Adaptation to an Intracellular Lifestyle by a Nitrogen-Fixing, Heterocyst-Forming Cyanobacterial Endosymbiont of a Diatom

The symbiosis between the diatom Hemiaulus hauckii and the heterocyst-forming cyanobacterium Richelia intracellularis makes an important contribution to new production in the world’s oceans, but its study is limited by short-term survival in the laboratory. In this symbiosis, R. intracellularis fixes atmospheric dinitrogen in the heterocyst and provides H. hauckii with fixed nitrogen. Here, we conducted an electron microscopy study of H. hauckii and found that the filaments of the R. intracellularis symbiont, typically composed of one terminal heterocyst and three or four vegetative cells, are located in the diatom’s cytoplasm not enclosed by a host membrane. A second prokaryotic cell was also detected in the cytoplasm of H. hauckii, but observations were infrequent. The heterocysts of R. intracellularis differ from those of free-living heterocyst-forming cyanobacteria in that the specific components of the heterocyst envelope seem to be located in the periplasmic space instead of outside the outer membrane. This specialized arrangement of the heterocyst envelope and a possible association of the cyanobacterium with oxygen-respiring mitochondria may be important for protection of the nitrogen-fixing enzyme, nitrogenase, from photosynthetically produced oxygen. The cell envelope of the vegetative cells of R. intracellularis contained numerous membrane vesicles that resemble the outer-inner membrane vesicles of Gram-negative bacteria. These vesicles can export cytoplasmic material from the bacterial cell and, therefore, may represent a vehicle for transfer of fixed nitrogen from R. intracellularis to the diatom’s cytoplasm. The specific morphological features of R. intracellularis described here, together with its known streamlined genome, likely represent specific adaptations of this cyanobacterium to an intracellular lifestyle

Replication Data for: Extracellular Electron Transfer by Shewanella oneidensis Controls Palladium Nanoparticle Phenotype

Biological production of inorganic materials is impeded by relatively few organisms possessing genetic and metabolic linkage to material properties. The physiology of electroactive bacteria is intimately tied to inorganic transformations, which makes genetically tractable and well-studied electrogens, such as Shewanella oneidensis, attractive hosts for material synthesis. Notably, this species is capable of reducing a variety of transition-metal ions into functional nanoparticles, but exact mechanisms of nanoparticle biosynthesis remain ill-defined. We report two key factors of extracellular electron transfer by S. oneidensis, the outer membrane cytochrome, MtrC, and soluble redox shuttles (flavins), that affect Pd nanoparticle formation. Changes in the expression and availability of these electron transfer components drastically modulated particle phenotype, including particle synthesis rate, structure, and cellular localization. These relationships may serve as the basis for biologically tailoring Pd nanoparticle catalysts and could potentially be used to direct the biogenesis of other metal nanomaterials

Phaeodactylum tricornutum UTEX640. Whole cells.

<p>Fig 1a. TEM thin section of an oval cell across the transapical plane. A single valve is observed at the top of the cell. Fig 1b. SEM of an entire cell after critical point drying, with a visible siliceous valve. Fig. 1c. SEM of a critically point dried cell in girdle view. There was no valve on either side.</p

Phaeodactylum tricornutum UTEX640. Valve biological interior (side towards the cell) from acid cleaned material.

<p>Fig 3a. Largely intact valve showing greatly thickened longitudinal costae around raphe. Fig. 3b. Central area of a broken valve showing recurved proximal raphe ends.</p

Phaeodactylum tricornutum UTEX640. Valve biological exterior (side towards the environment) from acid cleaned material.

<p>Fig 2a. Largely intact valve illustrating simple proximal endings of raphe in exterior view. Fig 2b. Broken valve in external view. 2c. Higher magnification of central area of the specimen in 2b. The arrow indicates the position of the recurved raphe ending, best seen in broken valves or in interior view.</p

Simplified maximum likelihood tree.

<p>The optimal maximum likelihood tree with crown clades collapsed. Numbers at nodes are bootstrap support values of 50% or above. The solid line clades or terminal strains represent taxa which could not be rejected by the AU test as sister to <i>P</i>. <i>tricornutum</i> plus <i>G</i>. cf. <i>exigua</i>.</p