer, is synthesized by several bacteria (all Gram-positive), one archaea and one eukaryote. PGA has diverse biochemical properties, enabling it to play different roles, depending on the organism and its environment. Indeed, PGA allows bacteria to survive at high salt concentrations and may also be involved in virulence. The minimal gene sets required for PGA synthesis were recently defined. There are currently two nomenclatures depending on the PGA final status: cap, for 'capsule', when PGA is surface associated or pgs, for 'polyglutamate synthase', when PGA is released. The minimal gene sets contain four genes termed cap or pgs B, C, A and E. The PGA synthesis complex is membrane-anchored and uses glutamate and ATP as substrates. Schematically, the reaction may be divided into two steps, PGA synthesis and PGA transport through the membrane. PGA synthesis depends primarily on CapB-CapC (or PgsB-PgsC), whereas PGA transport requires the presence, or the addition, of CapA-CapE (or PgsAA-PgsE). The synthesis complex is probably responsible for the stereochemical specificity of PGA composition. Finally, PGA may be anchored to the bacterial surface or released. An additional enzyme is involved in this reaction: either CapD, a gamma-glutamyl-transpeptidase that catalyses anchorage of the PGA, or PgsS, a hydrolase that facilitates release. The anchoring of PGA to the bacterial surface is important for virulence. All cap genes are therefore potential targets for inhibitors specifically blocking PGA synthesis or anchorage

Evolution of the diatoms: insights from fossil, biological and molecular data

Molecular sequence analyses have yielded many important insights into diatom evolution, but there have been few attempts to relate these to the extensive fossil record of diatoms, probably because of unfamiliarity with the data available, which are scattered widely through the geological literature. We review the main features of molecular phylogenies and concentrate on the correspondence between these and the fossil record; we also review the evolution of major morphological, cytological and life cycle characteristics, and possible diatom origins. The first physical remains of diatoms are from the Jurassic, and well-preserved, diverse floras are available from the Lower Cretaceous. Though these are unequivocally identifiable as centric diatoms, none except a possible Stephanopyxis can be unequivocally linked to lineages of extant diatoms, although it is almost certain that members of the Coscinodiscophyceae (radial centrics) and Mediophyceae (polar centrics) were present; some display curious morphological features that hint at an unorthodox cell division mechanism and life cycle. It seems most likely that the earliest diatoms were marine, but recently discovered fossil deposits hint that episodes of terrestrial colonization may have occurred in the Mesozoic, though the main invasion of freshwaters appears to have been delayed until the Cenozoic. By the Upper Cretaceous, many lineages are present that can be convincingly related to extant diatom taxa. Pennate diatoms appear in the late Cretaceous and raphid diatoms in the Palaeocene, though molecular phylogenies imply that raphid diatoms did in fact evolve considerably earlier. Recent evidence shows that diatoms are substantially underclassified at the species level, with many semicryptic or cryptic species to be recognized; however, there is little prospect of being able to discriminate between such taxa in fossil material