“SCATTERED CP ‐NUCLEOIDS” IN DIATOMS EXPLAINED: BACTERIA ‐ INSIDE THE ENDOPLASMIC RETICULUM ‐ PIERCE THE PLASTIDS OF PINNULARIA

Geitler's (1937) observation on the regular perforation of the chloroplasts of Pinnularia nobilis and the lack of a typical pyrenoid is supported using cytochemistry, and fluorescence & electron microscopy. Chloroplast cavities and channels harbour bacteria, their DNA elicit DAPI‐fluorescence. Previous speculations on the DNA‐positive, achromatic dots in the plastids of several Pinnularia species being “scattered chloroplast nucleoids,” and the consequent separation of diatom plastids in “primitive” and “advanced” types, are thus refuted. Bacteria are rod‐shaped and as suggested by their TEM‐profiles gram‐negative proteobacteria. They occur inside the endoplasmic reticulum throughout the cell, in interphase prevailingly in the space between the periplastidial membrane (PPM) and the CER membrane. Clusters of bacteria inside ER‐cisternae near the nucleus in preprophase, and near the new plasmalemma after cleavage, indicates a cell‐cycle dependent translocation within the diatom. Cavities in the plastids seem to be created by a combination of mechanical and chemical forces, in the sequence: alignment, attachment, deformation and lysis, initially without an obvious disruption of the PPM and the chloroplast‐envelope. Our future goal is to identify the bacteria and their metabolic function in the hereditary association with the plastids in Pinnularia. Their intracellular, but extraplasmatic, location may reflect an ancestral constellation, and the apparent worldwide distribution may substantiate this view.